AD587LW - 10V precision travel standard

[Andreas]

Overview:

Hello,

I will describe a 10V travel standard based on the AD587JQ.
Inspired from the US calibration club from Jason (cellularmitosis).
The cirquit is based on the proposal from Lars Walenius a enhanced SVR-T cirquit.
So the project name had to be AD587LW.

Goal is to have a temperature compensated cirquit (with NTCs)
and the good long term stability of a AD587JQ.
I intentionally wanted to have a travel standard which gets shipped "hot".
So no room for a heater and only minimal battery consumption. (around 5 mA)
Further targets: keep it below 5 cm heighth so that it can be shipped as letter within germany.
And weight should be below 500 g.

As Lars told me that the former discontinued AD587LQ had around 2 ppm/K
I will have to select among the JQ devices the best with < 2 ppm/K.
First measurements turned out that most of the currently sold devices have around 7-8 ppm/k and
some (3-4 within 10 devices) have near 2 ppm/K or even below.

So with temperature compensation (factor 10 enhancement over a small +/- deg C range)
a device with 2 ppm over my lab temperature range and 2 ppm ageing/year after a run-in time of 5-10 kHrs
seems to be feasable so with 1 ppm reserve for noise it could have around 5 ppm over a year.

- The device will have a switching power supply from a USB-charger
  (so do not measure during charging)
  with 12xNiMH AAA batteries for about 150 hours. (when new).

- The temperature compensation cirquit.

- a buffered output (highly recommended on a review by Lars)
  so mis-treating the output will only have minor changes on
  the output voltage after exchange of the LTC2057.

- and a controller which is able to measure temperature
  (for logging by RS232) and battery voltage
  and switch off the power supply before the batterie goes dead.
  Of course the connection to the PC is galvanically isolated.

work in progress: posts will be updated soon

[Andreas]

schematic: (first version)

the whole schematic is attached as .pdf.

It consists of several parts:

- the battery charger unit.
  Intention is to use every USB charger with USB-mini connector.
  Including a mobile power bank.

- the LT1763 based voltage regulator to keep the AD587 and
  the output buffer under constant conditions.
  Can be switched off in case of low battery.

- The AD587 part which is essentially the schematics that Lars published here:
  https://www.eevblog.com/forum/metrology/best-out-of-the-box-10v-reference/msg1197526/#msg1197526
  except that I replaced the passive NTC for temperature read out by a active read out.

- A LTC2057 based output buffer (to avoid damages to the AD587 in case of short cirquit etc.).

- And a battery monitor with a own power supply
  which samples the temperature and the battery voltage
  and sends the info to a PC by serial connection.

with best regards

Andreas

[Andreas]

power supply:

a) USB-charger.
Design goal was to have a charger that can be used with every (0.5A) USB Port/charger with a Mini-USB connector.
I first wanted to have one of those tiny SOT-23-5 1.6 - 2.5 MHz step up converters with tiny 4.7-10uH inductor.
The problem was that I did not find any device which can reliably limit the input current to below 500mA under all conditions.
Overloading a USB-port with more than 500mA can lead to a tripping of the polymer fuse.
After once having tripped it will have around double the initial on state resistance which may lead to a unreliable port for higher loads.

So I finally ended in a good old MC34063A design strictly layed out according to data sheet for a minimum input voltage of 4.2V (5V nominal) and
A maximum output voltage of 18.1 V to charge 12 NiMH cells via a diode to 1.45V/cell at room temperature.
The output voltage can be adjusted by P21 to the exact value of 18.1V measured @ room temperature between TP 4 + TP5.
NTC R32 gives a negative temperature compensation of around -3mV/cell/deg C which compensates the cell voltage.
So at high battery temperature the charging voltage (and thus current) is reduced preventing that the cells are cooked on hot summer days.

The current limit is done by resistors R34+R35 to a value below 0.5A.
I first wanted to use 3*1 Ohms in parallel for around 0.9A peak and 0.5A average current.
But in certain load situations the MC34063A goes nearly in a continuous on-state, so you have
Effectively to limit the peak current to 0.5A-0.6 max to get maximum 0.5A input current.
This ends that only R34 (1 Ohms) + R35 (1.5 Ohms) are populated for the 3 resistors.
Remaining charging current is around 80-90mA max for the 12 AAA cells (850-1000 mAH) so around C/10.
So when charging empty cells it will need around 14 hours until the full cell voltage is reached.

All was tested on a perf-board before making the final layout.

b) batteries
As batteries 12*AAA NiMH cells are choosen.
Giving between 13.8 - 17.2V usable output voltage range.
Main restriction for the size was the aluminium case height.

c) linear low noise regulator.
Here I am using my standard LT1763 cirquit with nominal 14V output voltage for 12 NiMH cells
To stabilize the output voltage for the AD587 which needs minimum 13.5V as input voltage.
This measure is mainly to keep the self-heating of the AD587 constant and improve PSRR.
The LT1763 can be switched off in case of too low battery voltage to protect the batteries.

with best regards

Andreas

Update: some scope measurements on the perf-board cirquit with full load conditions
(either full load current at 18V or near shorted output at around 6V).
URSC = voltage over current limiting resistor.

[Andreas]

AD587 section and selection criterias

The AD587 section corresponds to the enhanced SVR-T cirquit of Lars.
  https://www.eevblog.com/forum/metrology/best-out-of-the-box-10v-reference/msg1197526/#msg1197526

The input voltage is 14V (due to battery supply)
 so above the minimum 13.5 V spec in the AD587 data sheet.
C13 filters the wideband noise.
C12 + C17 are against EMI. (partly missing in the original SVR-T cirquit).
P3 is used to adjust the output voltage: with a reduced trimming range
so that R8 or R7 is needed to do the raw adjustment.
P2 is used to adjust the T.C. together with either R5+R6
 (SMD NTCs on the bottom side of the AD587) or alternatively with R1-R3 as through hole NTCs.
TP1-TP3 are testpoints for adjustment.
If the voltage difference betwee TP1+TP2 is zero then the T.C. compensation is also zero.
T.C. can be adjusted in both directions +/- by around 4 ppm/K/V according to the voltage difference of TP1 + TP2.
Of course this correction works only in a small (around +/- 5-10 deg C range).

R22 is for sensing the temperature of the reference. (will be displayed or logged on a PC).

For the selection of the AD587 references I bought some samples  and measured the T.C. and 0.1-10 Hz noise.
Main goal is to select a device with less than 2 ppm/K.
And a normal noise behaviour: i.e. below 6uVpp for the 1/f noise and low "popcorn" noise (can be seen only partial with the 0.1-10 Hz LNA).
Since the popcorn noise hurts more than the T.C. (which can be adjusted in a small temperature range) I decided the following for the "uncertainity score".
The T.C. in ppm/K is improved by a factor 10 by the adjustment.
 So every ppm/K (10uV/K) counts only 1 uV/K in the final score.
The measured maximum popcorn noise (you never get the maximum) counts * 2 in the final score.
score (uV) = T.C./10 (uV/K) * 10 deg C + 2 * max measured popcorn noise (uV).

To get a final spec of 5ppm/year with 1-2 ppm drift/year I have to select devices with a uncertainity score of less than 30uV which is reached when having 2 ppm/K and 5uV popcorn noise. A good device should stay below 20uV in the score.

So from the listed AD587 I decided to use AD587JQ#03 for my first cirquit and AD587JQ#14 for the 2nd travel standard.
#22 which is a excellent device that needs no T.C. adjustment will be used for a special purpose.

with best regards

Andreas

[Andreas]

output buffer:

the output buffer is my standard cirquit around U5 which I also use for my buffered LTZ references.
R15/C20 stabilizes the cirquit against capacitive loads e.g. the EMI-filter (or long cables).
R17 also compensates voltage drops of R15.
R13 is equal to R17 to compensate errors by input bias currents.
C18 gives a additional filtering for broadband noise above 340 Hz.
The LTC2057 can easily deliver up to 10 mA (near 30 mA in short cirquit condition).

At the output there is also a EMI filter which can be populated differently.
Either a common mode choke L6 (51uH Würth WE-SLM 744242510) together with C14+C15.
(usefull more for the lower frequency range)
Instead of the common mode choke L6 also 2 ferrrites L4+L5 (BLM31PG601) can be populated
which are useful around 100 MHz. (FM-frequency range).

I have built 2 samples one with the ferrites #02
and one with the common mode chokes #01 to make different tests.

The common mode choke has 2*0.32 Ohms max DC resistance so it adds
around 0.1 ppm max error when measuring with the 10 Meg input impedance of the meter.
The ferrites have 2*0.09 Ohms max so adding less than 0.02 ppm error.
https://www.conrad.de/de/line-filter-smd-51-h-032-2500-04-a-wuerth-elektronik-we-slm-744242510-1-st-1087399.html
https://www.conrad.de/de/smd-ferrit-600-l-x-b-32-mm-x-16-mm-murata-blm31pg601sn1l-1-st-442583.html

There are also 2 options for output connectors.
I use the CLIFF FCR7350 connectors. (gold plated)
https://www.conrad.de/de/sicherheits-laborbuchse-buchse-gewinkelt-stift-o-4-mm-schwarz-cliff-fcr7350b-1-st-419667.html
but as cheaper option also the Hirschmann connectors PB-4 can be used:
https://www.reichelt.de/Buchsen/PB-4-RT/3/index.html?ACTION=3&GROUPID=5920&ARTICLE=76865&SEARCH=pr%C3%BCfbuchse&START=0&OFFSET=16&

with best regards

Andreas

[Andreas]

battery monitor + temperature measurement

the battery monitor is built around a PIC12F1840 which has all the ingredients that are needed for a low power voltage comparator with hysteresis and a temperature measurement.
- 10 Bit ADC used ratiometrically with VDD as reference for measuring the temperature sensor on GP1
- the ADC is also used together with the 2.048V reference to measure the divided (R23/R24) battery voltage
- the hysteresis is then done in software
- a low power watchdog timer (used all 250ms to wake up the processor and make measurements all 1 or 2 seconds)
- A UART on GP0 which outputs the measured values to a terminal or a dedicated sampling software.
- a internal precision RC-oscillator (1%) for the UART

Firmware description:

The watchdog wakes the processor all 250 ms (nominal with factor 0.6/1.6 as tolerance)
In on state every fourth wakeup (in off state every 8th to save battery) a measurement is done.
GP5 is switched on to supply the temperature sensor and to switch on the voltage divider for the battery measurement.
Also the ADC and internal 2.048V (nominal) reference are switched on.
First the 33K NTC which is linearized by a 27K resistor for the 10-40 deg C temperature range
 (max 0.3 deg linearity error) is measured ratiometrically to VDD.
Then the battery voltage is measured.

The NTC is corrected (linear) by a slope factor (-0.93) and a offset (75,9 deg C for ADC zero)
to get a value in tenths of degree C which is converted to decimal and output as ASCII value on the RS232.
The battery voltage (divider+internal ADC reference) is also corrected by a factor from EEPROM.
Since I first wanted to use the 1.024V internal reference the calibration factors and measurement range 20.6V
are calculated from this value. A overall correction by factor 2 is done before the value is converted to decimal
and output as ASCII value on the LED.

; Sample #1+#2 EEPROM constants
;
    de  0xD2,0x49   ; Correction 20/19.6*20.6/25.6V*65536
    de  0xC2,0x73   ; Temperature offset 75.956 deg*655.36
    de  0xED,0x05   ; Temperature slope 0.925857

;  nominal values
;    de  0xCE,0x14   ; Correction 20.6/25.6V*65536
;    de  0xC2,0x73   ; Temperature offset 75.956 deg*655.36
;    de  0xED,0x05   ; Temperature slope 0.925857
;

the corrected values are also used for the calculation of the on/off state (hysteresis) of the reference.
when the voltage is above 15V (1.25V/cell) GP2 is set high to switch on the supply of the reference via LT1763.
Below 13.8V (1.15V/cell) GP2 is set low to switch off the reference and protect the battery.
Between 13.8V and 15V there is also a voltage drop detection (0.6V per minute)
to detect if a single cell in the stack is flat and needs a quick switch off.


Cirquit details:

C9 compensates for the parasitic capacitance of the LED and speeds up the edges.
T2 is used to switch of the voltage divider of the battery measurement. (and save current)
T2 is a logic level FET with < 1.5V threshold voltage. So at 3V minimum supply voltage a voltage of up to 1.5V can be measured at the source of T2.
Power supply of the battery monitor can either be done by a 5V voltage regulator which needs around 4uA idle current or by a BF545C FET which is switched as voltage regulator with his gate pinch off voltage (around 3-5V depending on load). I usually use the FET because it needs zero idle current.

J2 can be used to program the PIC in cirquit or adjust the EEPROM constants. I use a PICKIT3 for this.
If T1 is used I activate the power supply from PICKIT. 

with best regards

Andreas

Edit 30.03.2018: attached the firmware for the PIC and the PC-software with the necessary DLLs if you do not have VC6 installed.
Of course also a terminal program (e.g. HTERM) can be used for the first steps on the PC.

Edit 05.05.2018: Attached bug-fixed PC-Software (Rev 1: no readings in off-state of the AD587LW reference)

[Andreas]

PCB and construction

Edit: 30.03.2018 attached documentation with BOM, population plans and corrected front plate drill plan of Rev A PCB.

Edit: 01.04.2018 attached reworked BOM after hints from Lars

Edit: 07.04.2018 Description of PCB

The core of the PCB is the additional shielded area around the AD587.
Shielding is done against air drafts and also somewhat against magnetical influence by a TEKO 4020 tinned steel housing.
To make it completely shielded one would have also to put a additional cover from the bottom side.
(will see if it is necessary). Fixing of the bottom cover to the PCB can be done by soldering or by a additional screw.
The core area also gets some foam to further reduce air drafts. And also the bottom side of this area is filled with some foam to keep thermal gradients away from the COVAR pins of the CERDIP package of the reference.

On the lower side the output amplifier outside the shielded area is placed. In Rev B of the PCB there will also be a slot between the buffer and the core area to reduce thermal gradients in case of larger loading (or short cirquits) of the output buffer.
Already there a slot between voltage regulator LT1763 and core area for the same reason (different battery voltages).

The microcontroller (battery monitor) and charger are placed as far as possible from the reference to reduce cross talk.
Only the temperature sensor for sensing reference temperature is placed near the AD587.
Another temperature sensor for the battery temperature (to control the charge end voltage) is in the upper right corner.

[Andreas]

Erratas of PCB revision A of 15.01.2018 revised 30.03.2018
==========================================================
mayors:
- J1 drill positions do not fit together
  (connector pins are 0.2 mm too far against mounting holes)
- J1 not connected to case (via G2)
- L6 too close to capacitors C14,C15
- J3,J4 position in front plate drill plan 1.2mm too low
- Thermal isolation of reference against output buffer missing
- C18 connected to power ground instead of output ground

minors:
- C18 should be upgradable to 1 or 3.3 uF (10 Hz corner frequency)
  (7x7 mm outline footprint)
- testpoint for AD587 GND missing
- testpoint for 14V missing
- testpoint for 10V missing
- Pin1 marker LED missing
- D1 pin 1 marker missing
- D2 pin 1 marker missing
- component values: X7R marking missing on several capacitors.

changes for Rev B:
- reduce peak current in case of longer RS232 transmissions (47uF/25V)

[Andreas]

measurement results

1. power consumption from battery in "off state" measured over a 1K shunt -> 1 mV == 1uA.
    Average consumption in "off state" = 11uA (crude first software, could be further optimized if necessary).

2. power consumption from battery in "on state" measured over a 1K shunt
    Average consumption in "on state" = 3.5mA ~2mA for the AD587LW and ~1mA for the buffer.

[Andreas]

Revision B data of PCB. (30.03.2018)

AD587LWB_DOC_REVB.ZIP:
- Documentation with changed schematics, BOM, placing plan, drill plan for the front plate

AD587LWP_B.zip:
- Gerber data (RS-274x)
  note: there is no silk screen in the data (as it makes no sense to print over the SMD pads).

Notes 17.04.2018:
- I have not built a Rev B PCB by now. (So use data with care until I built the first devices).
- Against BOM I will try some different J-FETS (MMBF4416ACT-ND from Digi-Key) for the power supply of the processor.

Edit 22.04.2018:
- The MMBF4416A give only about 1.5/1.8V with a 1K load instead of the processor which is below the 2.3V needed.
   so I went back to my good old BF545C having more headroom. (2.5-2.7V)
- Charger, Processor supply, and 14V supply are running on 2 PCBs of Rev B. So only the AD587 part and the output buffer need to be verified.

with best regards

Andreas

Edit: if you use the gerber data it would be fine if you give me a note so that I know how many PCBs are in the wild.
        Free for non-commercial private use.

[Andreas]

reserve 2

[cellularmitosis]

       

[e61_phil]

Due to the internal diodes between the inputs of the LTC2057 a short of the output buffer will draw around 1mA out of the reference. Doesn't sound like a real problem. But one have to keep in mind, that the reference is only decoupled by the two 4k7 resistors.

[chuckb]

The new (Nov 2017) chopper OPA189 can handle full differential voltage at the input with no extra current draw on the zener. There are no input differential diodes.

[Andreas]

Due to the internal diodes between the inputs of the LTC2057 a short of the output buffer will draw around 1mA out of the reference.

I am in good companionship in this case.
even the venerable DATRON 4910 has this problem (when looking at the OP27 together with the 10K and 3K resistor) shorting even a LTZ1000 zener. (R120+R124+U111)
https://doc.xdevs.com/doc/Datron/4910_4911/4910%20c20090120%20%5B8%5D.pdf

The new (Nov 2017) chopper OPA189 can handle full differential voltage at the input with no extra current draw on the zener. There are no input differential diodes.

Good find as this is rare with precision OP-Amps. Slightly higher input bias current than 2057. But acceptable for this cirquit.

With best regards

Andreas

[Harfner]

When you log the temperature while in off state, perhaps you could also log humidity?

[bitseeker]

This looks pretty cool, Andreas. Thanks for putting it all in a nice thread.

[Andreas]

When you log the temperature while in off state, perhaps you could also log humidity?

Note: Logging is only done to a attached PC not within the board.
Humidity is not planned since I do not expect a large dependency on humidity.

with best regards

Andreas

[The Soulman]

Where will it travel to?

[Andreas]

Hello,

I have not decided finally since I first need a adjusted unit.
And want to do at least some stability measurements ...
But within EU would be a option. (without custom fees).

but be aware that I missed my 500 g target.
So postage will be at least for a 1000 g letter.
(so 7 EUR from germany to netherlands + eventually 2.50 EUR for a tracking number).
So I think for EU-shipments I will have to look for a different housing of the reference.
As below 500 g I have only 3.70 EUR for shipment.

With best regards

Andreas

[quarks]

very nice project

I am also located in Germany and you can count me in to make meassurements with 8.5 digit gear

[Andreas]

Hello,

Thanks, I will come back to your offer.

with best regards

Andreas

[Conrad Hoffman]

Nice! How important do you think continuous power is? I know for some references it matters. OTOH, my old Fluke 731s don't seem to care at all. They power up and are quickly at the same value as always. IMO, if the reference doesn't really need it, you save a lot of weight and complexity- more room for other stuff.

[Andreas]

Interesting question if it will have a influence on a AD587JQ.

for the LT1027DCLS8-5 I have a answer:

In ADC21 the readings (after 2:1 divider) on LTZ#4 (my most stable one)
decreased over 140 days from 3597.85 mV to 3597.81 mV and stabilized there.
I then switched ADC21 off for a longer time.
After that it started from 3597.83 mV and run opposite to 3597.85V (back to initial value).

Of course this behaviour depends on internal construction (mainly on die attach
which seems not to be the best on the newer SMD hermetically devices).
So it could be not necessary for a AD587JQ.

But yes I would expect from a constantly powered device that it stabilises somewhere.

with best regards

Andreas

[lars]

Nice! How important do you think continuous power is? I know for some references it matters. OTOH, my old Fluke 731s don't seem to care at all. They power up and are quickly at the same value as always. IMO, if the reference doesn't really need it, you save a lot of weight and complexity- more room for other stuff.

For the AD587xQ I don't think continuous power is very important. I enclose some long-term tests for AD587xQ, REF102,LT1021 and LT1031 in metal cases. All data is with my SVR ser.no 721 as reference. The first three graphs is with ten small plastic boxes 55x35x20mm with cheap binding posts from eBay with six AD587KQ or JQ from the nineties and REF102CM from end of nineties. The LT1021CMH is from France via eBay without date code. The fourth graph with REF102+LT1031 is an older box. All refs are compensated with NTC's (similar to AD587LW except LT1031).

In January to March 2017 I turned off the power to all 10 refs for three weeks and turned it on for three hours. I repeated this three times. On the AD587 and LT1021 it is difficult to see any effect. AD587-5 seems to have some seasonal effect. AD587-6 also may have a small seasonal effect. Both are AD587JQ with date code 9243 otherwise they share the same circuit as the others.

The last graph is older and you can see that I under two periods turned the power off for about 3 weeks between measurements. The interesting is the LT1031 that for me seems to be just a three pin version of the LT1021 but has a completely different drift.

But back to the battery in the AD587LW. I have not discussed this with Andreas as I still think it is a good idea to have a battery in the box for a travelling standard like this. The reason is to be independent from different Lab PS's and also if you enclose a small PS (adapter) it will still have a path to the mains as the DMM you compare it to, so you get a loop.

I haven't built in any batteries in my own boxes but has an extra box with two 9v alkaline batteries and an LP2951 set for 15V. I have used this when I a few times have travelled to cal labs and had the chance to test against old very stable Fluke 5700 and 8508's. I should say I have not seen any difference with battery or a small adapter during measurement but I think it is nice to have a battery to get good isolation.

During my travels to cal labs I have also had a min-max thermometer (and hygrometer) in the box as I think large temperature excursions are more of  a problem than power for the AD587xQ. Might even be important for the LTZ1000 as Mr. Pickering commented here:
https://www.eevblog.com/forum/metrology/the-calibration-of-hp-3458a-'gold'-version/msg1374690/#msg1374690

By the way it seems that my eight SVR-T's have drifted upwards about 0-4ppm in last six years and also my four main REF102CM's (that has been continuously powered 17 years, and not the same as in the graphs). So in average less than 0.5ppm/year. Probably the uncertainty is in the region of +-0.5 to 1ppm/year.

Lars

[Andreas]

During my travels to cal labs I have also had a min-max thermometer (and hygrometer) in the box as I think large temperature excursions are more of  a problem than power for the AD587xQ.

Thats a good idea.
I also thought to supply a simple thermometer. (so that we have a comparable basis) something like this (would not add significant weight to the package).

https://www.amazon.de/Budget-Digital-Thermometer-zum-Aufkleben/dp/B005LYXZRU/ref=sr_1_22?ie=UTF8&qid=1521273812&sr=8-22&keywords=aquarienthermometer&dpID=41LJ-coVlqL&preST=_SY300_QL70_&dpSrc=srch

But a Min/Max Thermometer would be even a better choice.
Which thermometer/hygrometer do you use?
On the other side I could improve the software with some logging algorithm by using the EEPROM as min/max storage.

with best regards

Andreas

[lars]

Hello Andreas,

I have used just cheap small LCD thermo/hygrometers. Noname bought from stores like IKEA. I have several old thermometers from IKEA that still is within about 0.2K after many years at room temperatures. I bought them for about 2USD.

As you already have temperature readings in the processor why not add min/max? I can´t even see that you need EEPROM for this, but it may be useful in case battery fails. What I don´t like by min/max is that you never remember when you reset it last time. Would be better to have just last month min/max in this case I think.

In my DIY GPSDO: https://www.eevblog.com/forum/projects/lars-diy-gpsdo-with-arduino-and-1ns-resolution-tic/?all I have included logging of temperature to the EEPROM every third hour for 18 days. This is of course depending of the amount of EEPROM you have available. Also in the GPSDO it is interesting to do regression analysis between the DAC value and temperature to get temperature sensitivity of the oscillator. I chose eight times a day to get good statistics but four could have been enough.

For your 10V std I think min/max is what you want. As it is battery powered also time since start might be valuable. In the GPSDO I have a seconds counter. As I have no battery backup to my always on rubidium GPSDO it is an indication of time between mains failures. So far my maximum time is about 30 million seconds (almost a year). I also increment a counter every third hour and store in EEPROM. I also stores number of restarts in EEPROM.

By the way, nice to see your update in reply#4 with your AD587JQ measurements.

Lars

[guenthert]

For the AD587xQ I don't think continuous power is very important.

That is also my experience.  I got a AD588 (it's my understanding it's the AD587 reference + a few OpAmps on a single chip) based reference from KKMOON (like https://www.tomtop.com/p-e0557.html).  It needs a bit more time to stabilize (say 20-30 minutes) than the LT1021 based DMMCheck, which is very quick to stabilize, but those Li-ion accumulator powers it for hours.

Those 4mm "Banana" sockets are IMHO more practical than the headers used on the DMMCheck from voltagestandard.com.  A combination 4mm socket/screw terminal (like many power supplies and bench multimeter offer) would be even more practical, as one could then use copper wire or low EMF spade lugs.

[Andreas]

Hmm,

that brings me to following idea:
one could store a clock with "working hours" which increments all 2 hours
(with 100000 write cycles this gives ~20 years until the processor has to be exchanged),
and writes the min and max values over lifetime together with the clock data.

A ring buffer with around 60 days back could store the raw ADC-values (to save storage place) of min and max values during one day.

Ok since the timing relies completely on the watchdog timer which is not very exact (can be factor 1.6 slower or faster than nominal) the 2 hours also are not very exact. But if you know the exact time when the clock was started and approximately the T.C. of the clock you can calculate back when a minimum or maximum has happened.

Ok it seems that the AD587 is more robust than I thougt. (is this also true if you transport it in winter or put it into the freezer). But now I have the continuous power and it will also help reducing stabilisation time.

With best regards

Andreas

[cellularmitosis]

Sounds like a good case for a "A/B" test!  Send both a powered and un-powered unit in the same package, and have the recipient measure each ref and also the difference between the two (using 100mV range).  You might be able to produce a plot over the course of a year which shows this difference grow and shrink.

[Andreas]

Hello,

Unfortunately I have forgotten to spend a switch for cold transport....
And I fear that only 2 devices (which might behave different)
have too low statistical basis to make a secure statement.
The better way would be putting one and the same device into the freezer powered + unpowered.
But even here it is not clear if we had a "one time effect" or really a systematic change.


The first PC-software for logging data is running

against a terminal program which logs only temperature + battery voltage
logging is done with time-stamps and the other file with 1 minute averages for the temperature.
So now I can do the T.C. adjustment when logging the output voltage too. (with time stamps).

with best regards

Andreas

[Andreas]

Hello,

first T.C. measurement on AD587LW#01 against internal temperature sensor.
The device is still only raw adjusted for output voltage.

To increase output voltage I have calculated R8 as around 12 Meg.
Since I had only 10 Meg at hand I used the 10 Meg instead.
The adjustment range of P3 is more than sufficient for this.

Difference TP2-TP1 changed by -30.64 mV due to R8.
So also T.C. curve has increased from initially -0.15 ppm/K to -0.056 ppm/K in average (18-33 deg C).
In my target range 18-33 deg C the change is around 1 ppm which is nearly perfect.
Ok I will adjust TP2-TP1 by another -13 mV to get near zero T.C. in the 18-33 deg C range.

Around 27-28 deg C there is about 0.4 ppm popcorn noise (2uV after 2:1 divider) visible.

The popcorn noise is also visible in the "stability/drift" measurement with near constant temperature over night.
I fear this will limit the accuracy more than the adjusted T.C. for this device.

with best regards

Andreas

[Andreas]

Hello,

today: raw adjustment of AD587LW#2 with AD587JQ#14

Ratio measurement on HP34401A showed 0.7193313 instead of 0.7195710 against my most stable LTZ#4.
Ratio is 333 ppm too low -> AD587JQ#14 is 333 ppm too high.
So I calculate R7 = 1700ppm/333 ppm = 5.1 Meg.
Having 4.7Meg in my drawer I installed it.
Shift on TP2-TP1 was -73.7 mV by this.
Adjusted ratio was very close to target. (so in my case the -1700 ppm/Meg might be -1565 ppm/Meg ppm in reality).

Mmm. What about T.C. adjustment before first measurement?
-1.35ppm/K divided by 4.2 ppm/K/V gives -321 mV difference between TP2-TP1.
-> adjusted this with P2
But now the output ratio has changed to 0.7194xx
So I have to trim this back with P3.
now the difference TP2-TP1 changed to -331 mV
-> again adjusted with P2
I think now it is ready for the first T.C. measurement.

But perhaps I should clean the board first and install the final thermal isolation ...

with best regards

Andreas

[Conrad Hoffman]

Don't underestimate the problems an unclean board can cause. Even fingerprints can be a problem, so wear gloves.

[Inverted18650]

Mr. Andreas,

Thank you for yet another great project. I have several voltage references that I am looking forward to testing and wonder if you would share your Gerber file for this PCB.  If you do, I will have a bunch made and share them with everyone here. Also, I am correct to believe that this setup would work well with many of the DIP 6, V-Ref IC's?

Thanks again Sir and looking forward to the next update.

[Andreas]

Don't underestimate the problems an unclean board can cause. Even fingerprints can be a problem, so wear gloves.

Hello,

I am usually cleaning precision board 2 times.
First time when having soldered the majority of the parts.

And a 2nd time (did this today) when having populated the adjustment parts.

I have several voltage references that I am looking forward to testing and wonder if you would share your Gerber file for this PCB.  If you do, I will have a bunch made and share them with everyone here. Also, I am correct to believe that this setup would work well with many of the DIP 6, V-Ref IC's?

I have no problems in parting with the gerbers in this case.
But: I will be not able to give support (or shipping programmed PICs) from my rare hobby time.
So this will be your task. 

And no you are not correct: the cirquit is optimized especially for the AD587.
It may work (with adaptions) for other (buried zener) references which have a linear +3/-1
ratio trim range (AD587 has +6%/-2%) and need no special impedance on the trim pin.
So especially those references similar to LT1021/LT1236 (which need a diode and a resistor)
will not work properly.

And there are still some issues with the first design which I want to solve before I
spread the design widely.

with best regards

Andreas

[Andreas]

Just some impressions:

- RS232 read out cirquit with photo transistor
- Thermal isolation Top + Bottom to reduce air drafts around the pins.
- Fix for missing ESD-Connection of USB-Connector

[cellularmitosis]

What are the details of that little metal box?

[Andreas]

further pictures

- measurement of TC adjustment
- guard pin (since I am on battery I can use the housing, isolated by bumpers, as guard)
- weight of #1 and #2 (bot too heavy for cheap transport limit of 500 g with packaging)
  on #2 there is still a missing screw

The TEKO 4020 box is mainly for equalizing temperature around the reference.
(reducing air drafts)

with best regards

Andreas

[Andreas]

Hello,

AD587LW#01 after first TC trimming.
Output voltage has gone up a bit by the trimming
(so I should have corrected this before a new T.C. measurement)

The result is amazing:
The regression curve over 18-33 deg C shows a zero T.C. (3 ppb/K)
with around +/- 0.4 ppm deviation due to noise and T.C.

with best regards

Andreas

[Andreas]

Hello,

first T.C. measurement on AD587LW#02
remember: I had pre-adjusted the raw T.C. of AD587JQ#14 of -1.35 ppm/K according to the formula of Lars. (-321 mV)

And Lars did a very good job with the formula: 

on the first run only 3.1 ppm change over 20 deg C span
In my reduced span it is only 2.1 ppm change over 15 deg C. so 0.14 ppm
 box T.C. with a average linear slope of 0.05 ppm/K.

So as expected a around factor 10 improvement to the raw T.C. of the reference.
The resulting error curve has a large 2nd order part.
So without changeing the curvature of the compensation there will be not much improvement.

Hmm do I have to increase or decrease R9 or even put a parallel resistor to the NTCs to compensate the 2nd order part?
@Lars: any quick idea? (otherwise I will have to modelise the cirquit somehow).

with best regards

Andreas

[MiDi]

Nice project!
Did you think about using Li-Ion or even Li-Po cells to reduce weight and space while maintaining the time to run on batteries?

[Andreas]

Yes I thought about that.

The AD587 needs minimum 13.5 V (+ tolerance of the voltage regulator).
LI-cells have 3V when empty -> minimum 5 cells needed if you want to use full capacity.
And of course I would want to use cheap 18650 cells for longer run time which have 46g/cell -> 230 g.
Whereas 12 NiMh AAA cells including the 3 battery holders are 200g.
Ok 16500 cells are only 28g/cell -> 140 g. without battery holder. So I could save ~50g which is not enough.

5 cells have 21V when fully charged. -> I cannot use my favourite LT1763 voltage regulator. (max 20V)
How about additional balancing cirquit, transport limitations, ...?
I hear you saying: use the step-up and a 1S configuration. But a running DC/DC converter near a precision cirquit is also not a good option and will lead to strange drifts due to EMI.
And most of the weight is in the housing. -> that would be the better way to save weight.

But there are further limitations for cheap transport.
Within Germany we have a 5cm limit for the height of a letter (2.60 EUR up to 1000 g)
From Germany to Europe there are mainly the weight limits 500 g = 3.70 EUR and 1000g = 7 EUR.

with best regards

Andreas

[MiDi]

5 cells have 21V when fully charged. -> I cannot use my favourite LT1763 voltage regulator. (max 20V)
How about additional balancing cirquit, transport limitations, ...?
I hear you saying: use the step-up and a 1S configuration. But a running DC/DC converter near a precision cirquit is also not a good option and will lead to strange drifts due to EMI.
And most of the weight is in the housing. -> that would be the better way to save weight.

Charging Li-Ion to 4V would only minor decrease capacitance, so 20V limit would not be that critical.
A balancer for Li cells is needed, but for NiMH balancing is needed too and charging is much more difficult (-dU/dT or better: peak detection).
Charging NiMH cells like Li cells is not recommended and trickle-charging at constant voltage or even very low current would damage the cells in a (short) period of time.
This seems to be quite critical in this application as it should be powered 24/7 and is trickle-charging most of the time.

12 cells of NiMH go down to 10.8V (0.9V/cell), far away from safe minimum 15V. Even nominal 1.2V result only in 14.4V, so not shure how you calculated the safety margin for discharge 

You are right: transportation of Li cells is restricted in several forms, a powered device cannot be shipped in normal way. No option for Li cells in this project, but not shure if the restriction of powered device is only valid for Li cells 

Yes, switching regulators are not suggested for such a precision gear, perhaps SC+LT1763 could be an option as the currents are quite low.
If the size of battery pack is reduced there is more room left in case, so the case could be shorter and the whole weight decreases.

But there are further limitations for cheap transport.
Within Germany we have a 5cm limit for the height of a letter (2.60 EUR up to 1000 g)
From Germany to Europe there are mainly the weight limits 500 g = 3.70 EUR and 1000g = 7 EUR.

If marked as "Warensendung" it is only 1,90 Eur/500g or 2,20 Eur/1kg within germany.
The prices to europe are the same as for (most of) the world, problem could only be the customs to pay for.
Not shure if there is an option to say it is only temporary out of germany/europe to avoid customs, as it would be if you travel with it, otherwise could be that customs has to be payed twice

[Andreas]

but for NiMH balancing is needed too and charging is much more difficult (-dU/dT or better: peak detection).
Charging NiMH cells like Li cells is not recommended and trickle-charging at constant voltage or even very low current would damage the cells in a (short) period of time.
This seems to be quite critical in this application as it should be powered 24/7 and is trickle-charging most of the time.

12 cells of NiMH go down to 10.8V (0.9V/cell), far away from safe minimum 15V. Even nominal 1.2V result only in 14.4V, so not shure how you calculated the safety margin for discharge 

Not shure if there is an option to say it is only temporary out of germany/europe to avoid customs, as it would be if you travel with it, otherwise could be that customs has to be payed twice

1. I do exactly that since over 7 years for my 6 LTZ1000 cirquits. Over the years I had to exchange exactly
   1 set of my first LTZ where the end voltage was too high in summer (with > 30 deg C in my lab).
   Sorry I would not call 5-7 years a short life time due to trickle charging.
   Additionally the cirquit is now improved with a NTC.

2. I use RTU cells where the remaining capacity below 1.15V (my switch off voltage) is less than 10%.
    (and you really do not want to have a NiMh cell going flat).

3. there is a possibility for enterprises with a EORI number. But you have to do some paperwork before shipping.

with best regards

Andreas

[MiDi]

  Take it as a big compliment, you designed the perfect and simplistic charge circuit for this pupose!
Really impressive, as (cheap) gadgets with NiMH and integrated chargers treat the batteries quite bad.
Could you please give the key specs for this charger?

[Andreas]

Hello,

description is here:
https://www.eevblog.com/forum/metrology/ad587lw-10v-precision-travel-standard/msg1449489/#msg1449489

For my LTZs I use a L200-based design and a 24V wall wart with the same specs except that there I use AA cells and thus a current limit of ~200 mA instead of ~90 mA.

The trick is to reduce the trickle current of NiMH well below C/100 .. C/50 when the cell is fully charged.
(1.45V @ 23 deg C with negative tempco of around -3mV/K)

With best regards

Andreas

[Andreas]

Hello,

continuation of story of AD587LW#01
https://www.eevblog.com/forum/metrology/ad587lw-10v-precision-travel-standard/msg1463587/#msg1463587

I have trimmed back the AD587LW#01 to 10.00000V (as good as I can in ratio mode of the HP34401A against my LTZ#4)
The result was that the T.C. trimming voltage changed from -43.7 to -44.9 mV.
Since I did not want to spoil the good result I trimmed back to -43.8 mV.

It is quite difficult to set those cheap 25 turn trimmers exactly below 1 mV since there is around a quarter to half turn play when changeing the direction. Perhaps I should have spent more money on the trimmers.
https://www.reichelt.de/Praezisionstrimmer/64W-50K/3/index.html?ACTION=3&GROUPID=3129&ARTICLE=2710&SEARCH=64W-50K&START=0&OFFSET=16&

On 30.03.2018 I measured around 15 ppb/K for the best fit curve (LMS) in my 18-33 deg C temperature range.
So I was in doubt: shall I really readjust this to zero T.C. or shall I accept this?

A repeated measurement on 31.03.2018 without any adjustments showed that
the (up to 6uV (3uV * 2) popcorn) noise was playing a trick on the measurement.
Resulting LMS curve showed a 1 ppb/K (0.001 ppm/K !) slope.
So now I know that noise will prevent a better adjustment.
(you shall not try to better adjust the AD587 as a LTZ1000)

For the first the adjustments on AD587LW#01 are done.
The ageing phase and drift determination can begin.

with best regards

Andreas

Edit: attached a screen shot from the measurement setup of data logging during T.C. measurement.
3 programs working together:
- one for ADC#25 which is measuring the 2:1 divided output voltage of the reference
- one to read out the temperature + battery voltage of the AD587LW#01
- one for reading the environment temperature in the chamber
ok I am doing T.C. measurments of 8G16 resistors in parallel so it is in reality the temperature of the 8G16 resistor which is near the AD587LW#01. Additionally the temperature (30 deg C) of the reference resistor.

[Andreas]

Hello,

I also did some wideband noise measurements with a measurement amplifier according to Linear Technology AN83 on AD587LW#02.
Thanks to branadic for the AN83 PCB.

According to the datasheet I will have to expect 160uVpp wideband noise for a 10Hz .. 1 MHz range with the 1uF capacitor on the reference populated.
The AN83 amplifier works from 10 .. 100kHz so for white noise I would have to expect a factor 3 lower noise.
But the 1uF capacitor already does a frequency shaping above 40 Hz. and a large amount of energy will be in the lower frequency range.

I additionally have a 340Hz low pass (100nF * 4700R) between output of the AD587 and the buffer amplifier.
I had to switch off all mains supplied gear near the measurement which of course is placed into a metal can cookies box to get the 50 Hz hum out off the measurement. The scope is supplied by a battery powered laptop.

So AD587LW#02 shows around 40uVpp noise in a 10Hz-100kHz frequency range.
FFT shows some decrease of noise above 30Hz. The 340Hz corner frequency has no effect.
So I should really put the output filter to e.g. 10Hz in a re-design.
The noise floor (-167 dbV) of the AN83 amplifier with a 50Ohms terminator on the input is shown as reference.

with best regards

Andreas

[Andreas]

by the way:

updated the documentation (BOM of the Rev A PCB) after hints from Lars.
https://www.eevblog.com/forum/metrology/ad587lw-10v-precision-travel-standard/msg1449494/#msg1449494

Will give the Rev B board documentation with fixed erratas soon.

with best regards

Andreas

[Andreas]

Hello,

since I urgently needed a further RS232 receiver for measurements I did a quick hack with a USB to TTL board.
This board I had developped years ago (when the B-Version of the FTDI chip was freshly on market).
It has 2 series Resistors of 1K to protect the 5V pins against shorts.
The TXD resistor is used as pull up (since TXD on TTL level is high in idle state).
Both pins (TXD and RXD) are thus shorted and the photo transistor is connected with Emitter to GND and Collector to the short.
As photo transistor a 3 mm plastic transistor BPW42 (ordered from Reichelt) is used.

Pictures
- USB to TTL board and Photo transistor
- Photo transistor wired with shrink tubes
- Connections to USB/TTL board and some wires for scope measurements
   Black shrink tube 3.2 mm to protect the photo transistor from unwanted back light.
- Measurements with 10:1 probe
- Completed cirquit (near final) still missing some hot glue to fix the cable

Some measurements:
- working transmission showing 25.2 deg C and 16.6V battery voltage
- the working range is not that what I am used to with such a transmission
  If the distance is too far (photo transistor inserted only with the head into the AD587LW) the transmission gets unreliable
  Similar also when the distance is too near
  (guess the axis of transmitter and receiver are off and light is not coupled to the receiver when being too near)
  Optimum distance for this pair is around 5 mm. So I fixed the backlight shrink tube at this distance.
- Rise and fall times show that there is not much room for improvement.
  Rise time is 25 us with respect to 208 us bit time.
  To increase sensitivity one would have to increase the 1K resistor but this would also increase the rise time.

So all in all I am not totally satisfied with the result (range of receiption)
But for the moment transmission works.
I will have to do some improvements later.

with best regards

Andreas

Edit attached: The simple "cirquit" with USB/TTL converter

[Andreas]

I did also a reverse engineering of my old "free style" wired receiver with discrete devices attached to a RS232 cirquit.

https://www.eevblog.com/forum/metrology/ad587lw-10v-precision-travel-standard/?action=dlattach;attach=407109

The cirquit is here (most probably it was a SFH309 as photo transistor.

And measurement at the RS232 output shows a much better signal quality
(near equal rise-/fall-times in the 1 us range) and much wider receiption range
from zero mm to the end of the light tube.

Originally I have a BC558A populated in the "Draht-Igel".
The cirquit diagram (perhaps as basis for a small PCB)
shows the SMD equivalent BC858A which has a different pin out.

with best regards

Andreas

[Andreas]

Story of a 2nd order TC-calibration on AD587

31.03.2018

I wondered if I would have to increase or decrease the value for R9 to get rid of the 2nd order term.
So a little bird whispered in my ear that if I set R9 to zero I would get rid
of most of the 2nd order term. (Thanks Lars for the info).

But how to calibrate with the new situation?
Formerly I had the value 4.2 ppm/V/K for the T.C. correction.
Now the total resistance has about halved. (2 * 680K + 1M5 with the 1M5 set to zero).
Ratio measurement against my LTZ#4 reference has changed from 0.7195709 to 0.7195168
due to the shorting of R9.

So what to do?

I decided in a first step to adjust the ratio with the T.C. (not the gain) pot back to 0.7195709.
This seemed to me the most plausible adjustment.
The more as the difference voltage for the T.C. was -0.1443 V which is about half of
the voltage before shorting R9 with -0.321 V. (so corresponding to half of the total resistance).

01.04.2018

The result is shown on measurement of 01.04.2018
I got a over-compensation of the T.C. which has now +1.33 ppm/K
instead of -1.35 ppm/K of the naked AD587.

So 2.68 ppm/K change by -0.1443V difference voltage.
But that´s much more than the double value of 4.2ppm/K/V.
It´s more like 18.6 ppm/K/V

Ok next adjustment with 18.6 ppm/K/V and the naked value of -1.35 ppm/K.
This gives -72.7 mV for the T.C. adjustment pot.

02.04.2018

Result of the previous adjustment gives +0.11 ppm/K.
But for the calculation of the new value I accidently used the box value of 0.15 ppm/K
So with the 18.6 ppm/K/V I got a delta correction value of +8.1 mV
resulting in a adjust value of -64.7 mV

03.04.2018

Hmm I do not really get closer to target. Now LMS value is -0.104 ppm/K.
So another error gets clear: I have underestimated the non-linearity of T.C. adjustment
which came by shorting R9. At the local point it is around 26.5 ppm/K/V for +/- 0.1 ppm/K.
Hoping that for the small change it will be not too non-linear I adjusted
the T.C. voltage difference to -68.6 mV.

04.04.2018

now regression curve slope has -0.064 ppm/K in the 18-33 deg C range.
So most probably I do not only have a non-linearity problem but also
the problem that the difference voltage is influenced by temperature.
(and every time that I adjust I have different temperatures).

So shall I leave it as it is? (better than many unadjusted LTZ1000 cirquits).
The perfectionist in me says: there should be a improvement possible ...
(perhaps as "delta calibration by 2 mV" under more stable temperature conditions.)

with best regards

Andreas

[Andreas]

Hello,

after some trimming back and forth the final result after 6th trimming is -27 ppb/K for the regression curve of AD587LW#02.
The adjustment is not easy due to the non-linear behaviour and the play of the trimmer.
And also the temperature dependancy does not make it easier.

This time I measured a 17.5 uV (1.75ppm) change over 15 deg C (116 ppb/K Box value).
Without the popcorn noise/nose near 22 deg C it would only be 12uV (1.2 ppm).

Tomorrow I will make a repeated measurement to see if all is stable.

with best regards

Andreas

[Andreas]

Hello,

Ups forgot to mail the follow-up measurement.
But there is nothing special to report.
The popcorn noise has moved a little up in the temperature axis,
and so the regression curve shows -51 ppb/K

25 deg C value has gone down by 2uV (10V range) or 1uV after 2:1 divider
but this is around the standard deviation of  my measurement device. (LTC2400 based ADC).
So the AD587LW#02 looks to be stable over temperature cycles.

As next I will do ageing drift measurements against my LTZ-devices.

With best regards

Andreas

[Andreas]

Hello,

now I have nearly built 2 PCBs of the Rev B PCB.
- Battery charger and power supplies are running. Still missing AD587 and output buffer.

Documentation including gerbers is here:
https://www.eevblog.com/forum/metrology/ad587lw-10v-precision-travel-standard/msg1449499/#msg1449499

some impressions:

[Andreas]

Hello,

I also built another version of my discrete receiver:



This time with a new bougth BPW42 as photo transistor.
Obviously the photo transistors which are made now have more sensitivity than the old SFH309. (or was it the BC557C that I had on hand as transistor?)
So I had to use 2 1K resistors in parallel to R1 and R2 to get a good distance range for the receiver, and avoid saturation of the photo transistor.

with best regards

Andreas

[lars]

Thanks Andreas for publishing  a second test of #2. I am glad to see it is repeatable.

I must also say I am glad to see the "pop corn noise" or "jumps" of about 0.8ppm. It is far to easy to think it is just the "normal 0.1-10hz noise" of about 0.4ppm p-p that even for quite long times might be 0.1-0.2ppm p-p if looked on a DMM with longer integration times.

I haven't seen these jumps on all 10v ref IC's with buried zeners I have, and I have a lot I have followed for long times, but on many irrespective of type as AD587, REF102, LT1021, LT1236 etc. Of course the LM399 also have it. Maybe some don't have them or maybe I haven't looked at the right times. For some of my better 10v ref IC's I follow it might be very long, months or even years, between periods of jumps. I have seen 0.4ppm p-p jumps on LTZ1000 as well and I think TiN have reported also.

Sorry that I haven't helped you with more information on the temperature compensation but my health is not so good.

I have an excel program for calculation but will not share it as I don't have power to answer questions for it. Everyone should be able to setup his own from the datasheet of AD587. Assume the feedback Rf is about 6k, the Rl about 14k and the series resistor Rt to pin 5 about 125k. Take the values for the NTC at eg 15, 20, 25, 30 and 35°C and make your own formula or put it into spice. As seen with #2 with a negative temperature coefficient and for me a normal second order coefficient the resistance in series with the NTC should be lower than the NTC. For AD587 with temperature coefficients close to zero (+-1ppm/K) it is difficult to use this scheme to compensate for the second order coefficient normally found around room temperature. That were the reason the AD587LQ, that Joe Geller had with about +1-3ppm/K, worked well. Also Andreas as you saw the negative TC is not the best as the sensitivity of the trim gets to high. TC on the positive side, needing higher resistance for second order compensation, makes it easier to adjust.

Lars

[Andreas]

Hello Lars,

my largest fear for the repeatability was that the trim-pots loose their position during thermal cycling.

One thing that I have learned by the adjustment: never use too cheap trimmers when doing a analog adjustment in the ppb/K range.
For sample #3+#4 I have now used the Bourns parts from the BOM. Yesterday on first measurements
I found that the "play" of these parts is much lesser than on the cheaper ones.
Unfortunately I had finished already the REV B layout. With that what I learned on adjustment of #2
I would have probably increased the number of NTCs to reduce sensitivity from 4ppm/K/V to 2ppm/K/V or even lesser.
When I select the AD587 for below 2 ppm/K then even 1ppm/K/V would be sufficient.
But 8 NTCs below the AD587 is hard to place.

Yes you see more popcorn noise the closer/longer you look at it.
Formerly on my analog scope I took 15 shots with 10s duration on a digicam and evaluated afterwards. (very time consuming).
And some jumps were outside the screen, so could be not evaluated. (and could also be a mains line distortion so discarded).

Now with the hires digital scope I leave at least factor 2 headroom and zoom in if there is no event.
And it is so easy with a alarm-trigger to automatically measure 15 shots times 100s store them to a file and redo automatically from start.
So without intervention after some hours you have only to read the peak-peak values from the recorded screens.
(I am shure thay you could also automate the last step with a script).
So I have at least 5-6 * 15 * 100 seconds of recording time so probability to see a rare occuring jump is much higher.

Dont apologize: your tip with the direction and to completely short R9 was more than I hoped to get. Thank you for that.
The rest was PDCA. Ok I could have saved 2 days when not having intermixed the sign of the correction.
And I really hope that you regain your health so that we hear more of your experiments.

Thanks for the tips, I will keep that in mind for adjustment of #3 and #4.
so optimum selection criteria would be around +2 ppm/K for the AD587.
As I am running out of "good" AD587JQ I have just ordered 10 further devices to select from.

Ok if I am honest: even without 2nd order adjustment the result was better (0.14 ppm/K)
 than that what a typical 6.5 digit instrument has as tempco (0.55ppm/K for my K2000).
But as a volt-nut I wanted to see how far I can go.

with best regards

Andreas

[Andreas]

Hello,

finally found the time to evaluate some measurements.
So here first ageing measurement summary of AD587LW#01 for the first month after adjustment.

Measurement setup:
HP34401A is measuring in RATIO-Mode (Sense input to AD587LW#01 and my calibrated LTZs #3-#6 to the voltage input.
Then calculate back from ratio and calibration values to the 10V of the sense input.
First measurement Offset then AD587 (full scale) and then the ratios of LTZ3 ... LTZ6.
Each value is averaged over 5 minutes integration time with around 5 minutes stabilisation time after connecting to the next device.

K2000 is measuring in 10V range in parallel to the voltage input of the HP34401A.
ADC13,15,16,17 are also measuring with a 2:1 divider in parallel to the K2000.
The ratios (this time not simultaneous measured but one after the other) are also calculated back to the 10V.

Since temperature now is rising in my "lab" there is also some temperature dependancy.

So the average AD587LW#01 internal temperature during is also recorded in the
20180429_AD587LW_01_ageing_avg_instr.PNG diagram where the stray of the averaged
instrument readings and the overall average is shown.

The other picture shows a averaging over the LTZ readings with different instruments.

It seems that there was a initial drift by -1ppm until day 9 and then slightly increasing.
But since this may be also within my 0.2 - 0.3 ppm standard deviation of the ADCs
or the remaining T.C. of the compensated reference I will have to observe the further progress.

with best regards

Andreas

[Andreas]

Hello,

first T.C. measurement on AD587LWB#03 (B-revision PCB).
Used reference is AD587JQ#01 which was measured before during selection process
as having nearly -1.6 ppm/K average T.C. between 18-33 deg C.

From this and with the formula of Lars (4ppm/K/V) I pre-adjusted -399.5mV on TP2-TP1 difference.
The result is already nearly perfect for the 1st order T.C.
around 16 ppb/K for the best fit linear regression curve between 18-33 deg C.

But over the whole temperature range you can see that there is also significant 2nd order / 3rd order deviation from straight line.

So in the 18-33 deg C temperature range we have a box T.C. of 2.08 ppm / 15 deg C or 138 ppb/K

with best regards

Andreas

 

[Andreas]

Hello,

first 500 hours of AD587LW#02 after adjustment. (first + 2nd order T.C.)
(setup see above for AD587LW#01).
Up to now no clear trend.

with best regards

Andreas

[Andreas]

Hello,

during testing of Rev B PCBs I found out that my PC-software was not logging any measurements when the AD587LW is in off-state.
So updated to Rev1 here:

https://www.eevblog.com/forum/metrology/ad587lw-10v-precision-travel-standard/msg1449493/#msg1449493

The .dll needed for the software if you do not have Microsoft Visual C 6 (VC6) installed are still in the Rev0 package.

with best regards

Andreas

[e61_phil]

Hi Andreas,

thanks for your nice graphs, but would it be possible to plot the voltage as "ppm deviation"? That would be much easier to interprete in a short look (at least for me).

[Andreas]

Hello,

shure. I already also thought about this.
Its only another calculation step to convert the 1 uV steps into 0.1ppm steps.

I think I should also do a constant scaling so that the graphs can be better compared.
But I do not know how long this will go good If I begin to do some "traveling" tests
like putting the whole cirquit into the freezer or a "drop" test. (of course with a parcel around).

with best regards

Andreas

[Andreas]

Hi Andreas,

thanks for your nice graphs, but would it be possible to plot the voltage as "ppm deviation"? That would be much easier to interprete in a short look (at least for me).

Hmm,

ppm relative to 10V (where I think that I have adjusted but only LTZ4 and ratio mode of HP)
or ppm relative to the first measurement of each instrument (to remove stray in absolute value).

with best regards

Andreas

[e61_phil]

With traces from different instruments I would prefer your second solution. Everything relativ to its starting point.

[Andreas]

Thats also what I thought.

since I want to track the drift for some weeks
it will be better to use the relative to first measurement values.

The absolute value will be trimmed again back to 10V when the traveling starts.

with best regards

Andreas

[Andreas]

Hello,

And another weekly ageing measurement. This time of AD587LW#02
Still the ageing direction on this device is somewhat unclear.

But T.C. compensation seems to be worse than on#01. (raw T.C. was -1.35 ppm/K against -0.4 ppm/K of #01)
You can see a correlation with temperature besides the noise of the measurement devices.

with best regards

Andreas

[Andreas]

I had a strange effect on AD587LW#04

From selection I knew a T.C. of around -0.4 ppm/K for the AD587JQ#12 which was used.
So with my adjustment setup I set the voltage between TP2-TP1 to -100 mV.
During adjustment I recognized that the voltage had drifted away so much,
that I could not adjust it with the voltage pot.
So I had to exchange R7 to a lower value to achieve this.

After that I adjusted again T.C. pot and the final output voltage to 10V.
And made my first measurement. (04.05.2018)
But WTF had happened?
Instead of getting around zero tempco as on the references before I had around +2.3 ppm/K!!!
So clearly overcompensated.
I measured the TP2-TP1 voltage and it was around -700mV instead of the trimmed -100mV.
And also recognized the voltage was off a bit. But could again not adjusted with the output voltage pot.
I had to increase R7 again to achieve this.

After exchange of R7 I tried again to adjust T.C. pot to -100mV.
I recognized a non-linear increase of the difference voltage between TP2-TP1 below around -60mV.
I had to do several turns to get the -100mV.
Then measured the absolute voltages from TP2 and TP1 against output ground.
And there it was again: the difference of around 0.6-0.7V between TP2-TP1.
Turned the pot back again to zero and started from new to a difference of -100mV.
Again measured the absolute values to a difference of 0.7V as expected by the several turns of the pot.

So what happened here? Something oscillating? Defective Pot?
(unlikely since this time I used high quality Bourns)
Ok connected my analog scope to the output.
nothing oscillating.
the T.C. pot now behaved normal around a half turn for the -100mV.
Disconnected the scope: The effect was again there.

Tried my USB-Scope with Laptop on battery and disconnected earth ground.
The effect was there and a 50Hz ripple of about 20 mV visible.
Ok I had forgotten to disconnect the switch mode charger from my LTZ#4 which was used as
reference for the 10V adjustment in ratio-mode of the HP34401A.

I can still not explain what really happens there. Any Ideas? (picture of the adjustment setup attached).
But at least I know now not to use a switchmode charger when adjusting the AD587LW.

And the measurement of 08.05.2018 shows that with a correct pre-adjustment
a near zero T.C. (+0.12 ppm/K as average linear coefficient) is instantly available.

With best regards

Andreas

[Andreas]

Hello,

2nd order adjustment of AD587LW#03:

since the 2nd order adjustment on AD587LW#02 took several days and T.C. runs I wanted to shorten the adjustment by simulation.
According to Lars you can use e.g. a Excel sheet to calculate. But this time I wanted to improve my LTSPICE skills.

The AD587JQ#01 reference is modeled by a temperature dependant zener (voltage source) with a output amplifier with 6K over 14K nominal.
From the T.C. measurement on 08.04.2018 I got the Zener voltage and the 3rd order LMS coefficients as:

 VT=7.001525962 * (1 - (1.54472E-6*TEMP) - (1.52508E-8*TEMP*TEMP) - (8.73627E-10*TEMP*TEMP*TEMP))

Where the 7V is the raw zener voltage before amplifier and TEMP is the temperature difference to 25 deg C.

The measurement of 01.05.2018 shows that I have a convex 2nd order with 7uV height after 2:1 divider.
This corresponds to the LTSPICE simulation of about 13uV 1:1 at 10V 2nd order simulation with nominal values and 1.5Meg in series to the NTCs.

The NTCs are simulated as temperature dependant resistors with Steinhart & Hart coefficients generated by a EXCEL tool from the Vishay home page:
http://www.vishay.com/thermistors/ntc-curve-list/
http://www.vishay.com/doc?29130

From Lars I know that for convex 2nd order curve I have to reduce the 1.5 Meg resistor.
So I tried 680K 330K and finally 220K in the simulation.
Of course each time I have to reduce the TP2-TP1 voltage difference by the T.C. pot and adjust the output voltage to nominal 10 V.
In the diagram I have subtracted the 10V so that only the error curve is shown.

Finally I have only to read the difference TP2 - TP1 (=VTRIM) so that I know what to adjust at the reference.

A measurement on the weekend will show how near I come to the simulated "zero TC" value from the simulation.

with best regards

Andreas

[Andreas]

Hello,

first measurement with the adjustment data of the LTSPICE simulation.
Simulation and measurement correspond relative well except the shifts
up to 1 ppm (probably due to popcorn noise) at higher temperature.

I fear there is not much room for improvement now.
(popcorn noise determines mainly the deviation in 18-33 deg C range).
The average linear T.C. over 18-33 deg C is around + 0.017 ppm/K.

So the adjustment could be done also in a single adjustment
step if you have the raw T.C. of the naked AD587:

Measure the raw T.C. as 3rd order polynom. (already done in the selection step <2ppm/K)
Simulate (LTSPICE) to adjust R7,R8,R9 and the trimpot positions for 2nd order adjustment.
Adjust the resistors and pots.
Measurement for confirmation if we are close enough.

with best regards

Andreas

[Andreas]

Hello,

did repeated measurements without any further adjustments to get a "better" popcorn noise.
But obviously I got "better popcorn" noise.
The fatal is that during selection noise measurements no popcorn noise was visible.
Now the maximum value is often around 1 ppm or 10 uV.

With best regards

Andreas

[cellularmitosis]

 

[branadic]

Damn popcorn noise. I do like popcorn, but not in electronic components. You can watch minutes and hours on measurements, select a component and spend hours and dollars to build a circuit and in the end you find a jumper by accident. You have my sympathy.

-branadic-

[Andreas]

thats why you have to build at least 4 devices ...

But it is still in my target spec: 2(+1) ppm for T.C. and noise.

And if we need it better we can still use the Patent of Joe Geller (with a LTZ1000):
http://www.freepatentsonline.com/7382179.pdf

with best regards

Andreas

[Andreas]

Battery drain test on AD587LWB#04.

I started the test on 15.05.2018 (fully charged).

Batteries are 950 mAH NiMH.
Today the battery monitor switched off the around 3.8mA Load (AD587 + LTC2057 buffer)
after 9.5 days. (see voltage increase after minute 700 on day 10).

with best regards

Andreas

[HighVoltage]

*update
unfortunately, it appears the vishay xls is unable to open in open office, security? 
but i managed to find an alternative
https://edwardmallon.files.wordpress.com/2017/04/ntc-steinhart_and_hart_calculator.xls

Where can I find the Vishay original xls?
Thanks\

[3roomlab]

@ highvoltage, in this thread. andreas' post#70. the 1st vishay link i think.

[Andreas]

then i saw your model. not sure if it is too late to insert an .option?
this ".OPTIONS numdgt=8"

it seems a challenge to configure it to have a very wide and "usable" range. would anybody having the SVR-T know the usable compensation range it as built in? and what do they consider the output as "within compensation limits" 1uV ?
if i consider 3uV the usable error span, i could get 20C compensation range by using 3380K 10k NTC (11 units in series), 9C to 29C.
i also tried other combinations, 6pc 15pc resulting in diff range.

Hello,

thanks for the hint with the .option.   I already tried with the reftol and abstol values without success.

The official "spec" can be found here:
http://www.gellerlabs.com/Voltage%20References.htm

+/- 5 ppm short term transfer accuracy.
+/-50 ppm 6 month stability. with < 1ppm/K

If you look at the adjustment then it is done at +/- 1 deg C around calibration temperature.
http://www.gellerlabs.com/SVR_TempcoTest.htm

Consider that you are also limited by (popcorn-)noise which may be up to 1 ppm (10 uV).
So a 1 uV or 3uV limit is a target that is not easy to met.
Usually you have those values already as thermal voltages due to wiring.

with best regards

Andreas

[Andreas]

Hello,

I had some effects on my ageing measurements (will report later) so I did also a stability measurement on AD587LW#01
Setup is AD587LW#01 connected to one of my ADCs (ADC#25) with 2:1 capacitive divider over nearly 4 days. (5660 minutes ~94 hours).

First diagram
red: Voltage AD587LW#01 after 2:1 divider (in mV)
blue: internal temperature of ADC#25
red: internal temperature of AD587LW#01
x-Axis time in minutes
y-Axis ADC reading in mV after 2:1 divider with 1 minute integration time
Each grid represents 0.1ppm so a overall span of around 1.2 ppm for noise + drift.

The diagram over time (minutes) shows some jumps of around 0.5ppm (2.5uV after 2:1 divider)
which are also reflected in the 2nd picture (overlapping allan deviation)
starting with 0.3uV/5000mV = 0.06ppm short term stability increasing to 0.6uV/5000mV = 0.12 ppm
for medium and long term values.

Overall standard deviation is 1uV = 0.2 ppm (3rd picture)

with best regards

Andreas

edit:
and yes I have quite high environment temperatures in my lab in summer.
Even when you subtract the 2-3 deg C self heating of the ADC voltage reference.

[Andreas]

Hello,

here the first 2kHrs of AD587LW#01.
Short after the first kHr I had a unexplained jump of around 1 ppm.

With the stability measurement (see previous post) it now gets clear
that around 0.5 ppm can be explained by popcorn noise.
The trend line seems to be 0.6 ppm/kHr for the ageing drift at the moment.

I will have to check wether #02 is more stable.

with best regards

Andreas

[cellularmitosis]

Other than the blip these results look good!  Since the 587 is used as the AREF of an ADC, these charts indicate that it is drifting downward?

[Andreas]

Hello,

I am measuring the following with the setup:

1. offfset of all instruments (4 ADCs, 1 K2000, 1 34401A)

further with all instruments parallel (the ADCs are with 2:1 divider each):
2. AD587LW#01
3. AD587LW#02
4. LTZ#3
5. LTZ#4
6. LTZ#5
7. LTZ#6

after this I calculate the AD587LW values out of the ratio of the measurements based on the LTZ calibrations.

So a drift upward is either a drift upward of the AD587LW-devices or a simultaneous drift downward of all 4 LTZ references. (which is unlikely).

In the beginning of the diagram I used the HP34401A ratio mode to directly measure the ratio instead of the calculation.
But this seems to give not "better" results than the manual ratio measurements.
Otherwise you would see a difference for the last 3 measurements

with best regards

Andreas

[Andreas]

Another experiment:

tilting the AD587LW#01 reference

orientation in room
0 = (N) normal
1 = (L) placed on left edge
2 = (O) over the top
3 = (R) placed on right edge
4 = (B) placed on back side
5 = (F) placed on front side

with LM399 based references you can observe several ppm difference between the orientations.
(especially with bad thermal isolation).

on AD587LW#01 there is no correlation with the orientation visible (only popcorn noise).
So all I can say is that the tilting effect is < 0.7 ppm on this device.

with best regards

Andreas

[Andreas]

Hello,

some results from device AD587LW#2:

Ageing chart (now over 2 kHr)
The measured values are more stable than on device #1

A 150 hour stability measurement shows that there is also much less popcorn noise
(3 events captured during measurement).

Allan deviation shows instability of 0.5uV (after 2:1 divider so 1uVeff absolute) around 700 minutes (half a day).
No wonder if you look at the temperature readings with a span of 3-5 deg C during one day and 8 deg C over the 150 Hours.

with best regards

Andreas

[Andreas]

Hello,

another stability measurement this time on AD587LW#03.

Obviously there is some remaining T.C. of around 0.1 ppm/K between 22-30 deg C which was already visible on the previous adjustment measurements without much popcorn noise.
During 150 hours I have only 1 event near minute 6000 of around -0.4 ppm (the scale is 0.5 uV per grid after 2:1 divider so one grid corresponds to 0.1 ppm)

Overall standard deviation 1 uV after 2:1 divider so 0.2 ppm.
And also allan deviation stays below 1uV/0.2 ppm with again a low stability at 700 minutes = 1/2 day.
Ageing data is still not available on this device (has started now).

with best regards

Andreas

[Andreas]

Hello,

Further ageing measurement of AD587LW#01
around 3 kHrs from first adjustment.

partly I have extreme high temperatures in my lab
(outside the planned compensation range of 18-33 deg C).
The ageing trendline seems to be 4-5 ppm/year against my LTZ references, which is still rather high.

with best regards

Andreas

[Andreas]

Now the 3kHr measurement of AD587LW#02

this candidate is far quieter than #01
Standard deviation of the measurements is around 0.2 ppm against my best instruments.
(and that over a 9 deg C temperature span).
If I leave out the first 1kHr then standard deviation is even only around 0.15 ppm

with best regards

Andreas

[Andreas]

Hello,

now 4 kHrs on AD587LW#02

With best regards

Andreas

[Andreas]

Hello,

finally I found some time to adjust sample AD587LW#04.

From T.C. selection of the AD587 chips I already had the T.C. of -0.4 ppm/K within 18-33 deg C.
So the reference already was pre-adjusted according to the formula of Lars
with about 4 ppm/K/V between the testpoints TP2-TP1 to -100mV.

LTSpice simulation showed that a 2nd order adjustment would not give much improvement
for this device. (around 5uV over the complete range). And I would have to set R9 to near zero.
So the sensitivity for the T.C. trimpot would drastically increase which makes a adjustment impractical.

I then decided only to adjust the linear T.C.
with the -100mV it was reduced from -0.4 ppm/K to -0.135 ppm/K.
So I had to correct by a delta of +34 mV.
Since the voltage difference is also temperature dependant and was around -95mV
when measured, I adjusted to -60 mV.

Result:
-0.013 ppm/K for the approximation of resulting average linear T.C. within 18-33 deg C
and a variation of 9.3 uV (after 2:1 divider so 18.6 uV at the 10V reference)
over the 15 deg C temperature range resulting in a 0.12 ppm/K box T.C.

with best regards

Andreas

edit: clarified 2:1 divider

[Andreas]

And first ageing data of AD587LWB#04.

since adjustment has been done yesterday there is no "after calibration data" available up to now.
But in this case I recorded 2kHrs "before calibration data".
Due to adjustment there is a "jump to zero" at day zero by about 2 ppm.

But you can see that also with only preliminary T.C. adjustment the device was rather stable.

with best regards

Andreas

[Andreas]

32 hours stability check on AD587LW#02:

the stability check on 3 instruments (HP34401A, K2000, ADC25)
shows that AD587LW#02 has around 6-7 uV popcorn noise.
(common on all 3 instruments).
All instrument values have been averaged over 1 minute each
for the diagram to give comparable instrument noise.

But also the K2000 (which I have linearly temperature compensated by -0.55 ppm/K)
shows around 6uV popcorn noise.
The HP34401A has no correlation to temperature so it is used directly.
The ADC25 has 3rd order temperature compensation.

for each instrument Allan deviation and standard deviation have been calculated.
Interestingly it seems that ADC25 has best long term behaviour
even when the short term noise looks to the worst on the time diagram.
Ok the TC compensated K2000 is close on the very long time scale.

with best regards

Andreas

[e61_phil]

Interesting data, thanks.

Which dimension has tau?

[Andreas]

Hello,

tau is one minute.

Y-axis is in mV. (so 10e-3 corresponds to 1 uV)

with best regards

Andreas

[e61_phil]

Hi Andreas,

thanks. I wondered why your time axis in the first plot goes up to 2000min but the allan deviation goes only to 1000.

best regards
Philipp

[Andreas]

Hello,

my allan deviations always go only to half the time.

with best regards

Andreas

[Andreas]

Hello,

I was still missing a easily transportable (against the discrete solution from here: )
https://www.eevblog.com/forum/metrology/ad587lw-10v-precision-travel-standard/msg1462954/#msg1462954

reliably working (against the USB-hack which works only with good alignment of sender and receiver)
https://www.eevblog.com/forum/metrology/ad587lw-10v-precision-travel-standard/msg1469255/#msg1469255

read out cirquit for the AD587LW internal values.

Now I have one solution based on a FTDI FT232RL.
The cirquit is straight forward.
A standard application cirquit from the data sheet and a amplifier for the photo transistor similar to the discrete cirquit.
USB-connection is via a MINI USB connector so that the cable for charging can also be used for the read out cirquit.

I never thought that on such a simple cirquit something could go wrong. So I did not test the cirquit before ordering the PCBs.
After the first sample was built I found out that the output polarity of the amplifier was wrong.
(High active instead of low active).
While looking for a solution I found out that fortunately the FTDI designers already have sympathy with users messing up the polarity.
So you can program the polarity of each pin with the help of the FTPROG application.

https://www.ftdichip.com/Support/Utilities.htm#FT_PROG

Do not forget to power cycle once after programming the polarity so that it gets active.

... 3 weeks saved against ordering new PCBs.

with best regards

Andreas

[Andreas]

traceable calibration of AD587LW#02 and AD587LWB#04

On our Volt Nut meeting on 27.10.2018 I was also calibrating (not adjusting) 2 of the 10 V references.
https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg1921490/#msg1921490
The adjustment against my LTZ#4 reference has been several kHrs before.

Method:
- climated room (room temperature was 23.5 deg C humidity 30% rH).
- connect reference to HP3458A + K2002
- wait 1 minute for temperature egalisation
- measure values on HP3458A with 100 NPLC over 5 minutes and calculate average
- manually read the values on K2002
- reverse connect the reference connectors
- wait 1 minute
- 5 minutes measurement reverse connected.

Uncertainity of calibration:
- out of HP3458A protocol +/-2.7 ppm (27 uV for 10 V) unceratinity for calibration.
  Since calibration has been recently we can add the 3458A (option 02)
  90 day specs for error propagation of the reference calibration measurements.
  so we have 2.6 ppm of reading + 0.05 ppm of range resulting in another 26.5 uV uncertainity
  of the instrument within 90 days.
  As both errors (calibrator + 3458A) are uncorrelated I add them per square law.
  so 27 + 26.5 uV give +/- 38uV uncertainity for this calibration for a 10V reference.

- from K2002 protocol I have to use the 1 year specs (since calibration was more than 90 days ago)
  + the uncertainity of the 5720 calibrator which has been used.
  so 10ppm + 0.15ppm in 20V range give together with the 1.45ppm of calibrator 104uV uncertainity

Evaluation:
- calculate a correction factor for positive readings out of the calibration protocol
  on HP positive values at 10V were reading 0.55 ppm too low and negative values 0.21 ppm
  So correction factor 1.00000055 for positive and 1.00000021 for negative readings
  similarly the readings of K2002 were corrected by 0.99999753 / 0.99999532 for readings in 20V range

- AD587LW#2 3458A 5 minute averaged readings:
   9999.992163   corrected   9999.997663
  -9999.994063   corrected  -9999.996163
  final value (average of pos and negative reading)
  value 27.10.2018: 9999.996913 mV +/- 0.038

- AD587LW#2 K2002 (plausibility check) with manual readings:
   10000.0251    corrected  10000.0004
  -10000.0428    corrected  -9999.996
  final value (average of pos and negative reading)
  value 27.10.2018:  9999.9982 mV +/- 0.104

- similarly AD587LW#4 on K2002 result in
  3458A 27.10.2018: 10000.00212 mV +/- 0.038
  K2002 27.10.2018: 10000.00395 mV +/- 0.104


The K2002 and 3458A results are very close (within 0.2 ppm).
So I guess the actual uncertainity is much less than the 38 uV from calculation.

Error estimation for further transfers:

- noise: around 1 ppm
  AD587LW#02 0.6-0.7 ppm typical observed

- temperature: around 2 ppm within 18-33 deg C
  AD587LW#02 measured 1.4 ppm (box)
  AD587LW#04 measured 1.8 ppm (box)

- ageing 1-2 ppm/year typical for selected AD587
  AD587LW#02 measured ~ +0.4 ppm against my LTZs during the last 6 months.
  AD587LW#04 still to be evaluated

so all in all around 5 ppm additional to the 3.8 ppm from calibration.

with best regards

Andreas

[Andreas]

Another traceable calibration ...

last week I did another calibration on the AD587LW devices
This time against a Fluke 5520A calibrator.

Method: compare Fluke 5520A and AD587LW output voltage (grounds connected)
with a HP34401A as null voltmeter. Average 2 readings with one above and one below zero volts difference on the 34401A.

from calibration protocol Fluke 5520A of 14.05.2018

33V range
displayed: 10000.00  mV
actual:     9999.956 mV  44 uV or 4.4 ppm too low
calculated correction factor: 0.9999956
Uncertainity of 10V calibration according to protocol +/-2.6 ppm (+/-26uV)

We have to use 1 year spec for error propagation:

1 year +/-5 deg C spec Fluke 5520A 12 ppm/reading + 20uV
 10V:   120 uV + 20 uV = 140   uV

uncertainity of AD587LW (10V) calibrations (independant devices -> geometrical added)
 10V: sqrt(sqr(140)+sqr(26))   -> +/-143 uV
(due to the calibration history it should be much less but I have not all protocols)

Unfortunately the AC was off in the lab at that day so the temperature was
at the upper limit of the calibrator specification (28.6 deg C room temperature near calibrator).

AD587LW#2 5520A 28.6 deg C (room) / 30.7 deg C (int)

10000.01  mV -> corrected   9999.966 mV (Fluke) + 0.009 mV (HP)
10000.02  mV -> corrected   9999.976 mV (Fluke) - 0.001 mV (HP)

value 05.12.2018: 9999.975 +/-0.143 mV

so 2 ppm below previous calibration against 3458A and K2002

similarly:

AD587LW#4 5520A 28.6 deg C (room) / 30.1 deg C (int)
10000.01   corrected   9999.966 + 0.005
10000.02   corrected   9999.976 - 0.005

value 05.12.2018: 9999.971 +/-0.143 mV

so 3 ppm below previous calibration against 3458A and K2002

with best regards

Andreas

[Andreas]

Hello,

I added a 11V TVS diode some weeks ago (see red circle) which has low leakage @10V to protect the output against ESD and (hopefully since I did not actually test it) against a current limited calibrator (if shorted accidently).

As far as I can see the output voltage has not changed by adding the diodes on all 4 samples of AD587LW

For AD587LW#01 + #02 I have now 1 year of ageing data against my LTZ1000s (from which LTZ1000#4 is the most stable).

AD587LW#01 clearly fails my 2 ppm/year target. Its more like 3.5 ppm/year against my LTZ#4.
And ageing also does not look like it will slow down the next time. So I really think that I should do some artificial ageing measures.

The other references are much more stable.
AD587LW#02 is more or less a seasonal change +/- 0.5 ppm against LTZ#4. So within my 2 ppm/year limit.
I guess that this is due to humidity swelling of the EPOXY PCB which influences the AD587LQ reference.
And I think I should exclude LTZ#5 from the averaged diagrams as it spoils the average due to its ageing drift.


with best regards

Andreas

[iMo]

https://assets.nexperia.com/documents/data-sheet/PTVSXP1UP_SER.pdf

[Andreas]

Hello,

and the remaining references:

AD587LWB#03: This guy is somewhat suffering from the 2nd order T.C. which could not fully be compensated. So a temperature dependant drift +/- 1ppm over the last months is visible (so within the planned +/-2 ppm for temperature). And it is obvious that it will meet 1 year ageing drift of less than 2 ppm.

AD587LWB#04: It took long time from first measurement until I had calibrated T.C. + full scale on this sample. So only half year data since calibration (day 0).  Ageing drift was -0.5 ppm immediately after calibration.  But since then the device is very stable.

with best regards

Andreas

[Andreas]

Hello,

I made a little instruction leaflet for the AD587LW reference. (see attachment).

with best regards

Andreas

[Magnificent Bastard]

Sadly, I must report that one of the very best non-ovenized references in the world, the AD587, has transitioned to NRND [Not Recommended for New Designs].  It's now only a matter of time before it goes to EOL [End Of Life].

 

[Andreas]

Hello,

and the 2nd best one the 5V brother AD586 is now also NRND.
They are crazy.

with best regards

Andreas

[Macbeth]

Since Analog took over Linear and are consolidating, I expect to see the LTZ1000's and LM399's to be killed off in the not to distant future too. Who really needs anything other than a bandgap and then only up to 5V?

[Magnificent Bastard]

Since Analog took over Linear and are consolidating, I expect to see the LTZ1000's and LM399's to be killed off in the not to distant future too. Who really needs anything other than a bandgap and then only up to 5V?

There are an awful lot of LM399's that go into DMMs, so I don't foresee them killing that-- but the LTZ1000 simply does not have enough sales to justify keeping it in the lineup.  If they kill the LTZ1000, that will be a day of mourning for me...  Keysight will have to scramble for a new "super reference" for some of it's high end meters.  There are some manufacturers of voltage references (and other test equipment) that will have to do the same.  This is what happens when the "cake eaters" start making engineering decisions-- disasters ahead!

 

The AD587 was the last great unheated reference (with low LF noise) that you could buy in a hermetic package.  This decision might cause T.I. to fire up the hermetic packaging line for the REF102-- not as low LF noise as the AD587, but still a good part if you can get it in a hermetic package (that was discontinued long ago).

 

[3roomlab]

I think the next area of exploration is a cooler temperature instead of hotter
in the pickering pdf where they show low temperature drift advantage, there is a plotted point where it is 11k/1k 
maybe we just need a sweet spot low temperature which forces the drift to stop
maybe a new LTZ with TEC inside instead of heating, it hold the die at maybe -10C. maybe it will never drift or 1ppm per 10 years.

Since Analog took over Linear and are consolidating, I expect to see the LTZ1000's and LM399's to be killed off in the not to distant future too. Who really needs anything other than a bandgap and then only up to 5V?

actually thinking about the history of rivalry, isnt AD the ones dishing out fake stability specifications many years ago?

[Andreas]

I did it:  I put 2 of the references in the freezer over night  

the good news: they survived (packed only into a plastic bag to reduce condensing water) at -19 deg C setpoint temperature.

The other morning the battery indicator blinked: the reference is still running.

The bad news

AD587LW#01 drifted -4.8 ppm
AD587LW#03 drifted -6.2 ppm

against last measurement from sunday. (obviously hysteresis)

so this is a strong indicator that shipping during winter is not the best idea.
And obviously a star pattern is needed to check the reference after each travel.

How long will it need to remove the hysteresis without heating?

with best regards

Andreas

[3roomlab]

you going to try 8hr "degauss" ?

[Andreas]

you going to try 8hr "degauss" ?

Hello,

what do you mean with 8hr degauss?

of course I waited several hours from morning to evening until I measured the voltage output under room temperature conditions.
So I think I will have the offset for several months.

with best regards

Andreas

[FriedLogic]

How long will it need to remove the hysteresis without heating?

For a 'winter transfer reference' I'm not sure if you want to remove it.

The question might be more: how do you maintain it?
It's not really the same thing a bench reference that never sees anything but room temperature.

I find that doing a wide range temperature test on a reference which has not had this done on it recently can cause a shift, but subsequent cycles cause much less.

My thinking is that maybe if a reference might get low temperatures, it may be better to always or regularly give it a low temperature. There might be more variation than a reference which never gets wide temperature swings, but it might also get rid of large jumps in the output voltage.

I have plenty 587LQ's to play with, so I can set something up to test how they behave. I'm thinking of doing that with some LM399's too.

[3roomlab]

Hello,

what do you mean with 8hr degauss?


Andreas

the hot cold hot cold cycling?

[Echo88]

Since Analog took over Linear and are consolidating, I expect to see the LTZ1000's and LM399's to be killed off in the not to distant future too. Who really needs anything other than a bandgap and then only up to 5V?

There are an awful lot of LM399's that go into DMMs, so I don't foresee them killing that-- but the LTZ1000 simply does not have enough sales to justify keeping it in the lineup.

Who told you how much they sell and if its profitable to Analog to produce them?

[Magnificent Bastard]

Since Analog took over Linear and are consolidating, I expect to see the LTZ1000's and LM399's to be killed off in the not to distant future too. Who really needs anything other than a bandgap and then only up to 5V?

There are an awful lot of LM399's that go into DMMs, so I don't foresee them killing that-- but the LTZ1000 simply does not have enough sales to justify keeping it in the lineup.

Who told you how much they sell and if its profitable to Analog to produce them?

My Linear Tech (now Analog) Rep.  He didn't exactly come right out and SAY that, but "reading between the lines" of what he DID say is how I arrived at that conclusion.  This was said just before the Analog takeover.  Analog is a vastly different company than it used to be, and is a vastly different company than Linear Tech used to be-- I don't think they would hesitate one second to drop a device from their lineup if it was losing money-- and ESPECIALLY if they had to pump a lot of money into an old fab just to keep it going, so that a few old parts can continue to be produced.  The only thing that is saving us is contractual obligations-- which I am not privy to-- but I know they must exist.  One good place to find out is Keysight-- they would know if there are rumors about these parts going NRND.  Keysight would have to scramble to come up with an alternative to the LTZ1000 and the LM399--- I have something in mind, but to think Keysight would listen to ME is laughable.

[Crossphased]

How do you guys feel about AD588? Looking at specs it has better tempco than AD587, and slightly lower p-p noise. Its also available in Cerdip, although it is more pricey than AD587. Its pin programmable to +10,+5, +-5, -10  so it can potentially fill in for the AD586 or AD587. Anybody have experience with this ref?

[Andreas]

How do you guys feel about AD588? Looking at specs it has better tempco than AD587, and slightly lower p-p noise. Its also available in Cerdip, although it is more pricey than AD587. Its pin programmable to +10,+5, +-5, -10  so it can potentially fill in for the AD586 or AD587. Anybody have experience with this ref?

Hello,

not myself but Lars reported on 25.11.2015 on "volt-nuts" about the AD688:
https://www.mail-archive.com/volt-nuts@febo.com/msg02662.html
Of course it is only a single sample and "popcorn noise" can occur on every reference.
But external temperature compensation seems to be difficult.

But I would only use a AD588/688 if I really need dual reference voltages.
And I would shurely not use the internal OP-Amps (1uV/K) for generating the buffered outputs.

with best regards

Andreas

[Andreas]

Hello,

on our volt nut meeting in Stuttgart I got another 2 traceable calibrations for my best 2 devices AD587LW#02 and AD587LW#04.
One against freshly calibrated 3458A from Branadic. The other as transfer in 10V range of 3458A compared to the Fluke 7000.

Attached you find the diagrams with the measurement uncertainities.
It seems that AD587LW#02 has nearly no ageing drift whereas AD587LW#04 drifted 1 ppm down during calibrations.

with best regards

Andreas

[Andreas]

Hello,

how time passes by ...
in the mean time I have new traceable calibrations for AD587LW#2 and #4.
Unfortunately with wider uncertainity.

with best regards

Andreas

[Andreas]

Hello,

there is no noise or other things in this chart.
These are calibrations against different calibrated Instruments with different accuracy along the calibration chain.
The largest width is the uncertainity of the (2 different) Fluke 5520A´s together with the uncertainity of their calibration.

with best regards

Andreas

[Andreas]

Hello,

again: how time passes by ...
in the mean time I have new traceable calibrations for AD587LW#2 and #4.
(Thanks to branadic)

this time I am showing only the calibrations against 3458B (option 02)
with either 1 day or 90 day + calibration uncertainity

Drift of AD587LW#2 is +1.4 ppm over 2.6 years
Drift of AD587LW#4 is -0.9 ppm over 2.6 years

So not bad to the "design spec" of 2 ppm/year

with best regards

Andreas

[guenthert]

Huh?  The measurements of January 2020 got lost?

[Kleinstein]

The data from january 2020 are with a rather high incertainty (e.g. +-10 ppm) - so they would not really help much and fit the scale.

[Andreas]

Huh?  The measurements of January 2020 got lost?

Hello,

Nothing gets really lost.
I think selecting the calibration points with low uncertainity is better for drift estimations.

Attached all calibrations.
But be aware:
a) On measurements of 06.06.2020 we had a guard loop between 2 parallel instruments.
So the values here are ~1-2 ppm worse. (have verified this with my weekly measurements against 24Bit ADC16)

b) The measurement against Fluke7000 are not direct but as sequential ratio measurement.
So there may be additional measurement setup errors.

c) the measurements against Fluke 5520A is differential with a 34401A as Null-Voltmeter.
   And I cannot guarantee that it is always the same Fluke 5520A.
   
with best regards

Andreas

[Andreas]

Ups I did it again:

(putting one of my references into the freezer for ~8 hours).
see also:
https://www.eevblog.com/forum/metrology/ad587lw-10v-precision-travel-standard/msg2449794/#msg2449794

AD587LW#1 had drifted back to the old value before the first freezing.
So I wanted to know wether I could trim it back near inital calibration by freezing again
or wether the reference is hardened by first freezing so no new change would happen.

Temperature logger shows that -20 deg C where reached.

And: we have again a step in output voltage: this time by -6.1 ppm. (previously -4.8 ppm)

with best regards

Andreas
 

[iMo]

Why so steady drift upwards after so many years? It is a metal can, isn't it?
Wouldn't be better to get rid of all those settings resistors and thermistor around it and live with something like 10.034.256?? It is only a number..

[3roomlab]

so is it possible that there is a temperature where it does 0ppm drift?
maybe @ 5C ? 

[iMo]

It is a travel standard so messing with ovens/coolers is not the right way to go, imho. I asked above as the drift could be caused by surrounding resistors and trimmers. I think a naked 587 (with the buffer) and a diode or thermistor for measuring the temperature close to the chip with a calibration graph glued on the top of the box would be less prone to such long term steady drifts..

[Andreas]

Why so steady drift upwards after so many years? It is a metal can, isn't it?
Wouldn't be better to get rid of all those settings resistors and thermistor around it and live with something like 10.034.256?? It is only a number..

Hello,

it is a CERDIP8-package not a metal can.
Without those trimmers and termistors around we would have 1-2 ppm/K temperature drift.
So we would have annual variations of more than 20 ppm with my lab temperature range of about 18 - 33 deg C.

With the compensation I have only problems at very hot days (see those "dips" at above 30 deg C).

from my 4 samples 2 are drifting with about 2 ppm/year (which is not too much) the other 2 are much lower in drift compared to my LTZ-references.

with best regards

Andreas

[Andreas]

I think a naked 587 (with the buffer) and a diode or thermistor for measuring the temperature close to the chip with a calibration graph glued on the top of the box would be less prone to such long term steady drifts..

Hello,

I am dreaming of a small processor with 12 Bit A/D to measure temperature and outputting a correction voltage either per PWM or a 12 Bit DAC (LT1257). So with a 3rd order polynomal one could correct +/- 2ppm/K of selected AD587 over a 18-38 deg range with a resolution of  ~100 nV.

with best regards

Andreas

[iMo]

fyi - I've been using the vintage ADS1100 (6pin complete i2c 16bit adc) for converting temp from an LM35 sensor, compensating my 34401A

[DavidKo]

I think a naked 587 (with the buffer) and a diode or thermistor for measuring the temperature close to the chip with a calibration graph glued on the top of the box would be less prone to such long term steady drifts..

Hello,

I am dreaming of a small processor with 12 Bit A/D to measure temperature and outputting a correction voltage either per PWM or a 12 Bit DAC (LT1257). So with a 3rd order polynomal one could correct +/- 2ppm/K of selected AD587 over a 18-38 deg range with a resolution of  ~100 nV.

with best regards

Andreas

Maybe you can use similar approach as in AN86, only you need to add the code for the temperature measurement and correction.

[Andreas]

Maybe you can use similar approach as in AN86, only you need to add the code for the temperature measurement and correction.

The more interesting question is here: how can I monitor that the poor processor is actually doing that what is intended.
And not hanging e.g. into a reset-loop or forgetting the calibration constants while outputting the minimum/maximum possible voltage?

with best regards

Andreas

[guenthert]

The more interesting question is here: how can I monitor that the poor processor is actually doing that what is intended.
And not hanging e.g. into a reset-loop or forgetting the calibration constants while outputting the minimum/maximum possible voltage?

with best regards

Andreas

       The same as with any component.  If your VR is stable, you're going to assume all is peachy, if it jumps or drifts excessively you investigate.

     As for how to investigate software bugs, I don't think this application is much different from other MCU applications.   At first thought it seems even simple enough to be implemented as FSM, in which case the states could be enumerated and the index of current one output on otherwise unused pins (if that extra noise is deemed acceptable).

     And don't be afraid of reset-loops: the firmware of e.g. Keithley 181 always resets after each measurement cycle.

[miro123]

The more interesting question is here: how can I monitor that the poor processor is actually doing that what is intended.
And not hanging e.g. into a reset-loop or forgetting the calibration constants while outputting the minimum/maximum possible voltage?

with best regards

Andreas

i have few questions
1.Do you want only to monitor the temperature or do you want to do compensation? or just say it with other words does your application requires exact 10.00000V
2. How you gonna sense the temperature? I asking this question because i struggling to get it right. I've build TC oven I can easily achieve sub-milikelvin sensitivity 'mK' and reasonable long term stability. unfortunately the mesurement erors are several orders of magnitudes worse - e.g placing the probe few cm further away gives +-100..200mK, small smd component cannot be measured directly
I come to conclusion that sensing of temperature can only be done on silicon die - e.g. LM399, LTZ1000 or REF50xx 

[Andreas]

Hello,

with the AD587LW project I am compensating T.C. according to the suggestion from Lars Walenius in the analog domain.
This works good for a small temperature range and you can do linear and partly 2nd Order temperature compensation.

Of course a oven will give a much better stability but I also want to have a 4-7 days continuous operation on battery.

Since the temperature range of analog compensation is too small for my "lab" temperatures it would be good to have a 3rd order temperature compensation.
But if you do this you also have to be shure that the compensation is "bullet proof".
So I actually want to monitor that the temperature compensation is working right.
(on a oven you also have a LED indicating that the setpoint temperature is reached):

For temperature sensing a good thermal isolation of the NTC/Voltage reference area against environment is essential.
Most of the temperature is distributed by the component leads. (so at best connect NTC ground to Ground pin of AD587)
And also selecting devices which have a low T.C. (< 2 ppm/K) in the temperature range of interest helps to maintain stability.
SMD components have usually lesser stability than DIP/CerDIP packages.

with best regards

Andreas

[magic]

I asking this question because i struggling to get it right. I've build TC oven I can easily achieve sub-milikelvin sensitivity 'mK' and reasonable long term stability. unfortunately the mesurement erors are several orders of magnitudes worse - e.g placing the probe few cm further away gives +-100..200mK, small smd component cannot be measured directly
I come to conclusion that sensing of temperature can only be done on silicon die - e.g. LM399, LTZ1000 or REF50xx

You could try to measure chip temperature by monitoring its quiescent current, but I don't know how much TC it has and it could be too low to be practical.
Other than that there seems to be no hope for AD587. Somebody torn down a few units and even the "factory test pins" had no internal connection.

[Andreas]

Hello,

somewhat off topic here but has to do with AD587:
Branadic asked me wether the zener reference voltage (visible on Pin 8 ) is typically "better" in T.C. than the output voltage.
I found this question very interesting. Especially because there is the Joe Geller Patend using a LM399 connected to Pin 8 to improve the T.C. of a AD587 to below 1 ppm/K.
https://patents.google.com/patent/US7382179B2/en


So I made a measurement.
My K2000 (temperature compensated) is connected to Pin 8 (which is filtered by a 1uF MKS02 capacitor).
My ADC27 is connected to the 10V output of the reference.
My 34401A is connected to both voltages measuring the ratio between VSense = 10V and Input = 7V

Here the results of 2 samples
AD587JQ#04 has a relative high T.C. of ~7ppm/K within 10-40 deg C
AD587JQ#08 has a relative low  T.C. of <2 ppm/K within 10-40 deg C

AD587JQ#4
Vout  ADC27  225.3 ppm / 32.4 deg C = 6.96 ppm/K  9998.078 mV @ 25 deg C
Vz    K2000  229.8 ppm / 32.4 deg C = 7.09 ppm/K  7132.876 mV @ 25 deg C
Ratio 34401A  4.44 ppm / 32.4 deg C = 0.14 ppm/K
calc  ratio   4.81 ppm / 32.4 deg C = 0.15 ppm/K


AD587JQ#8
Vout  ADC27  54.8 ppm / 32.9 deg C = 1.67 ppm/K 9999.375 mV @ 25 deg C
Vz    K2000  63.2 ppm / 32.9 deg C = 1.92 ppm/K 7136.059 mV @ 25 deg C
Ratio 34401A  8.6 ppm / 32.9 deg C = 0.26 ppm/K
calc  ratio   9.0 ppm / 32.9 deg C = 0.27 ppm/K

here the initial drift was rather high so I had to wait 2 temperature cycles until the values stabilized somewhat.

In both cases the output voltage divider has a relative low T.C. (well below 1 ppm/K).

With best regards

Andreas

[branadic]

Thanks Andreas for performing this measurement and presenting the results. So that literally means, that the third order component in the t.c. is actually coming from the temperature compensation at the zener diode itself and is not added by the output stage, which makes it hard if not impossible to compensate for, sad news.

-branadic-

[Andreas]

Hello,

some update on ageing drift data of my 4 AD587JQ references now after ~ 4 years (>1400 days)

The drift data is relative to my LTZ references.
Most stable reference is LTZ#4 (green line in charts).

#01: this device I had 2 times over night in freezer (which gave ~-5ppm jumps) otherwise the drift would have been ~10 ppm in 4 years.
#02: ~4 ppm in 4 years against LTZ#4. so pretty stable.
#03: was 1 time in freezer (see jump) otherwise near zero ageing drift but some seasonal changes due to internal temperature changes.
#04: also nearly no ageing drift against LTZ#04 but rather sensitive to temperature when internal temperature rises above ~30 deg C

with best regards

Andreas

[mawyatt]

Hello,

somewhat off topic here but has to do with AD587:
Branadic asked me wether the zener reference voltage (visible on Pin 8 ) is typically "better" in T.C. than the output voltage.
I found this question very interesting. Especially because there is the Joe Geller Patend using a LM399 connected to Pin 8 to improve the T.C. of a AD587 to below 1 ppm/K.
https://patents.google.com/patent/US7382179B2/en
Andreas

The Joe Geller idea is very clever, and your measurements suggest that the AD527 doesn't contribute much additional temp drift with the amp and gain resistors, as the dominate TC source seems to be the buried zener.

Wonder why National did not have a single chip that basically combined the LM399 with on-chip heater and the AD587 amp resistor network to get to 10 volt output? Or maybe they did and I just didn't know about it?

Anyway, thanks for the post and now you've created an intriguing question of whether this LM399 and AD587 combo might be an attractive alternative to a LM399 with discrete amp and resistors??

Best,

[Andreas]

Hello,

with the AD587 output ratio I do not have the flexibility that I want.
(converting from 6.6 to 7.2 V to exactly 10V).
Even if the ratio stability is rather good.

So I´d rather use a high resolution DAC with specified ratio drift for that task.
(So similar to Ian Johnstons approach).

with best regards

Andreas

[Kleinstein]

The LM399 is running quite hot and this can effect the stability of resistors.
A stable 10 V reference is not such a common application that it needs a special chip for this. There is still the option to have a discrete zener, resistors and temperature regulation that includes the resistors.

NS had a LM369 zener reference with 10 V scaling, but without a heater. So somewhat similar to the AD587.

[mawyatt]

50 years ago or so National had a Hybrid 10V reference but don't think it had a heater either. We used these a lot, but recall they were somewhat expensive.

I'm sure there was a very good reason for not including the heater, otherwise they would have done it!!

Best,

[kleiner Rainer]

50 years ago or so National had a Hybrid 10V reference but don't think it had a heater either. We used these a lot, but recall they were somewhat expensive.

I'm sure there was a very good reason for not including the heater, otherwise they would have done it!!

Best,

National Semiconductor LH0070 - there are two versions of it: a hybrid module (there comes the "LH" for hybrid from) built around a LM329 reference, some laser trimmed resistors and a LM741 and a newer version built around a LM169, but still on a hybrid.
I got a sample of the older version during my internship, forty years ago - it is still working and of course well aged.

Link:

https://www.richis-lab.de/REF09.htm

I had a link with pictures of the first version, but it seems to have disappeared.


Greetings,

Rainer

[Andreas]

Hello,

obviously I missed to repeat 1/f noise measurement after population of the transient zeners in ~April 2019
So after doubt of IMO I repeated the noise measurement to see if there is any increased noise by the zeners.

I could not see any significant change in 0.1 - 10 Hz noise.
The values in the table are averaged values out of 15-19 repeated measurements with 100 sec measurement time each.

with best regards

Andreas