AD587LW - 10V precision travel standard

[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