Author Topic: AD587LW - 10V precision travel standard  (Read 43771 times)

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Online AndreasTopic starter

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Re: AD587LW - 10V precision travel standard
« Reply #25 on: March 18, 2018, 07:37:25 pm »
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
 

Offline lars

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Re: AD587LW - 10V precision travel standard
« Reply #26 on: March 19, 2018, 08:37:36 pm »
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
 
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Offline guenthert

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Re: AD587LW - 10V precision travel standard
« Reply #27 on: March 19, 2018, 09:09:59 pm »
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.
 

Online AndreasTopic starter

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Re: AD587LW - 10V precision travel standard
« Reply #28 on: March 20, 2018, 08:15:24 pm »
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
 

Offline cellularmitosis

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Re: AD587LW - 10V precision travel standard
« Reply #29 on: March 20, 2018, 08:33:20 pm »
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.
LTZs: KX FX MX CX PX Frank A9 QX
 

Online AndreasTopic starter

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Re: AD587LW - 10V precision travel standard
« Reply #30 on: March 20, 2018, 10:14:15 pm »
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
 

Online AndreasTopic starter

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Re: AD587LW - 10V precision travel standard
« Reply #31 on: March 22, 2018, 10:03:10 pm »
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
« Last Edit: March 22, 2018, 10:21:43 pm by Andreas »
 
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Online AndreasTopic starter

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Re: AD587LW - 10V precision travel standard
« Reply #32 on: March 23, 2018, 08:01:32 pm »
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
 

Offline Conrad Hoffman

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Re: AD587LW - 10V precision travel standard
« Reply #33 on: March 25, 2018, 02:24:56 pm »
Don't underestimate the problems an unclean board can cause. Even fingerprints can be a problem, so wear gloves.
 

Offline Inverted18650

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Re: AD587LW - 10V precision travel standard
« Reply #34 on: March 25, 2018, 07:12:26 pm »
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.

Online AndreasTopic starter

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Re: AD587LW - 10V precision travel standard
« Reply #35 on: March 25, 2018, 08:45:44 pm »
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.  8)

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


 
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Online AndreasTopic starter

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Re: AD587LW - 10V precision travel standard
« Reply #36 on: March 27, 2018, 05:34:25 am »
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

 
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Offline cellularmitosis

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Re: AD587LW - 10V precision travel standard
« Reply #37 on: March 27, 2018, 05:52:23 am »
What are the details of that little metal box?
LTZs: KX FX MX CX PX Frank A9 QX
 

Online AndreasTopic starter

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Re: AD587LW - 10V precision travel standard
« Reply #38 on: March 27, 2018, 06:22:42 am »
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
 
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Online AndreasTopic starter

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Re: AD587LW - 10V precision travel standard
« Reply #39 on: March 27, 2018, 08:51:55 pm »
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
 
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Online AndreasTopic starter

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Re: AD587LW - 10V precision travel standard
« Reply #40 on: March 29, 2018, 05:23:46 pm »
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

 

Offline MiDi

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Re: AD587LW - 10V precision travel standard
« Reply #41 on: March 29, 2018, 08:01:04 pm »
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?
 

Online AndreasTopic starter

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Re: AD587LW - 10V precision travel standard
« Reply #42 on: March 29, 2018, 08:59:47 pm »
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
« Last Edit: March 29, 2018, 09:01:47 pm by Andreas »
 

Offline MiDi

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Re: AD587LW - 10V precision travel standard
« Reply #43 on: March 30, 2018, 03:01:03 pm »
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  :o

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.

Quote
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 ???
« Last Edit: March 30, 2018, 03:05:41 pm by MiDi »
 

Online AndreasTopic starter

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Re: AD587LW - 10V precision travel standard
« Reply #44 on: March 30, 2018, 05:41:55 pm »

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  :o

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

 
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Offline MiDi

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Re: AD587LW - 10V precision travel standard
« Reply #45 on: March 30, 2018, 06:18:46 pm »
 :D 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?
 

Online AndreasTopic starter

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Re: AD587LW - 10V precision travel standard
« Reply #46 on: March 30, 2018, 07:35:40 pm »
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
 

Online AndreasTopic starter

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Re: AD587LW - 10V precision travel standard
« Reply #47 on: April 01, 2018, 04:41:11 am »
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.

« Last Edit: April 01, 2018, 05:11:27 am by Andreas »
 

Online AndreasTopic starter

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Re: AD587LW - 10V precision travel standard
« Reply #48 on: April 01, 2018, 05:04:28 am »
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

 

Online AndreasTopic starter

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Re: AD587LW - 10V precision travel standard
« Reply #49 on: April 01, 2018, 12:34:30 pm »
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

 


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