AD587LW - 10V precision travel standard

[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-