Couple Questions about Reference Divider

[Crossphased]

Hello,

I've got (5) AD587KRZ burning in, they've been going for around 500 hours right now. After they've burned in for 1000 hours I'll be making a few 10V standards out of the better performing ones.

At the moment I'm putting together a circuit to divide the 10V down to 5V, for use as voltage reference for ADCs. The LTC1043 is being used. I put together a sample circuit that I'd welcome advice on:



The LTC2057 was selected for low bias current to buffer the output from LTC1043. I wasnt sure if the RC filters were necessary or not. Any suggestions or reccommendations would be appreciated.

Also I have a few questions regarding the implementation:
- would it be a good idea to trim out the offset from LT1001?
- the sampling capacitor C7 was partially wrapped in copper foil for guarding. I'm trying to figure out the best pin on LTC1043 to connect the guard to. As far as I can tell it would be pin 10 is that right?
- only 1/2 of the LTC1043 is being used for the divide by two... would it be a good idea to connect the other half in a divide by 2 configuration as well and connect in parallel to lower output impedance?

Thanks for any advice, cheers

*this was my first schematic in kicad if theres anything you would have done differently that is welcomed as well!

[Kleinstein]

The buffer circuits(both at the input and output) are wrong this way. As shown the OPs will be on the edge to oscillation. The capacitive load driving circuit is usually a little different: another capacitor as direct feedback and an additional resistor for the DC feedback. In addition the  resistor at the OP't output is usually smaller, more like 22-150 Ohms.

For the driver at the output, it really depends on the ADC, if the buffer is suitable this way.

The LT1001 has a low offset to start with. So there is not much gained (lower TC) from an extra adjustment - it is likely adding more trouble.

C2,C5,C10 are a little odd. Usually it only needs 1 local decoupling cap (e.g. the 100 nF) the larger cap stage can usually be shared.
If paranoid, one could consider an extra filter (ferrite, resistor) in the supply to the LTC2057 and than a larger local capacitor.

[Andreas]

- would it be a good idea to trim out the offset from LT1001?
- the sampling capacitor C7 was partially wrapped in copper foil for guarding. I'm trying to figure out the best pin on LTC1043 to connect the guard to. As far as I can tell it would be pin 10 is that right?
- only 1/2 of the LTC1043 is being used for the divide by two... would it be a good idea to connect the other half in a divide by 2 configuration as well and connect in parallel to lower output impedance?

a) no you will use the trimpot at the AD587 to trim overall value.  (and have no chance to adjust to zero without negative power supply).
b) yes connect to pin 10
c) in my opinion no: you also increase leakage currents, charge injection  etc. And you have a buffer at the output anyway.

to avoid oscillations either connect pin 6 directly to pin 2 of the OP amps or use the additional components (as suggested from Kleinstein).

with best regards

Andreas

[Crossphased]

Thank you both for your kind assistance

Is this kind of what you had in mind?


Or would it be preferred to have resistor and capacitor feedback on the OP?

[Andreas]

Is this kind of what you had in mind?


Or would it be preferred to have resistor and capacitor feedback on the OP?

a) yes

b) this one is wrong:
    R? has to be connected to C3
   Time constant of  R? * C? has to be larger than R1 * C3

Solution b) also compensates load currents through R1 (if there are any). So it can be more precise.

with best regards

Andreas

[Crossphased]

Ah ok I gotcha-
I see what your saying. If time constant in the feedback loop is too fast it will constantly chase its tail trying to correct the output voltage and oscillate. Ok, changing the values now.

I started to install LTC2057, but it has an odd package. I have the 10-MSOP version, but it doesnt seem to have the footprint of other MSOP devices I've seen. In the past I've been able to fit MSOP chips on TSSOP breakout boards. The LTC2057 has tighter pin spacing though. Here's LTC2057 on TSSOP breakout board:


and here it is on MSOP-8 breakout board, you can see pin spacing  is still to tight:


here's purchase bag:


Forgive me for my lack of knowledge but is MSOP not a standard pin spacing width?

[flittle]

I think it is the .5mm standard and the boards you have are .65mm.
This should work: https://www.ebay.com/itm/4-Sets-SOT23-MSOP10-UMAX10-to-DIP10-Adapter-Board-w-Pins-SMD-to-DIP-5-95mm/132799481297

[Crossphased]

I See, thanks very much. Weird that pin spacing is different for MSOP8 vs MSOP10. I guess maybe more pins desired in same amount of space? Ok well I learned my lesson from now on SOIC packages

[Crossphased]

Here is AD587 data after 500 hours. Temp hasnt varied a whole lot but you can see voltage change with temperature. There is no temp correction or anything like that in the circuit. Just +15V and a decoupling cap to each reference. How does this compare to data you guys have seen from other AD587 references?

[Andreas]

Hello,

If I look at the 10 hours and the 500 hours data there is nearly no ageing drift visible.
Some results of AD587 are here:
https://www.eevblog.com/forum/metrology/ad587lw-10v-precision-travel-standard/msg1750307/#msg1750307

AD587LW#01 is drifting around +0.5ppm/kHr
AD587LW#02 has nearly no drift over 4 kHrs.

with best regards

Andreas

[Crossphased]

Andreas,
Thanks very much for the link. I'm trying to understand what the graphs represent- what is the time scale?
I'm not sure what the first graph represents (in post #88), are you comparing the AD587 to other LTZ references in this graph?
In the second graph it looks like you are displaying the AD587 voltages as measured by different instruments. It looks like you have a talent for building precise ADCs! Do you divide the AD587 voltage down before reading by ADC or are you measuring between 587 and LTZ?

Also another question- how do you correct readings for temperature, for comparison to each other? Or maybe your circuit is temperature compensated with NTC?

Cheers

[Crossphased]

Also just finished putting the  circuit together. For the moment I replaced LTC2057 with OPA227, it will have to do until I get LTC2057 in proper package:

[Crossphased]

Here is update schematic:
Again if any values look incorrect or odd please let me know, cheers!

[Andreas]

Hello,

the feedback paths of the OPs are still wrong.
-> they should happily oscillate.
(you should have only re-wired the capacitor and not the resistor).

What type of 1uF capacitors do you use (Mylar or Polypropylene ?)
Which leakage resistance do they have (or discharge time constant).

with best regards

Andreas

[Crossphased]

Hi Andreas thanks for the feedback,

The capacitors I used with LTC1043 are these: http://www.rubycon.co.jp/en/catalog/e_pdfs/film/e_mpk.pdf, I'm using a value of 1uF.
The datasheet lists the insulation resistance as:
C≦0.33μF：25000MΩmin
C＞0.33μF：7500ΩFmin

I'll measure the leakage current is tonight. I matched the capacitors, they are within .5% of each other in capacitance.

About the feedback path- I guess I am misunderstanding you. I didnt change the wiring of the feedback path, only the values of the capacitors. How would you connect and what values would you choose?

[Andreas]

Hello,

not necessary to measure, they are good enough.
(looked for me like some cheap <500s Time constant values)

for the cirquit see figure 3 here:
https://www.analog.com/media/en/analog-dialogue/volume-38/number-2/articles/techniques-to-avoid-instability-capacitive-loading.pdf

with best regards

Andreas

[branadic]

About the feedback path- I guess I am misunderstanding you. I didnt change the wiring of the feedback path, only the values of the capacitors. How would you connect and what values would you choose?

[Crossphased]

Thank you Andreas and branadic for your kind assistance, it is very much appreciated

I understand what you mean now, I fixed the circuit. The link was very helpful.
After fixing the feedback in the circuit I took some test measurements. First I used a power supply set to various voltages to get a feel for how the circuit behaves. The power supply wasnt perfectly stable so the measurements fluctuated a bit, only accurate to 3 maybe 4 digits. Here's the points of measurement:


Here is measurements with various voltages from variable power supply:


The op amps dont like the lower voltages without a negative supply. Thats ok I wont be using for low voltages! The OPA227 seems to have a offset throughout its range much worse than the LT1001, it will be good to replace with LTC2057.
Next the AD587 voltages were measured:


It looks like even with buffering the LTC1043 is around 400 uV short. But the OPA227 draws 100x more bias current (2.5nA) than the LTC2057 (30pA). Could this account for the discrepency?

Kind of interesting the LTC1043 output is low by 400uV, but the OPA227 is offset high by 330 uV so it almost evens out

[Crossphased]

ok just took some leakage measurements with 10V on the LTC1043 sampling capacitors. The leakage current was measured to be between .008 - .010 uA at 10V

[MiDi]

How did you measured the current - with dmm in series in uA range?

[Crossphased]

Yes that exactly

[Crossphased]

Andreas,
Is this about the performance you would expect from LTC1043? Or would you expect better performance when LTC2057 is installed?

[Andreas]

Hmm,

something is odd: 400uV is far from that what I usually have with a LTC1050 or LTC2057

With a 5V reference on input of the LTC1043.
I typically measure around 2.5V - 20uV to 2.5V - 25uV at the buffered output.
(2499.975 - 2499.980 mV)

Be carefully with EMI. If you have a switchmode supply (LED Lamps) near the setup
you can get easily errors of several 100 uV due to rectification effects
on the input or output diodes of the components.
Usually the measurement values are "touch" sensitive if you have EMI effects.

Perhaps you need a additionaly filter cap/RC-filter at the input of the first stage.

with best regards

Andreas

[Crossphased]

Hmm,

something is odd: 400uV is far from that what I usually have with a LTC1050 or LTC2057

With a 5V reference on input of the LTC1043.
I typically measure around 2.5V - 20uV to 2.5V - 25uV at the buffered output.
(2499.975 - 2499.980 mV)

Be carefully with EMI. If you have a switchmode supply (LED Lamps) near the setup
you can get easily errors of several 100 uV due to rectification effects
on the input or output diodes of the components.
Usually the measurement values are "touch" sensitive if you have EMI effects.

Perhaps you need a additionaly filter cap/RC-filter at the input of the first stage.

with best regards

Andreas

I see, thanks very much for the additional information.

Ok I touched around a bit, the only thing that is touch sensitive is the sampling capacitor with the foil shield. When touched, output of LTC1043 drops by 200uV. I'm now going to put RC filter on the LT1001 input. Do you think it would be helpful to put ferrite beads on power supply lines?
Also, if nothing is connected to the LT1001 input, it likes to saturate to the positive rail. Would it be a good or a bad idea to put something like 100K resistor pull down on the input to LT1001?

Cheers

[David Hess]

The last example here shows how to drive capacitive loads however I often get better results using a series RC network on the output to ground which is simply a standard aluminum or tantalum electrolytic capacitor.  The large capacitance, typically 10 to 100 microfarads but smaller is possible in some cases, combined with the output resistance produces dominant pole frequency compensation and the relatively large ESR prevents emitter/source follower oscillation so do not try this with a low ESR capacitor unless you add series resistance.  Comparators can be compensated to operate as operational amplifiers in the same way.

Would it be a good idea to trim out the offset from LT1001?

Only if it is a significant source of error.  Do *not* trim the LT1001 offset to correct for other errors in the system as this will degrade its offset voltage drift.

I would consider using an LT1097 instead because of its lower input bias current and overcompensation pin but if you already have an LT1001, just use it.

- the sampling capacitor C7 was partially wrapped in copper foil for guarding. I'm trying to figure out the best pin on LTC1043 to connect the guard to. As far as I can tell it would be pin 10 is that right?

The LTC1043 datasheet discusses this but I do not remember the details.  Follow the instructions there.

- only 1/2 of the LTC1043 is being used for the divide by two... would it be a good idea to connect the other half in a divide by 2 configuration as well and connect in parallel to lower output impedance?

This would probably be a waste of time.  I might try it with the switches operating in anti-phase.