Help, offset voltage cannot be reset to zero

[daisizhou]

Hallo ,every one。

I copied the following circuit,When I adjust the offset voltage to zero,I will connect TP1 and GND,Adjusting the R3 potentiometer,Let TP2 and TP3 display zero position.After completion。

Adjust R2 so that TP4 also shows zero position.Continue adjusting R1 so that TP5 also shows zero position.Finally, adjust R4 so that TP6 also displays zero position

During the process, I found it difficult to make TP4 display zero position,What I mean is to keep TP4 always showing zero position，Because I wait for a period of time after completing, the zero position after completion will shift again with the extension of time,After TP4 is offset, TP5 and TP6 will also be offset。
I tried many times, but every time it was the same, I don't know where the malfunction occurred,Did I purchase a counterfeit operational amplifier chip.If you know, please let me know. Thank you

[daisizhou]

Has anyone found out why?

[tggzzz]

I am not going to search for the data sheet for a random IC, U15, to find out what it is.

I am not going to try to reverse engineer your circuit to guess what you want it to do.

However, it looks like you have electrolytic capacitors and ceramic capacitors C8, C50 in your signal path. You should be aware of leakage currents, dielectric absorbtion and non-linear C-vs-V effects.

[Vovk_Z]

I tried many times, but every time it was the same, I don't know where the malfunction occurred,Did I purchase a counterfeit operational amplifier chip.If you know, please let me know.

That's not an ultra-precision circuit and there isn't such a real thing as a 'Zero'. You'll always have some non-zero values. Power up a circuit, let it temperature stabilize (IDK, 30 minutes) then make 1 iteration. Let it sit some time (several minutes more) and make another iteration. That's it. Voltages will drift a bit that's ok.

[Marco]

R37 doesn't exist in the datasheet suggestion for nulling, though I can't intuit how relevant that is.

Also multiple pots to null the same thing is just asking for problems.

[daisizhou]

IC15-ad834 is a multiplier.
C50 is not installed in the actual circuit

[daisizhou]

S3, S2, S1 connected to DG485DJ.
If an offset occurs, it will cause me to be unable to display the calibrated zero position.
I don't know where the failure occurred

[daisizhou]

This is a radio frequency power measurement circuit. Two 100 ohm resistors in front form a 50 ohm load.
AD834 is used to measure the square of voltage or the square of current

[daisizhou]

R37 looks like a current limiting resistor

[Marco]

R37 looks like a current limiting resistor

The nulling leg of the pot is meant to be connected to a steady rail though, not something that flaps in the wind with opamp current consumption.

The effect might be too small to be relevant of course.

[daisizhou]

Please refer to the steps and functional instructions for calibration below

[Terry Bites]

Component tolerances at the first (differential) amp will generate offsets beyond the opamp trim range.
Are you operating outside the CM range of the 711?
–VS + 4 +V – 2!

[daisizhou]

Thanks.

How can I test the CM range of 711?

[Terry Bites]

Measure the voltages at the opamp inputs with a scope or DMM.
see AD711 spec sheet.

Use CAD and SIM packages to draw your schematics. Qspice is free.

If you have R2 trimming the DC balance, there's no point in adding R3 nor I suspect R1 or R4. Invest in a good multiturn pot for R2.
Loads of trimpots are never a good thing.
The 6V supply to the 834 output bias resistors must be regulated for stable gain and offset trim.
You might do better to use regulated 5V for Vcc and -5 for Vee.

Squaring circuits of this type have a very "bouncy" response with unscheduled DC making an ugly appearance. See attched with a solution for this.
The Leyden Jars C50 C51, what value are they? Don't use electrolytics here, they leak like hell creating more DC offsets, some MLCCs or film would be better.

Capacitance tolerance of C10, C11 will badly mess up the ac CMRR of your diff amp.
You have to make the diff amp inputs with identical AC and DC characteristics.
The CMRR will be made even less accurate without precision resistors.
I'd separate the lowpass from the squaring output circuitry if you can.

I've got an idea sketched out here that might make life easier. Do the DC bal at the bias resistors and then use the diff amp.
You can also go a step further by taking the difference of currents instead the difference of voltages. Both untested circuits provide 1V fullscale.
That is 1V when there is a 4mA difference in current at the mutiplier outputs.
Use two amps to make a good 4 pole filter with all the gain you like. https://tools.analog.com/en/filterwizard/ (too easy)

Maybe go for an AD8604 quad.

If you are using your circuit as part of an RMS to DC converter there are better techniques.
$10 ore less (on ebay) gets you an AD8361 Or just do it in code!

[daisizhou]

OK,Thanks.
Before measuring the circuit, I will send one physical image and one original factory image respectively.
I copied the circuit 1:1,It seems that the original factory used 4 layers of PCB, while I used 2 layers of PCB for production.

R2, R3, R4, and R1 are all multi turn potentiometers of the same model as before.
I also copied the same circuit for output 834, but I didn't modify it.
C50 and C51 were originally empty and not installed, so I did not install them either.
I also chose the same capacitor for C10 and C11, please see the picture







Clear images here-->https://we.tl/t-WuXzjOnIkn

[Terry Bites]

What is the circuit function?

[daisizhou]

Can anyone assist me?

[daisizhou]

This is my measurement data

TP4 always experiences offset after a period of time,I don't know what caused the change in TP4.

[amyk]

Those trimpots say BOURNS but perhaps they're fake? Drift has been a known problem with the fake (and closely-named) ones. Measure their resistance over time:

https://www.eevblog.com/forum/chat/bonens-(bochen-also-works-here)-counterfit-bourns-trimpot-analysis/

[daisizhou]

Thanks.

I have only heard that there are fake chips.
BOURNS potentiometer is fake first time I heard about it, give me some time I will test it

[Vovk_Z]

Do you do your measurements with input closed (short-circuited)? Because input IC has quite large frequency range and will always gather some signal.

[daisizhou]

Yes, I will connect the TP1 position and GND together.Because the calibration manual clearly states.
I will connect the AD834 input and GND together every calibration

[daisizhou]

There is no big problem with BOURNS adjustable resistors。
TP2-TP3 has always been stable.
Changes started with TP4,That is AD711JN.