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Simple opamp circuit linearity problem
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Unixon:
I just came across a weird problem with non-linear amplifier behavior on a large scale while doing a simple receiver module for 4-20mA current loop.
Basically what I needed the circuit to do is to convert voltage from 0.4-2.0V range to 0-5.0V and 0-3.3V range for two output channels.
A textbook circuit for non-inverting configuration with negative offset does Vout=m*Vin-b conversion with m=3.125 b=-1.25 for 5V channel and m=2.0625 and b=-0.825.
For the testing I've fed the circuit with ramp signal and plotted Vout against Vin for both channels, positive rail was 5.0V and the negative rail was 0V @GND.
The result was that Vout(Vin) significantly deviated from linear function way before positive rail at 5V channel and even earlier on 3.3V channel.
My first though was that LMV358 selected out of convenience was way out of line with its biases and tried few other RRIO op amps, but with nearly the same result.

So the question is what am I missing here? Probably it's something so obvious that I should have known this from the very beginning.
Zero999:
Try removing D1.
Unixon:

--- Quote from: Zero999 on February 18, 2019, 06:36:18 pm ---Try removing D1.

--- End quote ---
Do you think this 3.3V zener starts leaking enough current to drive the input away even well below 2V?
SiliconWizard:
This is likely.

From the datasheet of this zener diode series: https://www.onsemi.com/PowerSolutions/product.do?id=1N5913B
you can see that the 3.3V variant has a huge max leakage current of 100µA @1V! I don't know which one you use, but those low-voltage zener diodes leak like crazy. So with a 10k series resistor...

Just measure the voltage @AIN to make sure.
magic:
Just checked my Vishay BZX55 series datasheet, leakage is specified only at 1V. I'm surprised too, but your feedback networks seem alright and you tried r2r CMOS chips without any improvement.
Note that both channels show identical nonlinearity at different scale, suggesting the input may be wrong.

You can probably get away without IC2, TL431 has very low output impedance compared to those kiloohm resistor networks.
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