I have made a target board using STM32G484VE . I am using 11 ADC pins spread across ADC1 - ADC5. I have introduced a 3.3V Zener BZT52-B3V3 as a protection at all ADC inputs, but all the ADC voltages read 0.6V less from the actual voltage ? Any reason ?

[BSUJA.1]

I have made a target board using STM32G484VE . I am using 11 ADC pins spread across ADC1 - ADC5. I have introduced a 3.3V Zener BZT52-B3V3 as a protection at all ADC inputs, but all the ADC voltages read 0.6V less from the actual voltage ? Any reason ? The zener is introduced between the LM358 opamp output and MCU ADC input .( SEE ATTACHEMENT )

[Peter BENSCH]

You accidentally put an MC34072 in the circuit instead of an LM358. It would also be a bit more clearly visible if you had drawn the opamps with the usual triangles, so that one does not have to take the data sheet to hand first.

The first opamp is connected as a voltage follower, but the capacitor C80 must not be connected like this. If you wanted to build an integrator, the circuit would look quite different.

The second opamp is connected as a non-inverting amplifier with a gain of 1...3, which can result in a higher voltage than the ADC input can handle. Now you have connected a zener from the output of the opamp against GND, but neither zeners are really suitable for limiting a voltage (one would use a TVS instead), nor is the circuit useful, because you do not limit the current though the zener.

You should revise the current circuit and have a look at the exact requirements (input voltage range to be processed, range of the frequency to be measured, possibly filter functions to be realized, how to couple an opamp to ADC inputs, etc).

Regards

/Peter