Overvoltage at ADC input

d23 · ‎2017-07-24

Posted on July 24, 2017 at 16:23

Hello,

I am using a STM32L051C8. In my circuitry there is an OPAmp's output leading via a 2k2 resistor to an input of the STM-chip which is configured as ADC input. The STM-chip is supplied by 3.3 V and the OP by 5 V, rail-to-rail.

As long as the OP's output voltage is below 3,3 V everything is okay. Slightly above 3,3 V I still get values of 4095, but from 4 V on or more the ADC-values are decreasing again, about 3800 or so.

Is my resistor with 2k2 to small to keep the limit under 4095? Or how can I maintain a limitation without supplying the OP by 3.3 V?

Thanks

Dieter

Vangelis Fortounas · ‎2017-07-25

Posted on July 25, 2017 at 18:25

hehe Like a 3 years old child drawing!!

valentin · ‎2017-07-25

Posted on July 25, 2017 at 23:33

That's why I usually try to supply the adc amplifier from VDDA so no matter what happens, the ADC can never get any voltage higher than VDDA.

That requires of course to adjust the opamp gain accordingly. Maybe you don't even need an opamp? The appnote somewhere specifies the input current drawn from the ADC so you can calculate how many LSBs error you'd have to expect.

Saves you the money and board space for the opamp and gets rid of the overvoltage issue.

Dvorak.Peter · ‎2017-07-25

Posted on July 26, 2017 at 00:34

Agree.

I would use 2k and 3k3 for the output voltage divider.

To eliminate the resistor error from this circuit, just use the divider output (ADC IN) for your opamp feedback circuit.

(the output divider will be in your feedback loop)

Peter

waclawek.jan · ‎2017-07-26

Posted on July 26, 2017 at 07:16

the input voltage would move to the same positive direction and would be always higher than the VDDA,

Sure.

so the 4095 value should be kept.

Not necessarily - you are still violating the DS constraints, which are there for a reason.

JW

d23 · ‎2017-07-26

Posted on July 26, 2017 at 08:19

Peter,

using a voltage divider is certainly a safe and effective method to keep the higher voltages than 3.3 V away from the input pin. FB from there to the OP's input also good, then it is needless to change any other components.

But there are other reasons, why this solution is not that pleasant to me. Indeed my OPamp is rail-to-rail, but you always have some linearity errors near to its supply voltage. Secondly this supply voltage is not very stable and can vary from 4.5 V to 6.0 V or so, thus I have to design the voltage divider for 6.5 Volts to avoid rail-to-rail-nonlinearity. And then on the other side, if I have 5 V again by exemplaric variation I will get nonlinearities for very much lower values, about from 3150 and above. Finally I just use this range 0 ... 3150 or so.

I tried the solution with a series resistor of 2k2 and a Schottky diode, this works perfectly.

Thanks anyway for your help.

Dieter