Noisy non-inverting op amp with gain selection

Question

I need to measure input voltage by 16bit ADC with 2.5 V reference. There are two different input voltage levels: 0 - 100 mV and 0 - 30 V. I designed the circuit below which should work in the following way:

• Range 0 - 100 mV: Optocoupler OK1 is off and OK2 is on, so that input voltage flows via resistor R1 to op amp which works as non-inverting amplifier with gain 25.
• Range 0 - 30 V: Optocoupler OK1 is on, so that resistors R1 and R2 form a voltage divider. The optocoupler OK2 is off, so that op amp has only 24k resistor in the feedback and works as voltage follower.

My problem: When I select the range 0 - 30 V, the output is very noisy. When I sample it by 16bit ADC, the noise is several hundreds of ADC divisions. Also I noticed the bigger is the input voltage, the bigger is the output noise. When I replace R3 by 0R, the noise is gone, but the circuit is not able to measure the second voltage range :(

EDIT: I switch the circuits by optocouplers, but I could not find them in the schematic editor, so I drew MOSFETs instead.

Thank you for suggestions. Klasyc

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

Finally it turned out that the op amp was oscillating. The solution was to add 10 nF capacitor in parallel with R3. Anyway, thanks everybody for your suggestions.

Answer 2

When I select the range 0 - 30 V, the output is very noisy.

Yes, it's possibly because the IRF530 has 250 pF drain-source capacitance and this pushes the overall gain back to being high at higher frequencies. The frequency where the gain starts rising is: -

F = \$\dfrac{1}{2\pi RC}\$ where R is 1 kohm (R5) and C = 250 pF.

I calculate this as being 637 kHz.

You need to consider using a different approach to avoid this unforeseen noise.

Alternatively, it could be that noise on the gates is being amplified, particularly M1 because this should be on in low-gain mode. Also, given that your supply rails are +/- 3.3 volts, I suspect that you may be controlling the MOSFET gates with 3.3 volts and, for an IRF530 this is just enough to partially activate it and any noise on the gate will be easily amplified. Read the data sheet for the IRF530.

If on the other hand you didn't bother to change the default settings for the schematic editor you used to draw your circuit and the default is an IRF530 then consider yourself chastised for wasting my time!