# In a DC voltage sensing circuit, what's the need for capacitors on input and output?

This circuit uses a voltage divider to lower the voltage, and then uses an op-amp as a buffer (with some gain). If the input signal is always DC, why do I need these capacitors? The way I understand it, they'll act like very high resistances.

C22 may be useful if the DC input voltage contains an AC ripple or noise, that needs filtering. C23 at the OpAmp output is not a typical design. Many OpAmps can not drive such a capacitive load and overshoot or oscillate. Unless there is a very good reason, don't do that.

• Just to add to your comment, this comes from a datasheet on the LM324: "...are unity gain stable for 10nf CAPACITIVE load. When the load capacitance increases, the increased capacitance at the output pushes the non-dominant pole to lower frequency in the open loop frequency response, lowering the phase and gain margin. Higher gain configurations tend to have better capacitive drive capability than lower gain configurations due to lower closed loop bandwidth and hence higher phase margin."
– jonk
Jun 16, 2022 at 3:02
• Thanks! If so, how does one design these capacitances? I mean what can we assume as the frequency for the AC noise? Jun 16, 2022 at 3:21
• @nottherealfaraday The corner frequency of this low pass filter is 1/(2 * pi * R * C), but here you must calculate R27 and R29 as if they were in parallel assumed that the source inpedance is low compared to them. From the AC perspective they are considered parallel.
– Jens
Jun 16, 2022 at 3:38
• @Jens But my question is.. what corner frequency should I design the circuit for. Forget the values in the schematic; they're placeholders. Jun 16, 2022 at 4:59
• Depends on what you want to measure. If you want to exclude mains frequency, then maybe 10Hz might be reasonable? How fast are you sampling and how fast do you expect your voltage to change? Jun 16, 2022 at 11:48

If the input signal is always DC, why do I need these capacitors?

Alas, the input signal is usually not DC, because various source of interference superimpose their own AC currents on top of the DC voltage. Say a nearby cellphone - very common! The AC currents may even come from differences in the ground potential on the breadboard, for example.

In the ideal world, the capacitor would be unnecessary. The real world is not ideal.