Electronic – Where to place a RC filter regarding decoupling capacitor

capacitordecouplingfilterremote control

I would like to add a low-pass RC filter (500Hz) to the power rail of a TLV2782 op-amp. Datasheet of TLV2782 recommends two decoupling capacitors: 0.1uF and 4.7uF and they are currently placed on the VDD pin of the TLV2782.

Where should I place the RC filter? Should I place it before those decoupling capacitors or after? Should I consider that the decoupling 0.1uF capacitor is the capacitor of my RC filter (then forcing a resistor value) or should I consider the decoupling and the filter independently?

Any advice would be appreciated.

Best Answer

Typically, the decoupling capacitor is the C part of the RC filter, and the non-ideal inductance and resistance of the PCB traces is the R part. Sometimes (and this is not terribly common), a very small resistance or inductance will be placed like this:

schematic

but if you have so much noise on your power rails that a 500 Hz filter is necessary, you probably have bigger problems. The datasheet lists the supply voltage rejection ratio as 75dB minimum, power supply noise should not affect your measurement very much.

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