The output stage of my circuit is a 2nd order inverting low pass filter. The input should be negative (DC), thus yielding a positive output (DC). It is possible that the input could be positive, but I don't want a negative output, so I grounded the negative pin of the op amp in the filter. It's not rail to rail but I should never be close the the bottom rail anyway.

My only real question was, am I supposed to still put bypass caps on the negative supply pin, which is grounded? That seems odd, because it would just be a cap from ground to ground, and seems useless, but I could be wrong.

Here's a picture showing part of my schematic:


The bypass caps network is not shown, but present, with 4.7uF and .1uF caps to ground.

  • \$\begingroup\$ When you have 11 rep you will see it on the ask question page, but circuitlab has a nice tool for drawing and simulating circuit, it can make it easier to post these designs and make sure they work. \$\endgroup\$
    – Kortuk
    Feb 28, 2013 at 17:04

1 Answer 1


No, you don't put a bypass cap on a pin that is already connected to ground. The bypass cap should be between the two power pins of the opamp, physically as close as possible.

It would be good for the bypass cap to have its own connection to the grounded power pin. In other words, don't run the bypass cap current accross the ground plane. Keep the high frequency currents thru the bypass cap off the ground plane in as small a loop as possible, then connect one point of that loop by the negative supply pin to ground.


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