I have built an instrumentation amplifier out of two op-amps in a quad op amp (MCP6L04T). The input signal is coming from a low-impedanece source (~17k) and most of the information in that signal is below 10 Hz. Although I can see the outline of the desired signal, it is made of a bunch of peaks - as if the op amp's supply keeps getting cut off every time it reaches its intended output value. Why is it doing this? Both scope caps were taken at the output of the op-amp, before the filter, but they can also be seen after the filter.
The other two amplifiers in the package are being used as follows: one is a non inverting amplifier for the output of this amplifier, and the last is a peak detector for the output of that amplifier.
This is running off a fresh CR2032 battery. A previous design that used a dedicated instrumentation amplifier IC had the same input and output filters and power supply and worked flawlessly.
The op-amp has a 1 uF bypass cap right next to the pin, and a 10 uF cap further away. Both are ceramic. Looking at the supply pin with a scope revealed a fairly smooth supply - it doesn't jitter with these weird spikes.
FYI the paralleled resistors are there to increase the overall tolerance of each pair of resistors. It's giving me an extra ~3 dB of CMRR without the cost of 0.1% resistors.