I'm using an AD8421 instrumentation op-amp, and I need to adjust the DC level of the output. As per the datasheet, the VREF input pin needs to be driven by something that asserts a voltage (low output impedance), and they recommend a voltage follower amp. So, I'm doing just that:
simulate this circuit – Schematic created using CircuitLab
The 0.1uF caps were mostly out of paranoia/habit.
I'm wondering: do I really need bypass capacitors for this op-amp? The op-amp is supposed to do a good job at filtering power supply noise/ripple at low frequencies, and this op-amp will not be subject to sharp transitions or any other high frequency signals; at high frequencies, any other op-amp that would produce noise have their own bypass caps so that noise does not propagate to the rest of the circuit.
Now, speaking of power supply noise (since I'm using a DC/DC switching power supply), I then realized that the noise will directly leak to the output of the AD8421 through the input signal of the LF356. I'm thinking I should replace the simple voltage follower with a low-pass filter at, say, 5Hz or so (e.g., place 10uF instead of the 0.1uF caps in the input circuit). However, the fact that the 8421 datasheet does not mention anything like this makes me wonder whether I'm being too paranoid? Any comments?
Best Answer
Yes, you need capacitors to filter the voltage at the output of the potential divider because you don't want to transfer power supply noise through to the reference pin of the AD8421. You will probably only need one capacitor from the non-inverting input to ground; i.e. you don't need two as you have indictated.
(Alternative) Yes, you need op-amp power rail decouplers.