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September 22, 2011


Gerard Regnier

It's best to look at the final circuit in the frequency domain. You may be surprised at what you see as compared to what you thought you'd see when looking at the overall poles and zeros within a complex closed loop.

Frank Walker

Putting a low pass filter in the feedback path can cause problems, especially if the bandwidth is not sufficient to prevent overshooting (as seen in your graphic.)

Several reasonable alternatives exist:

1) Raise the bandwidth of the "low pass feedback filter" to be somewhat higher than the bandwidth of the opamp, at least enough so that the phase margin is not an issue.

This is likely best optimized by "knowing" what signal is to be rejected, or at least what the lowest possible frequency component to be rejected would be.

Since the only real purpose of the resistor is to decouple the opamp output from the capacitance, I would suggest using the absolute minimum value that accomplishes this goal. Again, keeping the Tau of the "low-pass feedback filter" sufficiently small to prevent phase issues.

2) Often it pays dividends to simplify the circuit, in this case I would suggest simply omitting the resistor and capacitor due to the fact the opamp is in a unity gain mode over the intended region of operation. Simply hook it up as a voltage follower and if necessary, place a "can" over the opamp to shield it from the EMI. (Obviously taking steps to reduce or eliminate the conducted EMI.)

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