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OPA4188: Loading effects on opamp

Part Number: OPA4188

Hi,

I have LPF built using OPA4188 in differential to single ended configuration.

Gain = 1.3 (RF = 133k, Rin = 100k, CF = 220p)

Now there is an external RC filter 1kohm and 1nF at the output, may be to reduce the overall noise profile.

My understanding from the AC analysis using break the loop method.

1. Phase margin we should check at the output of opamp where loop is closed, not at the external RC filter output., please confirm

2. External RC will not affect the loading of opamp, hence it wont contribute toward instability, is that correct.

3. Phase margin at the o/p of opamp is ~84 deg, which means circuit is stable

4. External RC will add to the delay of o/p signal by its time constant and will not affect the loading of opamp. The external C can be increased up to acceptable delay without loading the opamp, is that right?

Please refer below circuit and graphs

       

Thanks in advance!

Regards,

Sunney

  • Hello Sunney,

    Regarding your questions, see my responses.

    My understanding from the AC analysis using break the loop method. Yes, that should work well.

    1. Phase margin we should check at the output of opamp where loop is closed, not at the external RC filter output., please confirm

    Yes, that is correct.

    2. External RC will not affect the loading of opamp, hence it wont contribute toward instability, is that correct.

    It could, but since the resistance in series with the 1 nF capacitor is high, 1 kilohm, it will have little effect on the phase margin.

    3. Phase margin at the o/p of opamp is ~84 deg, which means circuit is stable.

    Yes, this is a high phase margin and the circuit will be stable.  We usually try to design our circuits to have a phase margin of at least 45 deg to assure stability even though it should still be stable with even less.

    4. External RC will add to the delay of o/p signal by its time constant and will not affect the loading of opamp. The external C can be increased up to acceptable delay without loading the opamp, is that right?

    Yes, when the signal is taken from the point where the output R and C connect there will be additional time delay added compared to when it is taken directly from the op amp output. As mentioned, since the series output resistor is such as high resistance the C could be increased without much change in the op amp output loading. The primary effect is the cutoff frequency of that RC network moves lower in frequency as C is made higher in value.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • Thank you Thomas for detailed response.

    Regards,

    Sunney