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OPA2171-Q1: OPA2171-Q1 and LM2904-Q1 RC filter design when using as differential amplifier

Scarlett Cao
Expert 3630 points
Part Number: OPA2171-Q1
Other Parts Discussed in Thread: LM2904-Q1,

Hi team, 

Do we have any guidelines on how to design RC filter when using OPA2171-Q1 or LM2904-Q1 as differential amplifier? Customer found that too small R value influence response time, so they would like to know if there're some general rules for this

Thanks,

Scarlett

  • 0
    Guru 140200 points

    Hi Scarlett,

    in a differential amplifier design care must be taken to not ruin the common mode rejection by introducing imbalanced filter components. So the corner frequency should be as high as possible and as low as necessary. This keeps the impact on common mode rejection minimal. Also, use very low drift parts with very low manufacturing tolerances. +/-1% NP0 caps and +/-0.1% thin film resistors are recommended.

    Keep the corner frequency of differential filtering about 10...20 times higher than the corner frequency of common mode filtering. This also to minimize the impact on common mode rejection.

    Keep in mind that imbalanced input bias currents will flow across the filter resistors. So keep the resistor values sufficiently small.

    Can you post a schematic?

    Kai

  • 0
    TI__Genius 15410 points

    Hi Scarlett,

    In addition to what Kai said. If you are designing the RC filter on the output of the amplifier, care must be taken to not degrade the phase margin of the amplifier due to the capacitive load. The resistor in the circuit must be large enough to stabilize the amplifier. I recommend taking a look at "Capacitive Load Drive Using an Isolation Resistor" for information on how to size the resistor if the RC filter is on the output.

    Thank you,

    Tim Claycomb

  • 0 in reply to Tim Claycomb
    TI__Expert 3630 points

    Hi Kai, Tim,

    Thanks for your reply. Sorry I didn't explain in my question. It is for RC filter on the output of the amplifier. So If the amplifier is connect with DSP, there's no problem of capacitive load, right?

    Thanks, 

    Scarlett

  • 0 in reply to Scarlett Cao
    Guru 140200 points

    Hi Scarlett,

    by DSP you mean an ADC input? In this case a charge bucket filter is usually mounted in front of the ADC input and the OPAmp must be able to drive this filter. This can indeed cause issues and one would need to know the DSP and the charge bucket filter to decide whether this is stably working or not.

    Kai

  • 0 in reply to kai klaas69
    TI__Mastermind 18887 points

    Hey Scarlett, 

    I believe Section 3 should be helpful for you from this Buffer Op Amp to ADC Application Note. 

    All the best,
    Carolina

  • 0 in reply to Scarlett Cao
    TI__Genius 15410 points

    Hi Scarlett,

    The capacitor (C) of the RC filter will appear as a capacitive load to the amplifier. If the resistor (R) of the RC filter is not large enough the output may begin to oscillate due to low phase margin. With different values of resistors the response time will change because you are changing the phase margin. Lower phase margin will result in more overshoot but possibly a faster settling time if the phase margin is not too low. High phase margin design may result in an overdamped design. It is important for a design to have sufficient phase margin (>=45 degrees) to ensure a robust design.

    I recommend watching our TI Precision Lab Videos on Stability.

    Thank you,

    Tim Claycomb