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OPA317: Differential circuit analysis

Part Number: OPA317

Hello,

A previous engineer designed this OPA317 circuit and we could use help with an analysis.

We’d like to lower the fC on the differential filter, but the calculated capacitance becomes fairly large.  Do you know why they would have split the resistance on the input to the diff amp?   4.99Ω seems to be an awful specific value but it is not covered in his design document.  Why not place the filter cap across the inputs to the amplifier itself and save the two extra resistors?  Does this cause a stability issue with the amplifier at high f? 

I can share a more detailed schematic by email.

Thanks very much for your help. Keith

  • Hi Keith,

    it's difficult to say something useful without knowing the full schematic...

    When omitting the two 4R99 resistors you won't have any low pass filtering in this circuit, unless the unshown source on the left provides some source impedance.

    The 4R99 could have been added to make the unshown source work with the 100n load capacitance.

    Anyway, I think you can increase the 4R99 resistances at the expense of the 24k3 resistances. When replacing the 4R99 resistors with 300R resistors and replacing the 24k3 resistors with 24k resistors, i.g., you would get a low pass filter corner frequency of about 2.6kHz while keeping the overall gain unchanged.

    Kai
  • Hi Keith,

    You hit the nail on the head - stability. If the differential filter capacitor is connected directly across the OPA317 inputs the circuit will become unstable. It comes about from the pole introduced into the feedback loop by the 1.02 Meg feedback resistor, and the 100 nF capacitance connected between the inputs.

    I ran a stability analysis on the circuit with the 100 nF capacitor placed directly across the inputs and phase margin is only 2.8 degrees. Any perturbation in the circuit will start it oscillating. However, if the circuit is configured as you have shown in the schematic with the 24.3 k isolation resistors after the capacitor, the phase margin increases dramatically to 88.2 degrees. A completely stable condition.

    Certainly the 4.99 Ohm resistors could be increased while the 24.3k resistors could be decreased allowing for C177 to be lowered in value for a particular cutoff frequency. Just keep in mind doing so will lower the phase margin. As long as you keep the margin higher than 45 degrees the circuit should be good and stable.

    Regards, Thomas
    Precision Amplifiers Applications Engineering