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OPA2132 feedback resistors

Tom Saluzzo
Expert 1280 points
Other Parts Discussed in Thread: OPA2132, OPA132

I am using an OPA2132 for an input on a differential signal. I want to use 100K input and feedback resistors (unity gain). Can I use 100K resistors with this device? All of the application literature shows low value resistors in these locations.

  • 0
    TI__Guru 62700 points

    Tom,

    Using 100k resistors in OPA132 difference amplifier configuration will not only dominate the total noise (Vnoise of 100k resistor is 40nV/rt-Hz) but even more importantly cause instability of the circuit because of the interaction of 6pF common-mode input capacitance with 100k||100k resistance. This interaction will cause a zero in the close-loop gain, Acl, transfer function around f=1/(2*Pi*6e-12*50k)=~530kHz and decreasing the phase margin – see below.

    A lower phase margin, in turn, will cause a significant small-signal overshoot which in the actual circuit, due to normal process variation, may result in a sustain oscillation. 

    13mV overshoot on 20mV input step voltage as shown above translates into 65% overshoot which means just 18 degrees phase margin - much lower than the recommended minimum 45 degrees - see below.

    For more information, please review stability section in the on-line TI Precision Labs tutorial - click on the link below:  http://www.ti.com/lsds/ti/amplifiers-linear/precision-amplifier-precision-labs.page