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OPA548EVM: OPA548EVM differential gain

Chad Schneider
Prodigy 20 points
Part Number: OPA548EVM
Other Parts Discussed in Thread: OPA548

I’ve setup the OPA548EVM with dual power supplies (+/- 12VDC) and input signals using the differential amplifier setup in the user manual, Figure 8, shown below. I have J2 (Vin-) as a 20Hz 1Vp-p sine wave and J5 (Vin+) as a +3V DC signal. What is the equation to set the gain for this circuit? I want to amplify Vout as the difference between the inputs on J2 & J5. Can I replace the surface mount resistor at R2 with a 10kOhm potentiometer (wired to the pads) to vary the resistance and increase the gain as R1/R2 or do I need to vary both R2 and R5 together via one potentiometer? Alternatively, would it be better to increase R1 using a different Pot (say 100kOhm)?

Thanks.

  • 0
    TI__Guru* 93105 points

    Hello Chad,

    The transfer function for the OPA548 differential amplifier circuit shown in EVM Fig. 8 is:

    If, RF1 = R2 = R5 = R7, then:

    For best performance the four resistors must be very closely matched to assure the difference function accuracy. Rather than buying very expensive low tolerance resistors, you can purchase and use 0.1% tolerance surface mount resistors that really aren't expensive. The key is to use the 4 resistors directly next to each other, one after the other on the tape, rather than randomly picking them from a pile of 0.1% resistors. Doing so almost always results in very close resistance matching between them.

    If you wish to operate the circuit with gain then R2 = R5, and R7 = RF1, try to match the pairs as closely as possible. If you try to use a dual potentiometer to vary the gain it is unlikely that the matching between the potentiometer sections will be very good. In that case both the differential gain accuracy and the common-mode rejection will suffer.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 20 points
    Hi Thomas,

    Thanks for the detailed response. I'm not sure of the gain I'd like to end up with, so unless there's another way to vary the resistance values, I'll try a dual potentiometer for Rf1 and R7 on Monday and see where that gets me.

    Thanks, Chad