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OPA602: transfer function including parasitics, gain rolloff

Dan Johnstone
Prodigy 40 points
Part Number: OPA602
Other Parts Discussed in Thread: OPA145

Application is measuring unknown impedances using an variable feedback resistance (any of 8 resistors switched in with reed relays). The parasitics cause errors in the measurement. I have modeled parasitic series combination of C and R in parallel with the device under test (Rdut or Cdut), and in parallel with the feedback resistor (Rf). Also, have modeled the parasitics with gain rolloff. Labview fitting program is used to get the C and R parasitic values. However, the model must not be correct, since the fit values for the parasitics are not constant with different measurements of Rdut and Rf. Also using an input and output voltage follower. Measurements are from 20Hz to 250kHz. I expect some influence, at least from the phase due to rolloff of the 6.5MHz BW.

Is there a better model out there? I'm using one from "Design with operational amplifiers and integrated circuits"  by Franco, p266, eqn 6.15a. It has A(jf)=A0x1/[1+jf/fB]. I'm using a form of A0 similar to eqn 1.62, accounting for Rf and Rdut, as well as the input and output impedance (rd=>Zd, and r0=>z0), and substituting Zf and Zdut including parallel parasitics for Rf and Rdut.

  • 0
    TI__Guru* 93105 points

    Hello Kent,

    The OPA602 is a legacy op amp developed over 30 years ago. Its Pspice simulation model is nearly as old having been created in 1990. No newer model exists and there aren't any plans to update it.

    A lot has changed since 1990, and newer models model more electrical behaviors that more closely imitate the perfprmance of the actual silicon. Importantly, our newer models realize more representative open-loop gain/phase and open-loop output impedance (Zo) characteristics which are very important for accurate stability analysis.

    If you would like to simulate your circuit with a modern, accurate simulation model, I suggest using the OPA145 JFET model. It is a new, precision operational amplifier with similar electrical performances to the OPA602, but uses a fraction of the operating current. It has a very sophisticated simulation model that includes the ac characteristics I mentioned. You can find the model here:

    www.ti.com/.../toolssoftware

    Regards, Thomas

    Precision Amplifiers Applications Engineering