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OPA2182: Dual Integrator Hold Mode

Stephen Tomporowski
Prodigy 230 points
Part Number: OPA2182
Other Parts Discussed in Thread: ACF2101, , OPA192

Tool/software:

I'm attempting to create a dual integrator to measure distance from an accelerometer.  We have the ACF2101 but we need a larger capacitor for the integration time which is 2 seconds.  Schematic is here.  This circuit is an attempt to create a discrete ACF2101 with the same type of operation.

The problem occurs when going into 'Hold' mode.  U8A and U10A open up (U8B/U10B remain open).  However, instead of the voltage on the feedback capacitors discharging slowly through the large feedback resistors (note that in actual operation, we replaced R19 and R22 with 100M resistors), the voltage goes immediately to zero.  Calculation of time constant from scope traces says that suddenly a 50 ohm load is put on the capacitor.  According to the experts on the mux forum, the switch does not have any internal load.  So I'm assuming that the OPA2182 is doing something.  Have a chosen the wrong type of opamp?  What would be a better opamp to use?

Thanks...Steve

  • 0
    TI__Mastermind 19257 points

    Steve,

    The OPA2182 is a chopper amplifier.  Chopper amplifiers continuously calibrate the input offset voltage.  This calibration allows for very low offset voltage and offset drift.  However, the disadvantage of these types of amplifiers is that they have a bias current transient.  The bias current transient can be microamps in amplitude.  This transient can translate into an offset when used with large resistors.  It could also disrupt the operation of circuits like integrators as the integrator will integrate the IB-transient.  Optimizing Chopper Amplifier Accuracy explains the operation, benefits, and limitations of chopper amplifiers. 

    Long story short:  Yes, I do think that chopper type amplifiers may not be best for your application.  A common amplifier with good offset, and offset drift that may work for your situation is OPA192.

    I hope this information helps!  Best regards, Art 

  • 0 in reply to Art Kay
    Prodigy 230 points

    Art, Thanks.  I believe that the chopper amp is causing the premature discharge.  I'll look at the OPA192.  Steve