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OPA357: Peak detector

danielb
Intellectual 270 points
Part Number: OPA357
Other Parts Discussed in Thread: TINA-TI

Tool/software:

Hello,

I'm troubleshooting a peak detector circuit using two OPA357 op amps (please, see TINA file) and encountering unexpected behavior. As shown in the image, the output voltage increases over time, even though the input signal is held at 0 V. Supply voltage is within the recommended range for the OPA357. Could this be related to input bias currents, capacitor leakage, or something specific to the OPA357? Any insights or suggestions would be greatly appreciated!

PeakDet_OPA357.tsc

  • 0
    TI__Mastermind 28915 points

    Hello Daniel,

      Capacitor/diode leakage could be an issue as you suggested. It could also be a slight instability problem, which can cause increased error with no input. 

      Would you be able to replace R3 based on your Tina-TI schematic with a small resistance between 10-50Ohms? 

    Thank you!
    Sima

  • 0 in reply to Sima Jalaleddine
    Intellectual 270 points

    Dear Sima,

    I just have the experimental results showing this behaviour. Putting a high impedance (10Mohms) in parallel with the charging capacitor seems to solve the issue. Is this a good approach or you recommendantion is better?

    Thanks!

  • +1 in reply to danielb
    TI__Mastermind 28915 points

    Hello Daniel,

      Your solution makes more sense to what you are seeing with output voltage not hitting exactly 0 when input signal is held at 0V. With the diodes at the output and the FET after, there is not a DC to ground for the amplifier's bias current to go other than charging the capacitor over time. Adding a resistor in parallel with the charging capacitor will discharge the capacitor. A high impedance is ideal to not affect the rest of your circuit.

      Later in your measurements, if you see the output obviously oscillate and deviate from the input when the input rises, then it would be helpful to add the isolation resistor previously suggested. 

    Thank you!
    Sima