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OPA1679: Peak Detect Circuit

Nicholas Carley
Genius 11025 points

Part Number: OPA1679

I am trying to design a low cost peak detector. I am looking at using the TI OPA1679 and have ran into some problems.

When dealing with small signals (pulse width 1 to 10 usec, with amplitudes of 10-100mV) I notice that the output is higher than the input and when I deal with large signals, the output is lower than the input. Also I am finding that the output rails around 2 volts, whereas the rails are around 4 volts.  I am at a lost to explain why this is happening.

I have included a TINA project to demonstrate the issue. Looking over the OPA1679, I don’t see anything to indicate why this is happening. Can you explain what is going on and how to address it? If there is a an alternative part to will perform better (lower cost), I would be interested in considering it.

sbomac4c_peakdetect.TSC

  • 0
    TI__Guru* 77501 points

    Hi Nicholas,

    For some unknown reasons, your generator is not working or corrupted. 

    I deleted the old generator and create a new one. Please see if this is what you need. Please make sure that Vcm is operating in the linear region. 

    sbomac4c_peakdetect 06142021.TSC

    Best,

    Raymond

  • 0
    TI__Mastermind 28985 points

    Nicholas,

    The output rails out to 2V because the capacitor reaches 2V and acts like a common mode voltage to the second op amp. 

    This is equivalent to having 2-2.3V common mode as shown with the addition of the voltage source I added for demonstration below

    It stays at 2V even if the cap were to charge further because the first op amp will drive its output such that it keep its inputs no more than a Vos apart. Diode D2 is forward biased in this scenario and current is sunk into the Op Amp U1. This current is 199.95uA. Notice that the output voltage of U1is the negative voltage across D2 minus Vos. 

    Vout1 = -VD2-Vos = -614.3513mV

    In order to maintain a Vos voltage, U1 will sink more or less current as needed to maintain this relationship. I have attached the updated simulation below.

    2502.sbomac4c_peakdetect.TSC