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AGC with AD8338

I am trying to design an AGC with AD8338 along with AD704 as an Transimpedance amplifier.
I need the details on :
1) Attack time equation for AD8338 given in datasheet.
2) The connection details on connection for AD8338 as an AGC.
3) How to set threshold values for output and control the output based on threshold values set.
4) How to control Dynamic Gain range of the AGC.

  • Hi MNK,

    Good day!
    For your inquiries, kindly see below response.

    1) What details regarding attack time equation do you need?
    2) For the schematic, you may check our users guide. Just connect the GAIN pin to the DETO pin and apply 0V to 3V on VAGC pin.
    3) You can set the output by changing the voltage applied on VAGC pin. The RMS output voltage will be the absolute difference between VAGC pin and VREF. Example, if you have 2V on VAGC and 1.5V on VREF, then the output voltage will be 0.5V. Or if you have 1.2V on VAGC and 1.5V on VREF, your output will be 0.3V.
    4) When AD8338 is set to AGC, you will only have control on the output thru voltage applied on VAGC. May I know why you want to control the gain?

    Best regards,

  • Hi Franz,

    Hope you are having a Good day as well .

    Thank you for your prompt reply. I am a noob in regards to circuit design and These are the few issues in detail, I would appreciate your help and feedback on the following:

    1) I needed more information on the following equation from data sheet which states T is approximately 1.75 millisecond.

    T (sec) = 17,450 Ω × (285 pF + CDETO). but the units on RHS doesn't match with LHS.

    2)  Down below is the Schematic for my circuit: The problem I am facing is I tried changing the voltage at VAGC form 0 to     3V. but Vrms remains at 1.51V.

    3) The output of the AD8338 is out of phase with the input form the below figure.


    4) For my application I need the Gain to be set within a range like from 30dB to 80dB for that I need the control on the output so that it lies within the corresponding range. Any additional suggestions regarding circuit are very much appreciated.

    Best Regards,


  • Hi MNK,

    Good day! I checked the spice model for AD8338 and I have seen that AGC functionality is not yet modeled.

    I will update you once we have the updated model for this part.

    Best regards,


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