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Original question:

How to improve the accuracy of Peak Detector as high as posssible

THS4631: Peak Detector

Luigi Giunti
Prodigy 70 points

Tool/software:

Dear Support,

I am trying to design a peak detector, and the schematic I have attached seems like a good starting point. The input signal dynamics involve a variable pulse with a range that goes up to 12V, so I need to select the components to meet this requirement. Additionally, my signal can have a minimum duration of about one microsecond, so this configuration might suit my needs.

The MOSFET needs to be driven by a microcontroller operating at 3.3V. Do you have any advice on choosing the appropriate component?

Furthermore, I would like to better understand how this circuit works: why was a 22Ω resistor chosen in series with the capacitor? Can I choose any type of capacitor? How was the feedback resistor value calculated? Is there any associated technical note?

Thank you for your feedback on this matter.

  • 0
    TI__Expert 8380 points

    Hi Luigi, 

    The 22 Ohm resistor is just going to reduce the amount of voltage across the sampling capacitor. As the capacitor charges, the equivalent current will flow through the resistor, increasing the resistor voltage and reducing the total voltage drop across the capacitor to reach the target value. 

    You want to size your capacitor based on the amount of output current the amplifier can provide and how long you want to hold the value for. The larger the capacitor the longer it will hold the value as leakage current will discharge it when the output diode is not forward biased. However, a large capacitor requires a lot of current to charge up in a short amount of time so you need to size it appropriately that the amplifier can settle the correct voltage value on the capacitor when the peak signal arrives. 

    I will send this thread over to our microcontroller team to suggest the best device for controlling your FET. 

    Best, 

    Jacob 

  • 0 in reply to Jacob Freet
    Prodigy 70 points

    Thank you, Jacob, for your response.

    I would like to ask for more information regarding the capacitor. I read in an article that dielectric absorption can be a factor in choosing a capacitor, and the article refers to polyester or polypropylene capacitors. Is it correct to go in this direction to achieve precise measurements, or could a ceramic capacitor still be suitable for my application?

    Considering the dynamics of my signal, are there alternative op-amps I could use at a lower cost?

    Additionally, I noticed in the simulation that varying the resistance value slightly—by 50 ohms, for example—prevents the capacitor from charging up to the input voltage. Is it normal for the range to be so narrow?

    Regarding the feedback resistor, it seems to me that its purpose is only to protect the inverting terminal from the capacitor's discharge. Is that correct?

    I also look forward to your feedback regarding the choice of the MOSFET. Thank you again for your support.

  • 0 in reply to Luigi Giunti
    TI__Mastermind 21390 points

    Hi Luigi,

    Are you only wanting to control a single FET with the MCU? If so you can use a very cheap MSPM0C1103 (smallest package is 8-pins), if you have a little more needs I do recommend looking at our MSPM0 platform.

    Regards,
    Luke