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Original question:

OPA860: OPA860 sample& hold circuit

OPA860: OPA860 pulse detection circuit, continuous sine wave question.

SK
Intellectual 260 points
Part Number: OPA860

Hi Kailyn & Kai,

I am practicing with the OPA860 peak detection circuit that Kailyn post.

e2e.ti.com/.../opa860-opa860-sample-hold-circuit

The circuit is consist of two-stage of OPA860. The first for a fast rise time and the second for a slow decay time.

The circuit can sample a pulse width of 1ns. It is really impressive.

I am wondering what will happen if the input instead of continuous sine wave.

I know that will output a very low frequency signal, even looks like a DC signal.

Below is my schematic, I used various frequency of sine, the same amplitude.

Why the output of DC voltage will drop when the frequency rise? Is it a bandwidth limitation?

OPA860 pulse detect.TSC

Thanks so much.

SK.

  • +1
    TI__Mastermind 25505 points

    Hello SK,

      Your understanding is correct that it is a circuit bandwidth limitation. Depending on the frequency range you are testing, the output will attenuate at higher frequencies. Below is an AC response at each output:

       I would like into this previous e2e thread for slower continuous sine wave inputs. Feel free to ask questions about this thread.

    Thank you,
    Sima 

  • 0 in reply to Sima Jalaleddine
    Intellectual 260 points

    Hi Sima,

    Thanks for your reply.

    What is the bandwidth of OPA860?

    I read the datasheet, the bandwidth of closed loop OTA + BUFFER is 375MHz typical.

    But the bandwidth of open-loop OTA is only 75MHz.

    What is the bandwidth in my application?

    Thank you,

    SK.

  • 0 in reply to SK
    TI__Mastermind 25505 points

    Hello SK,

      Your understanding is correct from the data sheet that the different sections of the device have different bandwidths depending on your device configuration.

      For your application, it depends on your circuit configuration. The above simulation shows your overall closed bandwidth which is around 1MHz. Kai’s circuit is around in the 10s of MHz. 

    Thank you,

    Sima

  • 0 in reply to Sima Jalaleddine
    Intellectual 260 points

    Hi Sima,

    It is funny. The circuit acts like a envelope detector.

    Let me attached some photos measured by an oscilloscope.

    The yellow trace is the input pulse.

    The blue trace is the sample and hold output using OPA860.

    Thank you,

    SK.