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OPA2355 to ADC: RC filter concern

I have built a 4 stage transimpedance amplifier, the first 2 stages are for gain and the last 2 for active filter. 

My signal is 1MHz with duty cycle of 50%. The active filter blocks anything above 1.5MHz (LP filter). My signal rise time is acceptable and I sample the signal using Maximum likelyhood detector approach (essentially sample when I can get the highest signal value).

The OPAMP is directly tied to ADC, there is no RC filter in between and so far I did not observe any issues, however somebody more senior suggested for me to provide an RC filter before the ADC. I am going to spin a new board and I like to ask masters if this does make sense.

The ADC I use has a 4pF capacitance with 1.5K switching resistance. The sampling time I am using is 100nSec. 

Thanks

  • Hi Frank,

    Yes, a low-pass, anti-aliasing filter is typically included at the input of an ADC.  Which converter are you using?

    Regards,

    Bart Stiller

  • In reply to Bart Stiller:

    It is a fast adc with 100nsec sample time, 1.5K switching resistance and 4pF capacitance. 

  • In reply to Frank QT:

    Bart,

    But pls remember, the signal is already filtered with active filter (1.2MHz LPF) so aliasing is not a concern this is more for stability of OPA355

    Thx, F. 

  • In reply to Frank QT:

    Frank,

    In the OPA355 datasheet you can find recommended output series resistance values for various capacitive loads (page 7), and the corresponding frequency response.  For a 4pF load, the recommended series resistance is ~88ohms.

    Regards,

    Bart Stiller

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