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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.
Yes, a low-pass, anti-aliasing filter is typically included at the input of an ADC. Which converter are you using?
It is a fast adc with 100nsec sample time, 1.5K switching resistance and 4pF capacitance.
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.
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.
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