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ADC and Filtering problem

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yasine79

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Hi all!

I get digital data from the ADC. the Sampling frequency of the ADC is 600KHz. the Analog signal has the frequency 150KHz. Should i built a digital bandpass filter a after the ADC to filter the noise (harmonics) and offset created by the ADC. what are the critical frequency that schold filtered out after the ADC.

The ADC output will be used to build the I and Q channel.

Thank you all
 

HMS1021

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As a minimum an analog low pass "anti-aliasing" filter must be place ahead of the ADC otherwise the harmonics (and other signals, noise) will be folded back into your band of interest. One there they are in-band signals and can not be digitally filtered out.
A digital band-pass filter after the ADC may be useful. This depends on your requirements. If your band of interest is 0 - 150kHz then a digital low-pass with a 160kHz cut off will provide a stop band in the region where the input analog filter may still be rolling off. If your band is narrower then a BPF at 150kHz will reduce the noise bandwidth thus lowering the noise floor improving SNR.
 

yasine79

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thank you for the answer. i biult an anti aliasing filter in the analog part. my frequency of interest ist just the 150 KHz. the Question that i could not answer.: should the band pass very narrow (more coefficient--->computation time!!), or it is enough to use for exemple 60 coefficielnts (wider filter-->less computation time).

thank you
 

HMS1021

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thank you for the answer. i biult an anti aliasing filter in the analog part. my frequency of interest ist just the 150 KHz. the Question that i could not answer.: should the band pass very narrow (more coefficient--->computation time!!), or it is enough to use for exemple 60 coefficielnts (wider filter-->less computation time).

thank you

It depends on several factors. If the signal is a sine wave the filter can be very narrow. Be careful to pay attention to clock and reference errors. If it is in a control loop delay is a issue to be watched. If it is a 150khz square wave or digital data the bandwidth requirement is 350 kHz + and your sample rate is to low.
 

yasine79

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Hi!
the signal is a sinee wave. in the first reply you wrote :*If your band is narrower then a BPF at 150kHz will reduce the noise bandwidth thus lowering the noise floor improving SNR.*. could give some names of literature where i could read about it. am a theoritical person :)

thanks for the help
 

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