AD974: Antialiasing filter

Document created by analog-archivist Employee on Feb 23, 2016
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Hi, Do I have to use for the AD974 an antialiasing filter (which
limits the bandwitdth of the analog input signal to half of the
sampling frequency) or the switched capacitor ADC doesn't need
something like that because of its design technology?

 

The AD974 still needs an anti-alias filter. In fact all converter which convert
between  the quantised digital domain and the continuous analog domain require
an anti-alias filter. It's a requirement of any sampled data system and
independent of the type of converter used. The reason you need an anti-alias
filter is to limit the bandwidth of the input signal in order to meet Nyquist
criteria.

BEWARE: Things are not quite as simple as adding a filter which limits the
bandwidth to half the sampling frequency. Remember that there is no such thing
as a brick wall filter. Take a quick  numerical example. The AD974 is a 200ksps
ADC and let's assume that the highest frequency you want to capture is 75kHz.
Ok so we design a low pass filter with 75kHz cut off frequency. With 200kHz
sampling frequency, Nyquist is 100kHZ, and the first image is at 150kHz. To
maintain accuracy you need a filter which will attenuate any signals at 150kHZ
to better than 74dB (in order to maintain a 12bit dynamic range). So filter
must roll off approx 70dB in one octave, that's a 23 pole filter you need. So
you can see that even to sample at frequencies close to Nyquist you need a
fairly expensive filter. What you can do is reduce the bandwidth such that the
first image appears at a higher frequency or consider using an ADC with higher
sampling rate and lower resolution. (AD9260 maintains 16 bit up to 2MHZ, AD9220
gives 10MHZ at 12 bit resolution).

Take a quick a look at our seminar notes for more background on sampling theory
and anti-alias filters etc. 

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