# Why analog anti aliasing filter is used before analog to digital converter when there is already a digital filter after ADC?

Normal data acquisition consist of:

1. Analog anti aliasing filter( Sampling frequency : $5\textrm{ kHz}$)
2. ADC - Digital Filter - (Sampling : 200K samples /sec)
3. Digital low pass filters Filters
4. DAC

Questions:

1. My question is why analog anti-aliasing filter is used when their is already a digital low pass filters after ADC to prevent anti aliasing.

2. If analog anti-aliasing filter have sampling frequency $5\textrm{ kHz}$ , the system will not take frequency greater than $2.5\textrm{ kHZ}$,

• then why ADC frequency is $200\textrm{ kHz}$?
• Doesn't analog anti-aliasing filter sampling frequency limit overall system frequency ?
• your analog anti-aliasing filter has a sampling frequency? – endolith Nov 15 '16 at 18:40
• @endolith perhaps it's a switched capacitor filter? – Peter K. Nov 15 '16 at 19:35

Question 1: The anti-aliasing filter before the ADC is exactly for the purpose of rejecting high frequencies, that will become lower frequencies (i.e. aliasing) after the ADC. The digital lowpass after the ADC cannot help here, as the aliasing has already happened. Consider this example:

• Your input signal is a sum of two sine waves, with frequencies 10kHz and 220kHz.
• After ADC you would find two sine waves: one at 10kHz, one at 20kHz (220kHz-2*Fs).
• Hence, you have aliasing occured, and no digital lowpass can remove this aliasing.

Question 2: Without more information on the system this cannot be answered. However, here are some thoughts:

• filter of 5kHz requires a sampling frequency of at least 10kHz (ideally). You state you only need 2.5kHz. I think you mix something here.
• in reality, no anti-aliasing filter is a perfect low-pass, hence its cutoff-frequency does not mean, that higher frequencies are perfectly blocked. Instead, they are more and more attenuated. To cope with non-ideal anti-aliasing filters, the sampling frequency should be higher than 2 times the cutoff. However, 200kHz from your example occurs still quite high for me.
• I answering question 2, you quoted i need only 2.5 KHz,, which i mean if at beginning you have 5 KHz analog filter, then input is limited to half of it which 2.5 Khz, then what is the use of higher sampling rate of 200 kHz ADC, which is very common in most of the standard data acquisition system. – user2121195 Nov 15 '16 at 15:44
• You stated: your anti-aliasing filter has 5kHz. Hence I your subsequent ADC needs to have 10kHz – Maximilian Matthé Nov 15 '16 at 15:46
• Re-read the comment man – user2121195 Nov 15 '16 at 15:48
• To prevent aliasing, the analog low pass filter has to have a cut-off frequency below half the sample rate. e.g. for a sample rate of 5 kHz, the filter cut-off frequency needs to be below 2.5 kHz. Using a filter with a cut-off of 5 kHz, the sample rate needs to be above 10 kHz to prevent aliasing. The higher the sample rate, to easier it is to make the low pass filter (making a filter with a sharp transition band is more costly). – hotpaw2 Nov 15 '16 at 17:00
• Okay i got it, if i wanna use 200KHz ADC at higher frequency than analog anti-aliasing filter, i can bypass analog filter. Its in the technical reference manual of data acquisition system. Page :51 www.dewesoft.com/download?file=SIRIUS_TechnicalReferenceManualV1.4.4.pdf – user2121195 Nov 15 '16 at 17:08

The higher the sample rate, to easier (and thus cheaper) it is to make the analog low-pass filter required to prevent aliasing (to limit aliasing to below your required noise floor). Making a filter with a sharp transition band is more costly. Often a much higher sample rate is cheaper to implement than a slightly sharper low-pass filter.