ADC KICKBACK FILTER DESIGN

Thread Starter

hoyyoth

Joined Mar 21, 2020
170
Hi Team,

I have a question regarding the KICKBACK Filter design in the ADS892xB data sheet. According to the datasheet it has three sampling rates.

ADS8920B: 1-MSPS =1Mhz
ADS8922B: 500-kSPS =500Khz
ADS8924B: 250-kSPS=250Khz

As per my knowledge the cut off frequency of the KICKBACK Filter is sampling frequency/2. Please correct me if I am wrong.

Please see the below figure obtained from the datasheet. If we calculate the cut off frequency of the RC filter given in the below
figure it is 7Mhz. Which is more than half of the sampling frequency of all three models.

May I know where I went wrong.

1626511712760.png
Regards
HARI
 

DickCappels

Joined Aug 21, 2008
7,989
It is not an anti-aliasing filter. the capacitors are there to clean up the sampling by allowing the fast charging of the ADC's internal sample and hold capacitor. Reread section 8.1.2.1 of the datasheet.
 

Thread Starter

hoyyoth

Joined Mar 21, 2020
170
It is not an anti-aliasing filter. the capacitors are there to clean up the sampling by allowing the fast charging of the ADC's internal sample and hold capacitor. Reread section 8.1.2.1 of the datasheet.
Hi Dick,
Thank you.
May I know how they got R value 2.2Ohm

usually when designing anti aliasing filter what I do is I take C 100 times the ADC sampling capacitor and calculate the resistor by
equating to Fs/2 .May I know is this method correct.

Regards
HARI
 

DickCappels

Joined Aug 21, 2008
7,989
All I know about this kind of filter is what I read in the data sheet. From memory your method (100x sampling cap) is very close to what was described in the data sheet and for all (little) I know your method is correct.

The 2.2 ohms, as a guess, is the output impedance or resistance of a device preceding the ADC.
 
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