AnsweredAssumed Answered

AD7730 with Antialiasing Filter

Question asked by vismay on Mar 19, 2016
Latest reply on Mar 30, 2016 by BrenS

Hi All,

 

In my Project, I am connecting AD7730 with Loadcell. I am using AD7730 in CHOP = 1, 100Hz Output Rate with Normal Second Stage FIR Filter. I am going to capture weight loaded on the pan hanged on loadcell. Now the signal that i want to capture is never gonna have frequencies on and above 1Hz (i.e. the desired signal would be always less than 1 Hz) . So I want to remove any of the frequency components on and above 1Hz. So I decided to put 2nd Order Passive anti-aliasing filter  as shown below

 

 

This filter's Single Stage's cutoff frequency is 4.37Hz and so final cutoff frequency of this complete filter is nearly 2.81Hz.

 

I got to read an article which mentions some limitations on putting anti-aliasing filter in Sigma Delta ADC.

 

(1)   http://www.edn.com/electronics-blogs/bakers-best/4395286/Delta-sigma-antialiasing-filter-with-a-mode-rejection-circuit

 

(2)   http://www.edn.com/electronics-blogs/bakers-best/4314103/Using-an-analog-filter-to-inject-noise

 

As mentioned in article 2, If i want to use anti-aliasing filter for Sigma Delta ADC then I have to target ADC's Output Rate as 3 db cutoff frequency of Anti-aliasing filter, which means I need to put R and C such that

 

f = 1/(2*PI*R*C) ,

 

f = 100Hz( as this is going to be my output rate from ADC)

 

But If I want to put anti-aliasing filter which i have drawn above which would have 2.81Hz as its -3db Cutoff Frequency, Is it possible ? Or there would be any dis-advantages of using it ? Please guide me.

 

Also in AD7730's datasheet it is written that its differential input are buffered so presenting High Input Impedence, and so it allows external low pass filtering. So Is that true for AD7730 , it doesn't matter much even if i put Heavy Filtering at its input ?

 

Regards,

Vismay

Outcomes