Hi All, I'm a HW engineer and need to design decimation filter for sigma-delta ADC. After reading some articles and discussion from internet, I knew decimation filter has 2 major function, downsample and 1-bit to multi-bit data conversion. As all replies for http://www.dsprelated.com/showmessage/2836/1.php, the output bit number seems the max value of 1s accumulation, ex. 12-bit for 4095 oversampling. But I found a sample code of decimation filter in datasheet of AD7401 that is a sigma-delta ADC( http://www.analog.com/static/imported-files/data_sheets/AD7401.pdf ). That decimation filter is implemented by a Sinc3 filter and 256 decimation rate. The accumulation bit number is 24-bit in that filter. I don't realize why it needs to 24-bit? Even all 256 sample are 1s, the max value is only 256. If I change the decimation rate to 512 or 128, should I change the accumulation bit number? Is there any rule for bit number decision for decimator output? This question confused me a lot.... Thanks for your kind advices in advance! Best Regards, pcchen

# Question for bit number decision of decimation filter's output

Started by ●May 6, 2009

Posted by ●May 6, 2009

Hi All, I'm a HW engineer and need to design decimation filter for sigma-delta ADC. After reading some articles and discussion from internet, I knew decimation filter has 2 major function, downsample and 1-bit to multi-bit data conversion. As all replies for http://www.dsprelated.com/showmessage/2836/1.php, the output bit number seems the max value of 1s accumulation, ex. 12-bit for 4095 oversampling. But I found a sample code of decimation filter in datasheet of AD7401 that is a sigma-delta ADC( http://www.analog.com/static/imported-files/data_sheets/AD7401.pdf ). That decimation filter is implemented by a Sinc3 filter and 256 decimation rate. The accumulation bit number is 24-bit in that filter. I don't realize why it needs to 24-bit? Even all 256 sample are 1s, the max value is only 256. If I change the decimation rate to 512 or 128, should I change the accumulation bit number? Is there any rule for bit number decision for decimator output? This question confused me a lot.... Thanks for your kind advices in advance! Best Regards, pcchen