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# AD7176-2 Digital Filter / sinc5 + sinc1

Hi, concerning the AD7176-2 Digital Filters I run into the spreadsheet AD7176_2_filter_model.xls provided by Analog Devices.

The spreadsheet results always varied from my understanding of a sinc5 + sinc1 filter.

The equation showed that the sinc5 + sinc1 filter is

3 x Moving average filter of the length 32

+ 1 x Moving average filter of the length of 16

+ 1 x Moving average filter of variable length depending on the output data rate.

Here is the Excel Code Analog Devices uses. 20*log(abs(

 (1/f_dec *SIN(f_dec *\$A44/Fm*PI())/SIN(\$A44/Fm*PI()))^(order-2) *   (1/f_dec2*SIN(f_dec2*\$A44/Fm*PI())/SIN(\$A44/Fm*PI())) *   (1/(f_dec*n_avg)*SIN((f_dec*n_avg)*\$A44/Fm*PI())/SIN(\$A44/Fm*PI()))

))

For example

 Name Symbol Value Unit Range MCLK Frequency Mclk 16,00 MHz 1 ..16.384 ODR[4:0] ODR 1 0..20 Sinc Filter Order f_ord 5 3 or 5 Single Cycle / Multiple-Channels 1 0 or 1 SINC3_MAP s3_map 0 0 or 1 FILTER[14:0] FilterReg 5000 1 to 32767

results in order = 5  and n_avg = 2 and f_dec = 32 and f_dec2 = 16

Questions:

So is the sinc5 part in reality a sinc3 + sinc1 filter? Or is there an error in the spreadsheet?

## Top Replies

• Hi da_lundi,

The AD7176-2 filter model is slightly modified Sinc5 filter. It is a combination of 4 x Moving average of length 32 + 1 x moving average of length 16. The first part contains three of the Sinc5 moving average terms. The second part models the shorter moving average and the third part combines the remaining Sinc5 term plus the output data rate dependent average term.

Regards,

Jonathan

• Hi Jonathan

thank you for the answer, however it is still not absolutly clear to me:

You are saying that

... It is a combination of 4 x Moving average of length 32 + ......

and

... The first part contains three of the Sinc5 moving average terms. ..

that is a conflict, or is it not?

Thank you for your Help!

Sören

• This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

Thank you,