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?
