1) On the datasheet I get the impression that the 'CHOP' variable affects the output rate of the sinc3 filter, and therefore the frequency response. So if SF = 512 and CHOP = 0, the response is as it is in the datasheet, but then setting CHOP = 1, will it not stretch the response out such that the first node is at 200Hz. I ask this because the excel model of this filter, which you provide on your website, the sinc3 filter seems to be unaffected by the value of CHOP. 2) For the calculation of the sinc3 output, the formula that you have used is a bit different than what I'd expect 3) How would I go about modelling the sinc3 filter in Simulink? 4) Is there any chance you could provide me with the FIR filter coefficients?
1)When chop is off and the FIR filter is not used, the AD7731 does the inversions needed for chopping. However, it does not do the averaging. You must take 2 subsequent conversions and average them. The excel model shows the frequency response before this averaging. However, the averaging that the customer performs will change the filter response. The averaging will introduce first order notches at half the output data rate and at odd integer multiples of half the output data rate. For example, with chop off, FIR filter not used, the output data rate is 600 Hz i.e. the sinc3 filter is used and is free running. With chop on and the FIR filter still bypassed, the output data rate is reduced by a factor of 3. This is because the sinc3 filter has a settling time of 3 conversions. So, with the analog inputs connected in one direction, a conversion is generated. The analog inputs are then swapped and another conversion is generated. These 2 conversions then need to be averaged by the customer to remove the offset. With chop on, the output data rate is 200 Hz. Therefore, the averaging will introduce notches at 100 Hz, 300 Hz, 500 Hz, etc. 2)The output data rate is listed in the datasheet. The modulator operates at MCLK/16. So, for a 4.9152MHz clock, the modulator operates at 307.2 kHz. The output data rate is fmod/SF when chop is off and the sinc3 filter only is used. So, when SF=512, the output data rate equals 307.2kHz/512 = 600 Hz. When chopping is enabled, the output data rate is reduced by a factor of 3. So, for the above conditions, the output data rate is 600/3 = 200 Hz. The excel model is correct. 3)The sinc3 filter is a standard sinc3 filter. 4)The FIR coefficients are the same for chop off and chop on.