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Question asked by chaganty on Jul 20, 2013
Latest reply on Oct 1, 2013 by KarenNE

I am using AD7766 (24bit at 128kSPS). I need to use the digital filter and have filter pass band frequency variable (from 10 HZ to 50 kHZ). This corresponds to 0.547 x ODR. I implemented the scheme where in I am changing FMCLK accordingly. For AD7766 the stop band without aliasing is up to FMCLK − 0.547 × ODR , with stop band attenuation being >70dB. For  FMCLK=14.56KHz (with ODR =1.82KHz,supposed to give me pass band corner frequency = 1KHz ) the attenuation is pretty good from 1.01KHz to 3KHz. [ But for my surprise the aliasing starts from 3KHz onward and at certain frequencies the signal in stop band is much lower than 70dB. As per the datasheet the aliasing should start from FMCLK − 0.547 × ODR which is equal to 13.56KHz. Similar performance is also seen for other ODR frequencies. Since my application requires different pass band corner frequencies, I am unable to insert different anti-aliasing RC low pass filters and I need to change the pass band corner frequency through keypad. The ADC output is connected to a 20 bit DAC (where I am measuring the AC output) through SPI using 32 bit 80MHz PIC micro controller. Please let me know, whether my understanding from data sheet is correct and if so what needs to be done for increasing stop band attenuation and reduce aliasing.