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AFE4490 ADC Anti-Aliasing

Robert McGurrin
Prodigy 50 points

Other Parts Discussed in Thread: AFE4490

Hi,

I'm trying to understand the anti-aliasing performance and filtering on the AFE4490 and I have several questions.
Can you please send me the frequency response or specs of the 500Hz/1kHz corner filter referenced in figure 54 of the spec?
What is the oversampling rate of the sigma-delta converter?
What is the order of the sigma-delta modulator?
What is the frequency response or spec of the filter at the output of the sigma-delta?  
Is it cutoff set based on the desired output sample rate (PRF/4)?
The spec says that the transimpedance filter has a low pass response determined by Rf and Cf: I interpret that as the filter has a cutoff frequency of 1/RfCf.  Is this correct?
Later the spec says in equation 1 that Rf x Cf <= RxSampleTime/10.  Where does that factor of 10 come from?
Can you please explain the averaging module in more detail?  Is this a moving average filter? Or does this module capture and hold the voltage and then average multiple samples of that voltage?
Thanks in advance
Rob
  • 0
    TI__Guru 50036 points

    Hello Rob,


    I apologize for the delayed reply.


    1. The filter is a switched RC filter. Depending on the duty cycle, the bandwidth of the filter around DC gets scaled down from 500 Hz (lower the duty cycle, lower is the bandwidth). Also there are going to be copies of the primary frequency response (around DC) around every multiple of the PRF. Finally there is an envelope (close to a sinc function) whose width again depends on (inversely proportional to) the duty cycle. So overall, it is quite a complex function.

    The details about the 22-bit del-sig converter is TI IP and will not be disclosed.

    6. The 3-dB bandwidth of the TIA is determined by Rf and Cf and is equal to 1/(2.pi.Rf.Cf). The switched R-C filter follows the TIA and its bandwidth is not determined by Rf, Cf (refer response to Qn.1)

    7. When you have a factor of 10, essentially you are giving 10 time constant for the TIA to settle to the pulse transients. It could be set to between 5-10.

    8. Enabling averaging causes the ADC to convert the voltage sampled by the switched RC filter multiple times to reduce the ADC noise. The sampling is done once (across the capacitance of the switched R-C filter) and the same sampled voltage can be converted multiple times by the ADC (each conversion taking 50 us).

    Best regards.