Precision Opamp Selection for AAF (2nd order Sallen-key)- 24 channels 16 bit DAS

Hi,

I'm working on 16 bit 24 channels DAS. Here are the specs;


Architecture : Single ADC multiple channels. In each channel one AAF is used.
Resolution : 16 bit
Signal swing : +/- 10V (20V)
Input signal frequency : 200Hz max
Number of channels : 24
Total time available for 24 channels for conversion : 800us [ each channel 33us approx]
ADC sampling frequency : 4 MSPS
Line : Differential
Required 3-dB point for AAF : 200 Hz
Order of the filter : 2
Attenuation at 75Khz : -96dB (min)
Settling time: < 2us [ After every selection of channels - It seems to be small when compare to total available time period for each channel (33us). It is because, to increase the per channel sample rate by taking multiple samples of a single channel. 30us is used for sampling each channels.]
AAF architecture: Sallen key
Input allowable offset : <1.2mV
Supply : dual +/-15(min)

24 channels are connected to ADC via MUX(ADG1207). So maximum transient loading occurs during the switching of channels which are connected to max and min value of input signal. Since Sallen-key architecture is used, there may be a need of additional RC filter at the output of AAF opamp.

Can anybody suggest an opamp for this particular application.

Thanks,

Jebas.

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  • Jebas,

      A couple of things to consider....

    --  Every year ADC prices come down, so I would consider  using three ADCs or more.  Although the ADG1207 is a low capacitance mux, it still has finite on resistance and capacitance.  To get to 16 bits of accuracy, you have to allow 11 time constants to settle.

    --  96 dB at 75 kHz?  With a second order filter, you only get to ~52 dB.

    --  With perfect muxes, op amps, and ADCs, you still have to worry about pc board layout.  Real easy to get crosstalk and wind up with a 10 bit system.  I would strongly recommend singles or duals, not quads, to avoid routing problems and crosstalk.  I would start with a minimum of six layer board.

    You also need an AAF after the mux before the ADC.

    Harry

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