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,

    I can think of over 60 opamps with a 40V/usec slew and less than 0.5mV offset that run off 30V rails. I would also be looking for a low input bias current as a 200Hz filter needs large RC time constants, which you would probably want to generate with large resistors. Have you considered having one AAF for each channel before the multiplexor? It makes the system design much easier.

    Anyway ADI offer the AD744, AD746, AD 845, ADA4627, and OP42

Reply
  • Jebas,

    I can think of over 60 opamps with a 40V/usec slew and less than 0.5mV offset that run off 30V rails. I would also be looking for a low input bias current as a 200Hz filter needs large RC time constants, which you would probably want to generate with large resistors. Have you considered having one AAF for each channel before the multiplexor? It makes the system design much easier.

    Anyway ADI offer the AD744, AD746, AD 845, ADA4627, and OP42

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