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 bitSignal swing : +/- 10V (20V)Input signal frequency : 200Hz maxNumber of channels : 24Total time available for 24 channels for conversion : 800us [ each channel 33us approx]ADC sampling frequency : 4 MSPSLine : Differential Required 3-dB point for AAF : 200 HzOrder of the filter : 2Attenuation 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 keyInput allowable offset : <1.2mVSupply : 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.
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.