Recently I am designing a Data Acquisition system which utilizes FPGAs and ADCs. As my project needs to have a good balance between price, performance and quality, I have looked into the market to find available medium-cost ADCs. My current selection is the single-channel AD977 ADC (100/200 Ksps, 16-bits where I need 12 Effective Bits (ENOB)) from Analog Device. Also there is the AD624 instrumentation amplifier which is again, single-channel but is great if you want to programmatically amplify your signal in the analog zone, not losing your dynamic range.
But my design requires 4 or 8 analog inputs. So what I thought was to place the ADG508 analog mux in the first stage of the DAQ, just before the near Hi-Z inputs of AD624 instr. amp.
However my instructor believes that this is a bad workaround, suggesting to use multi-channel ADCs which have the Analog MUX On-Chip which, reduce noise. Surely that would be a better choice, but if I am going to use a multiple channel ADC with the same parameters and quality of ADG508 (sample rate, SNR, SFDR, ...), I will be have to heavily increase the budgets!
I had some research on the internet for comparison of MUXes. Everyone just seem to compare various MUX ICs with respect to bandwidth and RDS(ON) (On-Resistance). From the noise aspect in Communications Circuits Engineering, the equivalent thermal noise introduced on the signal could be equivalent to sqrt(4 K T R B). But is that all the noisy source? Other people have also had the same question but in other senses, such as: How much is the noise of a multiplexer for analog inputs?
The question is: Is my discrete analog MUX solution a big deal from the signal integrity and noise aspect? Did anyone have these problems with the same approach? If it is really a bad idea, what would be other ways of solving this issue?
I have important design rules of analog systems such as grounding (Analog GND, Signal GND, Power GND, Digital GND,...) and shielding in my mind.
Please note that the input signal of each of the channels is thought to have these properties: about 10 KHz maximum frequency, maybe as weak as 1mV or even less, impedance in range of 10KOhm~300KOhm
Extra Note: Does inserting a buffer before the MUXes make things better?
Thanks to All