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Need help to drive a differential ADC inputs (AD7626)

Question asked by glias on Oct 10, 2011
Latest reply on Jan 31, 2012 by glias


I have a question about the ADC acquisition chain. I would use a AD7626 which have differential input and a VCM output produces by a voltage reference (4,096V).

My system is composed of two boards, one front analog and other digital one (with ADC).So the inputs need to be connected to a differential pair cable. The length of the cable is about 30 centimeter and the frequency of the analog signals are near of the 1MHz.


1- Since the environement is pretty noisy, I have a placed the differential driver stage on the analog board to avoid EMC effects : the cable is a differential pair.

Is it a good choice to have the differential driver stage on the analog board ? or does I must put it on the digital board instead ?


2- in case of placing the differential stage driver on the analog board, is is it better to have the VCM voltage reference on the analog board and then connected by a cable on the ADC reference input (with a low noise non inverter stage G=2) ? (see 1st solution file)


or another solution is to have the voltage reference near the ADC which also drive the VCM of the differential stage (non inverter stage with a gain of 1/2 on analog board) (see 2sd solution file)


or can I have a separate voltage references stages :

- one (2,048V) for the VCM for my differential stage on the analog front end board

- another one (4.096V) for the reference stage on the ADC  (see 3rd solution file)



3- in general does the best way to minimize EMC effects / noise of the cable,  is it better to place the buffer stage on the ouput of the "emiter", or place it on the input of the "receiver" ? (In my problem I mean the voltage buffer for reference voltage stages).


4- I sit allowed to have a differential driver stage with a different VCM reference voltage (for example 1V VCM for the differential stage) and for example a 2,048V for the VCM voltage (which is Vref/2) for the ADC ?


I hope that you could help me

best regards