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Questions met on designing ADC schematic and choosing components

Question asked by richiechen on Dec 11, 2012
Latest reply on Dec 19, 2012 by maithil

Hi all


I am trying to build an ADC circuit with AD7682. However, during the process, I met some problems:


1. What is the transient response of an ADC? Can I understand it as the settling time needed for ADC input to reach the new voltage?


2. When should I use digital isolator? I heard that digital isolator can help reduce digital noises injected into ADC analog input. As is shown in datasheet, the input digital current is only 1uA. Why do I need a digital isolator?


3. Comparing with ADR445, how is the quality of the voltage reference of the AD7682? I am also using AD5760, which is a 16 bit -10V--- +10V  DAC requiring external voltage reference. Which plan is better?

a.  Using ADR445 to provide voltage reference both for DAC and ADC

b. using internal reference of ADC for both ADC and DAC.


4. For the analog input driver of ADC, what are the concerns? I am thinking of using ADA4841 or ADA4004. Could you tell me the difference? (Voltage ranges are not a concern, since the system will contain +- 15V and 5V and 3.3V)


5. The 0.25 input bandwidth is achieved by using a larger resistor. However, the larger resistor itself will also generate noise.....It may be a concern in high precision ADC. What should I do about that?


6. As can be seen in EVAL-ad7682 (, there is a RC filter before input of ADC. Is it the anti-aliasing filter?


I understand that I have many questions. Any clue will be really appreciated.


Thank you.




Richie CHEN