Question
Q1.: I will only be using two of the four analog inputs on the AD7864. Do I
need to pull the two unused inputs low or is this done internally by the AD7864.
Q.: 2 I will be connecting an external reference (AD780) to the AD7864. The
external reference (AD780) chip has a 2.5V and a 3V option. Will the accuracy
and dynamic range of the AD7864 be improved by using the 3V reference option.
Answer
Question 1:
If not used, inputs should be connected together to ground.
Question 2:
The part is not designed to be used with a 3V reference. Anything other than a
2.5V reference will result the transfer function of the ADC generating errors.
So I can only recommend to use a 2.5V reference or the internal reference.