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.
If not used, inputs should be connected together to ground.
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.