We are using the ADC AD7725 in our design.In that we have tested the ADC in Unipolar mode and the output looks good with very low distortion. We are facing problem in testing of Bipolar configuration. Our Analog Input Range is -2V to +2V Single Ended Signal.
In Bipolar Mode, We are using the AD8138 OP-AMP to convert the input Single Ended signal to Differential Ended signal for AD7725 to work in Bipolar configuration. We have placed AD7725 in our main pcb and AD8138 circuit in another small daughter board placed on main board.We have made jumpers(length is 4cm) for connections between main and daughter board. We are observing large noise at the Output sample value while testing.
For Unipolar mode ,ADC is working fine .Also we are not using any Jumper.
We have not used the OP-AMPs mentioned in the Datasheet as per figure 14.Instead we have used AD8138 OP-AMP for Bipolar configuration.
Please let us know why distortion occurs.Also shall i use Ad8138 instead of datasheet specified.