Q
Our application is sensitive to offset voltage to keep leakage current
measurement dynamics as high as possible. AD7788 specifies typ.<3uV offset
but not input voltage, at which is measured. Could you specify it? If input
offset is say 3uV is tested at 0V common mode, should I expect, that it could
be more than 30uV at 1.2V common mode due to CMRR of 90dB? Similar to PSRR.
Does 90dB CMRR@DC and PSRR mean, that chopping technique is not able to
suppress offset (at zero input differential voltage) below say 10uV?
A
The offset is measured with an external short which is biased at AVDD/2. The
CMRR is measured with a differential voltage of 1V between AIN+ and AIN-. We
would expect the offset to be 3 uV typically at the higher common mode voltage
also. We would not expect it to degrade to 30 uV. If the input leakage current
of the device is also important, using a buffered part such as the AD7790 leads
to better leakage current compared with an unbuffered part.