Looking at the specs of AD8677 in datasheet Rev A on page 3 and 4, I see the
CMRR is 120 dB minimum. In the typical performance characteristics, the "PSRR
vs Temperature" (figure 18) and "CMRR vs Temperature" (figure 16) curves are in
line with the spec. But the “PSRR vs Frequency” (figure 29) and “CMRR vs
Frequency” (figure 28) curves start at much lower values: CMRR at about 110dB
and PSRR at about 90 dB. Which values can I trust?
The values in our specification tables are valid and ADI guarantees these
values. The additional charts are not part of the specification. They are hand
measured on the bench to show each curve characteristic.
The value in the spec table for CMRR is a large signal, DC measurement. We move
the common mode voltage by 20V and measure the change in offset voltage using
the classic op amp test loop. The graphs are AC small signal (
measured in the classic four resistor difference amp configuration with a
Caddock resistor array, 0.01% matching, so theoretically, the best we can do on
the bench is about 90-100 dB. At AC, if the input capacitance is 0.1 pF higher
on one input compared with the other, then that will make about 5-10 dB
difference at 10 kHz. Keep in mind that 120 dB is a ratio of 1 million, so 10 V
on the output would be 10 uV on the input. Microvolts of noise will interfere
with the measurement.