Hi all
I have also problem with my MDAC.
I have designed a fully differential opamp with UGB=680MHz, DC gain=67db, PM=61 deg, input common-mode voltage = 0.75v but the output common mode voltage =0.5v .This opamp is designed for a pipelined ADC and is used in the MDAC. The signal CM is also 0.5 volt
The problem is that this opamp doesn't work! I tried to test it in a simple Charge-Redistribution SHA circuit with sampling rate of 4 MHz, but both of the outputs are always saturated in the hold phase and consequently the differential output is zero.
I know that in theory, the opamp of a CR SHA doesn't need to have equal input and output CM voltage. So what is the problem?