I have a 89c51rd2 reading an analog channel via AD574 and then sending the value to the PC through serial port.

The MCU + ADC board is powered by a separate transformer with three windings to get +5, +12 and -12 volts and thus the DC ground of the board fully floats with ref. to the power ground of the total panel in which the PC is placed.

Things are fine as long as the serial cable is NOT plugged in. The moment it is done, the ADC values are all over the place. The reason for this is the fact that the MCU +ADC ground gets linked with the PC ground via the serial cable and that once again is connected to the power ground via the SMPS of the PC.

Any solutions or tips to tide over this problem ?

Thanks

Raghu