I need to measure from 100 to 250 VAC (60 Hz) with about 3-4 volts precision, with 1500 V isolation between the voltage source and the PCB. I have a working circuit, but it has a drift problem. After power is applied, for the first hour of operation the readings decline more or less exponentially before reaching a steady state.

The circuit uses a transformer to step down the voltage and provide isolation. The transformer secondary feeds a full-wave bridge rectifier, the output of which is buffered and low-pass filtered before feeding into an ADC input.

If I drive the rectifier input with a 9 VAC power supply, there's no drift. However the transformer gets warm after power is applied, and eventually gets pretty hot, so I think that's where the problem is.

The transformer is designed for this type application (so the manufacturer says - I'm not a transformer expert). It's a split-bobbin transformer. The turns ratio is 230:9, and the secondary is rated for 9 mA. The effective load on the bridge is 18 Kohm, so the secondary current should be about 0.5 mA. Why is the transformer heating up? Why does the temperature change affect the transformer's output?

Any advice is appreciated.