I have made a simple piezo-energy harvester. The piezo strip is connected to a full-wave bridge rectifier, then into a capacitor for storage. The voltage across the capacitor increases as expected, when measured with a multimeter - however drops rapidly due to the allowed current flow.
I then connected an INA126 single instrumentation-amp to allow the reading of the capacitors' voltage without current flow. A voltage-divider from 9V source supplies the opamps' V+ of 5V, and V- is ground, the same ground as the battery, as are the opamps. The voltage-divider has a capacitor in parallel to ground, and the Vout runs into an OP07CP precision opamp as a unity-feedback buffer for voltage regulation.
My problem is the voltage measured from Vout of the instrumentation-amp and ground, is an inaccurate reading of the capacitor to a direct reading across the capacitors terminals.
For example, the capacitor currently has a voltage of 1V, and the output of the IA126 with ref. to ground is a stable 3.6V.. And the voltage-divider output is roughly 5V.
PS There is no Rg resistor, to add gain over 5..