Electronic – capacitor discharging through oscilloscope’s probe

capacitoroscilloscope

I have a device and the output ac voltage is 1-2Vp-p with very low current.
I made a voltage quadrubler with a storage capacitor(output) in order to rectify the voltage and observe how much time is needed to charge the capacitor.

So, while the device's current is very low, if I connect my oscilloscope to the storage capacitor and let the device operates for two hours I will never see the charge waveform because the capacitor discharges through the probe.

Could you suggest me something to stop the flow of the current through the probes.
I thought to connect a resistor parallel to the capacitor and then connect the probes but it didn't work.

I upload a rough layout
circuit

Thanks

Best Answer

One option is to try and increase the input impedance to the measurement stage. An oscilloscope with a 10x probe has an input impedance of around 10 MΩ, but you could boost that another factor of 100 or more by using an instrumentation amplifier. For example, the AD8295 has an input impedance of 100 GΩ. This may not be enough depending on how slow the voltage charges, but will give you a time constant 10000x longer than that of an oscilloscope probe.

For illustrative purposes consider the following:

schematic

simulate this circuit – Schematic created using CircuitLab

You connect the two inputs across the output of your device, and end up with a single ended output you can connect to your oscilloscope.

If you don't need it to be differential then you can simply use a high-impedance op-amp configured as a non-inverting amplifier, which will give you the same benefits but at less cost.

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