Could you please provide a circuit diagram showing how the PicoScope is connected to the circuit?
Circuit diagram was sent in to our support e-mail:
When connected to the top circuit across the capacitor, the output seems to be affected by the picoscope; the time constant is reduced to 0.1 seconds versus the expected 1.0 seconds and the amplitude is severely attenuated. When the picoscope is connected to the output of the op-amp, there is no noticeable effect on time constant or amplitude; the picoscope correctly graphs the output: time constant is 1.0 seconds, and the capacitor charges to 1.0 volts. The input signal period is 20 seconds, so the capacitor has plenty of time to charge and discharge. I watched multiple charging and discharging cycles.
The reason you are seeing the effects that have been observed is that the oscilloscope probe has an Input Resistance and Input Capacitance as shown in the manual for the MI007 oscilloscope probe
At the 1x setting, the input resistance is 1 MOhm and the input capacitance is 70pF - 120pF. The resistor and capacitor combination provide a simple low-pass filter - given that capacitances in parallel are summed together, the cut off frequency for the signal will reduce. Also, the 1 MOhm resistance of the probe in conjunction with the 10 MOhm resistance in your circuit will result in an attenuation of the signal.
The op amp is providing a buffer to this effect and reduces the impact of reduction on the signal strength.