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PicoScope 2204A - PP906 affecting the circuit

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PicoScope 2204A - PP906 affecting the circuit

Postby dherdegen » Fri Jun 19, 2015 4:09 pm

I just started using my PicoScope 2204A and I am noticing that the scope is affecting the circuit. To test the impact, I connected the picoscope to a simple RC circuit with a 1 second time constant (10 MOhm and 0.1 uF). When connected across the capacitor, the voltage level was severely attenuated (my probe was switched to 1x), and the time constant was altered (severely shortened). Therefore, I inserted a non-inverting unity gain op-amp into the circuit and connected the picoscope to the op-amp output. Read from the op-amp output, the time constant and voltage levels were correct and not modified. I am surprised that the picoscope would affect the circuit so severely. I expect that I am doing something wrong. (I am using the beta unix software). Do I need to provide circuit isolation routinely when using the picoscope? I am not having the same symptoms with my bench o-scope.
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Re: PicoScope 2204A - PP906 affecting the circuit

Postby Hitesh » Mon Jun 22, 2015 4:06 pm

Hi dherdegen,

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.

Circuit diagram

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.

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