Picoscope AWG input affected by load circuitry

Thread Starter

maker_spence

Joined Apr 17, 2021
1
When testing my new PC oscilloscope, a PicoScope 2205A (25MHz, 1GS/s), I setup an RC circuit with a 2.2K resistor and a 0.1uF cap and used the AWG generator to input a 1kHz, 1V square wave input. The input square wave (plotted in red) is not the expected square wave, seemingly affected by the capacitor (as the input went flat after moving it and measuring a voltage divider).

Any idea on why my input waveform is distorted by the load circuitry?

Further info
* I measured the rise time of the square wave without any load circuity at 1.2us. The time constant of this circuit is 220us.
* I also used the 10x attenuator probes, and tuned the variable capacitor in the probes beforehand.

Screenshot:
Input voltage in red, capacitor voltage in blue
  • 3
 

jjw

Joined Dec 24, 2013
676
When testing my new PC oscilloscope, a PicoScope 2205A (25MHz, 1GS/s), I setup an RC circuit with a 2.2K resistor and a 0.1uF cap and used the AWG generator to input a 1kHz, 1V square wave input. The input square wave (plotted in red) is not the expected square wave, seemingly affected by the capacitor (as the input went flat after moving it and measuring a voltage divider).

Any idea on why my input waveform is distorted by the load circuitry?

Further info
* I measured the rise time of the square wave without any load circuity at 1.2us. The time constant of this circuit is 220us.
* I also used the 10x attenuator probes, and tuned the variable capacitor in the probes beforehand.

Screenshot:
Input voltage in red, capacitor voltage in blue
  • 3
According to the datasheet the signal generator has a 600 ohm output impedance and with the load it forms a voltage divider.
 

Irving

Joined Jan 30, 2016
2,058
As stated above, the voltage seen by the scope is impacted by the capacitor charge current through the 600ohm output impedance of the AWG. To get what you expected you need to buffer the AWG with an opamp to get a low output impedance of 50ohms or less.

Or change your RC to use a much smaller C and a bigger R >> 600ohm, eg 220k and 1nF (at 1kHz Zc will be 159kohm so use x10 probe @ 10Meg impedance)
 
Last edited:
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