Why is PS2204A resistance so low?

[Ben321]

I did some experiments and calculated that the scope's input resistance is about 1MOhm. That may seem high, but when I'm doing measurements on the output of a circuit, where the voltage I'm measuring is using a 470kOhm pull-up resistor, the scope's own resistance is quite low, and impacts the operation of the circuit. Most high quality voltage measuring devices (multimeters or oscilloscopes) have an input resistance of about 10MOhms, not 1MOhm. 1MOhm is typical of a cheap multimeter. Is there any way to improve the situation with the PS2204A, without buying a different model? In fact, do any of the Picoscopes have an input impedance of at least 10MOhms?

The voltage I'm measuring is the output of an open-collector based light sensor (phototransistor), who's collector is biased to 9 volts (when no light is falling on the sensor) via a 470kOhm resistor. The power source is a 9V battery, so when the phototransistor is off, the output voltage should be 9V, but it's only about 6 volts.

I've attached a circuit diagram to see the initial circuit I was working with (later, I used another circuit with the phototransistor removed to more carefully measure the resistance of the scope). This diagram should give you an idea of why it is important to have a very high input resistance for the scope measuring the voltage output of this circuit.

[Martyn]

You need to use one of these Passive Probes on the x10 range. That is the common way of handling this situation and are offered as an optional extra on the entry level 2204a, but supplied as standard with other scopes on our range.