Oscillicope Probing

Hello All,

I am aware that probing around circuit can lead to unwanted affects on the circuit due to the probe. Would someone be able to explain how an oscilloscope probe affects the signal you are trying to monitor? If you could point me in the right direction that would also be greatly appreciated.

Amir

The equivalent circuit of a 10X scope probe is like 10MegΩ shunted with 10 to 15pF. For DC measurements, the 10MegΩ is usually not a problem. Hanging an extra 10pF across an RF tuned circuit that might have 20pF of circuit capacitance to begin with can completely detune it...

You can add a test point to such a circuit by balancing out the scope probes parasitics, but this kinda rules out 'probing around' as the circuit required to do this would need to be a permanent part of the system.

akahrim said:

I am aware that probing around circuit can lead to unwanted affects

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When the probe slips off and shorts two traces there can be unwanted effects.

Or when you clip on the ground and realise theres a difference in the 'scopes ground (or any other device connected to it) and the equipment under test's ground.

dr pepper said:

Or when you clip on the ground and realise theres a difference in the 'scopes ground (or any other device connected to it) and the equipment under test's ground.

Click to expand...

My probes have burn marks on the ground clips. I work on things that live on the power line and ground does not equal ground.

In simple terms, try measuring a DC voltage with an output impedance of 10Mohms. When you try and measure the voltage using a 10M input impedance probe, you will load it. The resulting measurement in this case will be half of what is expected. Use a 1M probe and the error will be worse.

This is basic fundamentals about knowing your equipment and its limitations. We were taught this in first year OND electronics back in 1984 in one of our first classes. However you can use Thevenins equivalent circuits to measure the actual open circuit voltage with a bit of sums in this DC example.



Once the meter is attached, the voltage will be half what you expected.

In RF circuits, the problem is generally worse as the capacitance and inductance of the probe affect the circuit. High impedance RF probes are also very expensive.

Thank you all for the responses. It sounds like things can get a little confusing (at least for me) when measuring a DC signal that may have some noise. I am going to have to crack open some books and see if I can learn more on this topic.

Here is a useful paper I found that goes into depth on many of the thing you all kindly mention. Thanks again.