Electronic – the impedance limit for using 10X probes with oscilloscopes


I thought 10X probes are only used for impedance matching at high freq. circuits.

But I read in a text that they are also used in high impedance circuits to prevent loading. But isn't a scope's input impedance already huge? I mean what is high impedance in this case and how can 10X help the situation?

Best Answer

A typical scope input impedance is 1Mohm + 20pF. This is not huge, even with a 100k source I get 10% error DC and 20pF is only 8k impedance at just 1MHz. A 10x porbe makes the input 10meg + 2pF, much better. It also allows for tuning of rise times by trimming a small capacitor in series with the R in the probe.