Why two types of input impedance in laboratory instruments?

[GeoAVR]

Hi, in some laboratory instruments (i.e. Network analyzers) the provide two kind of input impedances : 50Ohm and 1MOhm // 20pF.

I understand the 50 Ohm , is the standard for maximum power transfer but why the 1MOhm parallel with 20pf?
Where is this needed and how it is exactly implemented inside the circuit?

Thank you.

 

[chuckey]

The high impedance is so the instrument can be bridged across high impedance tuned circuits.
Frank

 

[crutschow]

50Ω impedance is not primarily for maximum power transfer, it's to match the transmission line impedance of the connecting cable to avoid signal reflections,

The high impedance input is for where you don't want to significantly load the signal source and don't need to match a transmission line impedance.
The high impedance in an oscilloscope is typically used with a 10:1 probe to further increase the impedance and minimize loading the source.

 

[GeoAVR]

Thanks for the replies!

What about the parallel capacitance then? Is it needed or it is a parasitic capacitance?

 

[crutschow]

It's parasitic (stray) capacitance and not desireable, but is unavoidable.

 

[FvM]

It depends. In case of a 1 MOhm oscilloscope input, a specific range of parallel capacitance is required to make the probe compensation work.