Oscilliscope Restistance

[eat411]

I need to know if the resistance in an oscilliscope is high or not. i
believe they have a low resistance, but i am not sure why they would
need to be designed this way. I would appericiate an explanation.

thanks in advance

 

[mike]

I need to know if the resistance in an oscilliscope is high or not. i
believe they have a low resistance, but i am not sure why they would
need to be designed this way. I would appericiate an explanation.

thanks in advance

You are correct, they're designed that way.
I trust my answer was as clear as your question.
Wanna try again?
mike

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[Keyser Soze]

I need to know if the resistance in an oscilliscope is high or not. i
believe they have a low resistance, but i am not sure why they would
need to be designed this way. I would appericiate an explanation.

thanks in advance

The answer depends on the kind of signal you need to measure and type of
oscilloscope used.

A typical oscilloscope vertical amplifier input will have a 1M ohm input
impedance. The typical 10x passive probe will have a 10M ohm impedance.

For signals of 350MHz or less conventional passive probes are fine.

Oscilloscope designed for signals from 500MHz to 4000MHz will benefit
from using active probes.

Active probes have high bandwidth amplifiers at or near the probe tip.

The amplifiers output is designed to drive a 50 ohm cable to the
oscilloscope vertical amplifier input.

Oscilloscope that are intended to use only active probes will usually
have only a 50 ohm input impedance.

-----------

The basic concept here is that it takes a lot of power to accurately
drive a 500MHz signal down a 1 meter cable. Most circuits that use
signals at this frequency do not have the needed power and are adversely
affected when connected to the load of an oscilloscope vertical
amplifier input.

The ideal oscilloscope probe would have very high impedance, low
capacitance and disturb the signal very little.

As the signal frequency increases it becomes much harder to make a probe
that can do what is needed.

 

[Lewin A.R.W. Edwards]

I need to know if the resistance in an oscilliscope is high or not. i

believe they have a low resistance, but i am not sure why they would
need to be designed this way. I would appericiate an explanation.

They should have a high input impedance. You want to load the signal
under test as little as possible => you want as little current as
possible to flow through the scope.

 

[Mantra]

I need to know if the resistance in an oscilliscope is high or not. i
believe they have a low resistance, but i am not sure why they would
need to be designed this way. I would appericiate an explanation.

thanks in advance

Now that you've gotten mutually exclusive and contradictory answers,
here's the real answer.

It depends on your frequency of interest. If you can assume a lumped
equivalent model applies (generally under ~500 MHz) then you want your
probe top to look like a high impedance (i.e. invisible) to the
circuit under test. If you can *not* assume a lumped equivalent model
applies, then transmission line effects are important and you want to
assure an impedance match to avoid introducing reflections into your
scope circuitry or your circuit under test so you'll want the probe
tip to look like the charactistic impedance (typically 50 ohms) (i.e.
invisible). In other words: "yes".

MM