Re: Oscilliscope Restistance
From: Keyser Soze (noreturn_at_nothere.com)
Date: 10/06/04
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Date: Wed, 06 Oct 2004 09:34:13 GMT
eat411 wrote:
> 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.
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