[SI-LIST] Re: In scope measurement, why is the amplifier of an active probe in the tip of the probe, not at the oscilloscope in most cases.

From: Joseph.Schachner@xxxxxxxxxxxxxxxxxx
To: issue.lu@xxxxxxxxxxx
Date: Wed, 9 Jul 2014 10:58:05 -0400

The cable has capacitance.  If the amplifier was at the scope, the circuit 
would have to drive the (50 ohm characteristic impedance) cable. It takes 
significant charge to charge up a long cable.  In general, the less the 
probe disturbs the circuit under test, the better. 
What do you want to see?  a) The voltage at the scope input    b) the 
voltage at the probe tip     c) the voltage at the point where the probe 
tip is but without any affect due to the probe tip being connected. 
I'm willing to bet the answer is c.  Well, that can be done (short app 
note: http://cdn.teledynelecroy.com/files/appnotes/lab_wm772.pdf ) but 
that's done with mathematics after the acquisition.  What we really expect 
from the probe is b, the voltage at the probe tip, with as little affect 
of the probe on that voltage as possible.

There are two conventional ways to reduce the load on the circuit:

1) Place a resistor very near the probe tip.  If the resistor is 450 ohms 
and the scope channel is 50 ohms then the scope sees 1/10 of the voltage 
at the circuit.  Any capacitance of the cable and scope input is separated 
from the circuit by the 450 ohm resistor.  (Of course, could use 950 ohms, 
so the scope would see 1/20 of the voltage, for example, if the signal is 
large enough).  Detail: In old passive probes there is a variable 
capacitor across the resistor, for "compensation", that you adjust to get 
a good flat low frequency square wave.   That allows a lower impedance 
path for a step. If compensation is too low the transitions will seem to 
be too small and the signal will slowly rise the rest of the way to a 
final level.   If compensation is too high the response will appear to 
overshoot and drop back slowly to a final level.  Of course, that lower 
impedance path provided by the compensation variable cap increases the 
load on the circuit, but at DC that affect is not present.
 
2) Place an amplifier in the probe body as close to the probe tip as 
possible. The amplifier should have a very low capacitance input.  (That 
may mean it doesn't have big over voltage protection diodes, so treat your 
active probes with care). The circuit drives only the input to the 
amplifier, the amplifier drives the cable  to the scope.

If there is an amplifier in the probe tip there must be power supplied 
from the scope to the probe.  There probably is also a serial interface 
and at least a ROM in the probe that the scope can read over that serial 
interface, that would contain a calibration curve for that probe that the 
scope can compensate for. And maybe there will be a small processor that 
can react to commands from the scope to control gain and offset in the 
probe, and whatever else the manufacturer made it do. Probes with these 
capabilities can appear to give extremely accurate representations of the 
signal at the probe tip because they provide information to the scope that 
allows small non-flatness imperfections to be corrected.

I don't actually work on probes, I'm a software engineer, but I am 
familiar with them.

--- Joe S. 
Senior Software Egineer
Teledyne LeCroy


From:   steve ham <issue.lu@xxxxxxxxxxx>
To:     "si-list" <si-list@xxxxxxxxxxxxx>, 
Date:   07/08/2014 11:30 PM
Subject:        [SI-LIST] In scope measurement, why is the amplifier of an 
active probe in the tip of the probe, not at the oscilloscope in most 
cases.
Sent by:        si-list-bounce@xxxxxxxxxxxxx



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Follow-Ups:
- [SI-LIST] Re: In scope measurement, why is the amplifier of an active probe in the tip of the probe, not at the oscilloscope in most cases.
  - From: Aaditya K

References:
- [SI-LIST] In scope measurement, why is the amplifier of an active probe in the tip of the probe, not at the oscilloscope in most cases.
  - From: steve ham

[SI-LIST] Re: In scope measurement, why is the amplifier of an active probe in the tip of the probe, not at the oscilloscope in most cases.

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