[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: "Tom Dagostino" <tom@xxxxxxxxxxxxx>
  • To: <aaditya.kandibanda@xxxxxxxxx>, <Joseph.Schachner@xxxxxxxxxxxxxxxxxx>
  • Date: Wed, 9 Jul 2014 10:45:18 -0700

Input capacitance is the key.  All scopes with a 1MOhm input resistance
already have a high impedance input amplifier.  But to get to it the scope
probe's lead is in the way.  Putting an amplifier at the tip of the probe
now allows the lead from the scope to the probe head to be a 50 Ohm
transmission line.  Sending a signal down a low loss 50 Ohm line driven by
an amplifier is much easier than sending the same signal down the passive
probe's cable.

Older 10X passive probes had input capacitance between 8 or 9 pF and 20 pF
or higher.  At high frequencies these pretty much look like a short circuit
to any signal of interest.  The signal being probed has to drive a long
scope probe cable with its associated parasitics, ringing and need for
compensation networks.  

With a passive probe the input RC network has to be sized to compensate for
the input C of the scope plus the distributed C of the scope cable.  With
the amplifier in the probe tip the input network only has to deal with the
input C of the him impedance buffer.  This it is possible to have the
equivalent of only a few tenth of a pF at the probe tip.

Tom Dagostino

Teraspeed Labs
9999 SW Wilshire St.
Suite 102
Portland, OR 97225
USA

971-279-5325  Office
971-279-5326   FAX
503-430-1065  Cell

tom@xxxxxxxxxxxxx 
www.teraspeed.com 

Teraspeed Consulting Group LLC
16 Stormy Brook Road 
Falmouth, ME 04105
401-284-1827

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Aaditya K
Sent: Wednesday, July 09, 2014 10:24 AM
To: Joseph.Schachner@xxxxxxxxxxxxxxxxxx
Cc: issue.lu@xxxxxxxxxxx; si-list
Subject: [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.

What I can think of is flexibility. If we have amplifiers on probes, we can
use any type any scope any time, including two different types of probes on
same scope and its cheap, when amplifier goes bad, we need not send the
oscilloscope for repair, we could just send the probes. Hold true even if it
is necessary to replace oscilloscopes.
Please, Correct me if I am wrong.


On Wed, Jul 9, 2014 at 7:58 AM, <Joseph.Schachner@xxxxxxxxxxxxxxxxxx> wrote:

> 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
>
>
>
>  href:http://localhost:50374/Question/Details?id=3D10216
> 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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