[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: Martin Rowe <martin.rowe@xxxxxxx>
  • To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
  • Date: Wed, 16 Jul 2014 21:06:28 +0000

SI List members,

I'd like to write a piece for EDN (T&M and  DesignCon sections) based on this 
thread. Would you mind if I quoted you? Can post a preview link to the list 
before sending it live. If anyone would like to add relevant links or add 
anything else, please send them along. Even better, if you would like to write 
the piece, you're welcome to do so.

Thanks,

Martin Rowe
Senior Technical Editor, Test & Measurement
UBM Tech
1-212-600-3403
martin.rowe@xxxxxxx
EDN Test & Measurement Design Center
EE Times Test & Measurement DesignLine
twitter @measurementblue
   

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of steve weir
Sent: Thursday, July 10, 2014 1:45 AM
To: si-list@xxxxxxxxxxxxx
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.

Blame it on CAT II safety.  In order to avoid injuring or killing the probe 
user lots of clearance is required between the probe tip and the scope return 
ground.  The space increases the size of the induction loop compared to low 
voltage active probes, or a transmission line probe AKA resistor probe.

Steve.
On 7/9/2014 8:52 PM, Ivor Bowden wrote:
> Since we are discussing scope probes, I have related question.
>
> I note that when I use passive probes e.g. 10x on 1M input I see 
> crosstalk between them. For example, if probe "A" is on fast digital 
> signal, and probe "B" is looking at mV level analog signal, there is 
> noise on "B" channel correlated with digital signal switching. 
> Grounding both probes at same location using short e.g. <1 cm ground 
> leads not have much affect.
>
> However when I use resistor probe e.g. 950 ohm + 50 ohm coax to 50 ohm 
> input, I don't see this crosstalk, the analog signal (still passive 
> probe 10x 1M input) is clean. Same case for other (active) probes that 
> use 50 ohm termination.
>
> Why is there crosstalk with passive hi-Z probe, but not with resistive 
> probe?
>
> Thanks,
> Ivor
>
> On 7/9/2014 7:38 PM, steve weir wrote:
>> Getting the signal down the TX line isn't all that hard, it's keeping 
>> it from coming back that's been the struggle!  More directly to Tom's 
>> point is that the usable bandwidth of  a 1X probe is down in the 
>> 10MHz region.  10X probes use attenuation to buy equalization and 
>> advertised bandwidths up to 1GHz exist (for low impedance sources).  
>> But that still leaves the tricky problem of a relatively low Z signal 
>> zooming down the scope probe cable to a 1M amplifier input impedance 
>> having nowhere to keep going except reflecting back up the cable to the 
>> probe.
>>
>> A huge innovation was the introduction of a very fine high 
>> resistivity core wire for probe cables.  That serves a couple of 
>> purposes, one of which is to defeat skin effect issues, and the other 
>> is that it provides distributed loss to the signal reflection.  Those four 
>> probes for $25.
>> affairs offered on eBay tend to use simple copper cores and the 
>> reflection issue is very apparent.  Probes from any of the major 
>> scope vendors, or quality third parties like ProbeMaster all use 40 
>> gauge-ish ni-chrome core leads.
>>
>> If one has a low Z source like a PDN, then a transmission line probe 
>> to a 50 Ohms scope input amplifier is ideal.  Tek,  Agilent, and 
>> LeCroy have all offered and/or still offer attenuated transmission 
>> line probes that are still hard to beat with any amplified single ended 
>> solution.
>> If one does not have a low Z source then a quality amplifier as close 
>> to the signal source as possible is the only way to limit the amount 
>> of energy that diverts from the circuit into what is at sufficiently 
>> high frequency a transmission structure.
>>
>> Steve
>>
>>
>> On 7/9/2014 10:45 AM, Tom Dagostino wrote:
>>> 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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