FreeLists / si-list / [SI-LIST] Re: Low Load Probe Suggestions

[Doug Smith <doug@xxxxxxxxxx>]

Hi Mark,

As soon as you say the probe has a mainly capacitive input impedance, 
such as an active probe with < 1pf capacitance, there are some real 
problems. Any probe that has much over a few thousands of Ohms of DC 
resistance is suspect.

Until a few years back active probes did not work very well: >6 db 
gain peak and input impedance of about 20 Ohms in the upper octave of 
bandwidth, even with the smallest probe tips that came with the 
probes. The first active probe I am aware of that worked really well 
was the Agilent 1158a although there may be others in that time frame 
of about several years ago that also worked. The 1158a has a little 
red tip resistor of about 115 Ohms.

The secret to making a low capacitance probe work is that you need to 
put a damping resistor at the probe tip on the order of 100 to 200 
Ohms depending on the particular hardware used on the probe. Without 
this tip resistor it is not possible (at least in my experience) to 
make an active probe work with useable probe tips. Even a few 
nanohenries of inductance is too much.

Nowadays there are a lot of good active probes on the market that 
incorporate this feature, but many older models are still sold that 
have an input impedance on the order of 20 Ohms in the upper octave of 
their bandwidth with the smallest probe tips furnished with the probe.

I have run simulations that show a typical active probe without 
damping resistors not only affects what you see on the scope but 
causes potentially severe SI issues that you cannot see on the scope 
depending on the inductance of the signal source (IC package). This is 
the kind of stuff I cover in my seminars.

To my knowledge, I originated the idea of using damping resistors on 
probe tips a long time ago at Bell Labs and have used them myself for 
over 20 years now. BTW: damping resistors are not needed on resistive 
divider probes.

Doug

Mark Randol wrote:

> I realize this is like asking gear heads what motor oil they prefer, but
> for low loading what probes do you prefer?  I know what I've used, but
> just wondering what else might be available.
> 
> I'd prefer <1pF capacitance and at least 100kohm resistance.  Bandwidth
> up to at least 500MHz, preferably into GHz.  Low frequency should push
> towards 10kHz.  Tip voltages typically <5V, almost always <15V.  Some
> wiggle room is allowed in the 'specs'.
> 
> Regards,
> --
> Mark Randol, RF Evaluation & Application Engineer
> ON Semiconductor
> 901 S. Mopac Expressway
> Barton Oaks 4, Suite 343
> Austin, TX 78746
> 512-329-5640 (voice), 512-329-8151 (FAX)
> =20
> =20
> 
> 
>>-----Original Message-----
>>From: si-list-bounce@xxxxxxxxxxxxx=20
>>[mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Doug Smith
>>Sent: Wednesday, September 12, 2007 1:30 AM
>>To: emc-pstc; SI-List
>>Subject: [SI-LIST] Fluorescent light noise
>>=20
>>Hi Everyone,
>>=20
>>For years the electronic fluorescent light over the table I=20
>>do many of my experiments has been putting "fuzz" in some=20
>>scope measurements. So, I decided to measure what was going=20
>>on. This month's Technical Tidbit on my site is:
>>=20
>>Abstract: Electronic fluorescent lights are capable of=20
>>generating significant EMI. The nature of the EMI resulting=20
>>from internal switching used by such a light is presented.=20
>>The amplitudes measured are significant and could affect high=20
>>impedance analog circuits.
>>=20
>>The results are surprising in the amplitude observed at 48=20
>>kHz. Go to http://emcesd.com .  The picture of the scope=20
>>probe and fluorescent light is the link to the article.
>>=20
>>Doug
>>--
>>-------------------------------------------------------
>>     ___          _       Doug Smith
>>      \          / )      P.O. Box 1457
>>       =3D=3D=3D=3D=3D=3D=3D=3D=3D          Los Gatos, CA 95031-1457
>>    _ / \     / \ _       TEL/FAX: 408-356-4186/358-3799
>>  /  /\  \ ] /  /\  \     Mobile:  408-858-4528
>>|  q-----( )  |  o  |    Email:   doug@xxxxxxxxxx
>>  \ _ /    ]    \ _ /     Website: http://www.dsmith.org
>>-------------------------------------------------------
>>=20
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>>=20
>>=20
> 
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-- 
-------------------------------------------------------
     ___          _       Doug Smith
      \          / )      P.O. Box 1457
       =========          Los Gatos, CA 95031-1457
    _ / \     / \ _       TEL/FAX: 408-356-4186/358-3799
  /  /\  \ ] /  /\  \     Mobile:  408-858-4528
|  q-----( )  |  o  |    Email:   doug@xxxxxxxxxx
  \ _ /    ]    \ _ /     Website: http://www.dsmith.org
-------------------------------------------------------
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or at our remote archives:
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Old (prior to June 6, 2001) list archives are viewable at:
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