[SI-LIST] Re: Differential TDR

ZG,

As Yuriy also pointed out, you still will measure the differential 
response, even if there is no ground connection.  If we have a single 
'ground' connection for a balanced circuit, the 'ground' will have no 
fluctuation due to a differential excitation, so whether you connect it 
or not, it will not change the result.  Unbalanced circuits, however 
(for instance, if the two single-ended impedances are different, or if 
there is noticeable skew between the legs), will result in a residual 
signal on its 'ground' and therefore its stray coupling to cable and 
instrument ground will alter the result.  Bottom line: for well-balanced 
circuits and if you need only the differential response, the ground 
connection is not needed.  For all other cases you need the ground 
connection.

Regards,

Istvan Novak
SUN Microsystems


ZHENGGANG CHENG wrote:
> Dr. Novak,
>  
> Thanks for your reply.
>  
> If there is Gnd connection from differential probe to DUT, the 
> measured Zdiff will be still the same as that measured w/o gnd connection?
>  
> My concern is: w/o gnd connection to DUT, the signal return path will 
> be each other, (i.e., p reference to n, and n reference to p); when 
> gnd is added, signal might partially refer to gnd, the return path 
> might be different from w/o gnd situation. Then will the Zdiff still 
> the same?
>  
> ZG
> 2008/12/13 Istvan Novak <istvan.novak@xxxxxxx 
> <mailto:istvan.novak@xxxxxxx>>
>
>     ZG, Orin,
>
>     If the probe is really part of a TDR setup, I dont think
>     high-impedance probe is an option.
>
>     If the DUT is passive and well balanced, the differential probe
>     without ground connection
>     measured the differential response.  It can not, however, measure
>     the common-mode response
>     without a ground connection to the DUT.  So to answer the original
>     question, without a ground
>     connection to the DUT, only the differential impedance can be
>     measured.  And since without
>     ground connection the single-ended impedance can not be measured,
>     both Z_single and
>     Z_mutual will remain unknown.
>
>     Regards,
>
>     Istvan Novak
>     SUN Microsystems
>
>
>     olaney@xxxxxxxx <mailto:olaney@xxxxxxxx> wrote:
>
>         If you mean a hi-z diff probe, then the line needs to be
>         terminated with
>         matched resistors to ref ground.  If dual 50 ohm probes into
>         50 ohm
>         inputs, the instrument itself provides termination.  The
>         instrument
>         doesn't need to "know" what the single ended impedance is to
>         measure
>         Zdiff.  That's not to say that Zse or Zcm aren't also
>         available from the
>         typical setup.  The most important consideration for Zdiff
>         accuracy is
>         that measurement path delays be identical for both probes
>         (automatic for
>         a unitary diff probe), and that the probe grounds be tied to
>         each other
>         (not necessarily to other grounds) at the measurement end
>         (also automatic
>         for a unitary diff probe).  Also note that Zdiff can be well
>         defined even
>         if Zcm is not, hence the successful application of unshielded
>         twisted
>         pairs dangling in air and over heating ducts in the form of
>         CAT5 LAN
>         cabling.  The large swings in Zcm barely affect Zdiff.
>
>         Orin Laney
>
>         On Sat, 13 Dec 2008 12:22:54 -0800 "ZHENGGANG CHENG"
>         <zhenggang.cheng@xxxxxxxxx <mailto:zhenggang.cheng@xxxxxxxxx>>
>         writes:
>          
>
>             Hi all,
>             When using differential probe to do the TDR for
>             differential pair, if the
>             probe only has P and N tip no Gnd tip, does the TDR gives
>             accurate values?
>
>             Here is the concern:
>
>             Zdiff =2*(Z_single-Z_mutual), if no Gnd tip is used, how
>             does the TDR know
>             what's the single-ended impedance, and consequently how
>             does TDR know the
>             Zdiff?
>
>
>             Thanks,
>
>             ZG
>
>
>                
>
>

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