Tektronix P7313SMA, Technical Reference

Page: 26 / 62

Operating Basics
Shorting Termination on (-) Input.
An alternative single-ended measurement topology with a shorting termination on
the (-) input is shown. (See Figure 9.) The general equations describing the response and loading of this topology are also
shown. The equations for this topology show identical loading of the signal source when compared to the 50
Ω termination
topology. This is because the termination voltage, V
T
, effectively isolates input signal loading from the termination on
the probe negative input.
Figure 9: Shorting termination on (-) input
The measured single-ended signal response for this topology differs from the 50 Ω termination topology. The measured
AC voltage, V
DM
, is the same for both single-ended topologies, but the measured DC voltage is affected by both the
common-mode input voltage, V
CM
, and the termination voltage, V
T
.
In the special case where the termination voltage is set equal to the common-mode input voltage, the input signal DC
loading is minimized and the measured DC output voltage equals the full common-mode input voltage, scaled by the probe
attenuation. The intelligent TekConnect probe interface automatically accounts for the probe attenuation setting and a
TekConnect oscilloscope will display the full single-ended input signal when V
T
equals V
CM.
Although this topology displays the correct DC common-mode voltage, it also has a greater risk of exceeding the probe
dynamic range and overdriving the probe ampli
fi er.
14 P7313SMA Technical Reference