Operating Basics
52
P7380SMA 8 GHz Differential Probe Instruction Manual
Shorting Termination on (--) Input.
An alternative single-ended measurement topology with a shorting
termination on the (--) input is shown in Figure 22. 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.
50
Ω
50
Ω
V out
IN +
IN --
±
V
DM
50
Ω
V+
V--
V
T
V
CM
V
IP
V
IN
+
--
V
IN
=
0
V
IP
=
V
DM
2
+
V
CM
+
V
T
2
V
O
=
(V
IP
−
V
IN
) x A
V
I
L
=
V
CM
−
V
IP
50
DC loading on V
CM
source:
A
V
=
0.40 for 2.5X
=
0.08 for 12.5X
Atten
& V
CM
comp
+
--
V
O
=
Ꮑ
V
DM
2
+
V
CM
+
V
T
2
Ꮖ
x A
V
Figure 22: 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