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The adjustability of the termination voltage also provides measure-
ment flexibility for characterizing or stressing serial data signal
drivers. Because of the low impedance of the input termination and
attenuator network, the signal termination currents can become quite
large. Table 5 below can be used to calculate the DC common- - mode
voltages and currents at the probe inputs and termination voltage
driver under several common source impedance conditions.
Table 5: Common- mode voltage and current table
0 Ω
V
V
I
40.00 mA x V -- 40.00 mA x V
I
40.00 mA x V -- 23.33 mA x V
1
When inputs are AC coupled: V
The probe block diagram shows that the input termination network is
followed by an attenuator and V
attenuator is used to increase the effective input dynamic range of
the probe differential amplifier.
The P7380SMA probe has two attenuation settings, 2.5X and 12.5X,
that allow dynamic range to be traded off against signal noise. The
12.5X attenuator setting has the largest dynamic range; the 2.5X
attenuator setting has the lowest noise.
The V
CM
common- - mode voltage at the probe differential amplifier inputs
even with varying termination voltage and input signal DC
common- - mode voltage. This maximizes the differential mode signal
input dynamic range. The V
common- - mode input voltage range to be the same for both
attenuator settings as shown in Figure 17 on page 43.
P7380SMA 8 GHz Differential Probe Instruction Manual
Source impedance
compensation circuit automatically minimizes the DC
CM
1
50 Ω
0.5 x (V + V )
20.00 mA x V -- 20.00 mA x V
28.33 mA x V -- 11.67 mA x V
= V
, I
= 0, I
= 16.67 mA x V
I
T
I
T
compensation circuit. The
CM
compensation circuit allows the DC
Operating Basics
T
39
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