Tektronix P7313SMA, Technical Reference

Page: 28 / 62

Operating Basics
Single-Ended Measurement Procedure
The description of characteristics of the three alternative single- ended measurement topologies suggests the following
procedure for making single-ended measurements on serial data signals that require light DC loading, (for example, when V
T
= V
CM
):
First, determine the common-mode input voltage, V
CM
, of the single-ended signal by making a measurement with the 50 Ω
termination topology shown. (See Figure 8 on page 13.) With this topology and the Termination Voltage Select set to Auto
mode, the common-mode input voltage can be measured with a DMM on the Termination Voltage Monitor output pins.
Note that measuring the common-mode input voltage on the single-ended signal using this topology is more accurate than
using a differential measurement topology, where the measured common-mode voltage is the average between the two
single-ended signals that comprise the differential signal. The common-mode voltage for each of the single-ended inputs that
comprise the differential signal should be measured independently and recorded for use in the second step of this procedure.
Next, since the 50 Ω termination topology only displays half the common-mode input voltage, it is now necessary to switch to
the shorting termination topology shown. (See Figure 9 on page 14.) This can be done simply by changing the termination
attached to the (-) input from a 50
Ω SMA termination to an SMA shorting termination.
Since Auto mode only works with matched-source impedances on both probe inputs, it is also necessary to switch the
Termination Voltage Select to either Int or Ext mode. The termination voltage should be set to the voltage measured
in the
fi rst step. This can be done easily in Int mode, but requires a TekConnect oscilloscope that has support for probe
termination voltage select.
Setting the termination voltage in Ext mode requires the use of an external power supply and the accessory cables supplied
with the probe. Once the termination voltage has been set to match the DC common-mode input voltage, the complete input
signal is displayed with the shorting termination topology. This shorting termination topology, however, has the highest risk of
exceeding the probe dynamic range. Dynamic range calculations for single-ended measurements will now be described.
Single-Ended Dynamic Range
The dynamic range of the probe has been speci fi ed for differential measurements, as described in the differential
measurement topology section. When single-ended measurements are made, the input common-mode voltage is no
longer nulled out, but becomes a differential mode DC signal that must be within the input dynamic range of the probe to
be measured accurately.
The speci fi ed dynamic range for differential signals, which is expressed as a differential peak-to-peak voltage, can be
converted to a more conventional voltage range for single-ended signal measurements as shown. (See Table 2.)
Table 2: Differential to single-ended conversion table
Attenuation
Differential measurement
dynamic range Single-ended measurement
dynamic range
2.5X 800 mVp-p ±0.400 V
12.5X 3.6 Vp-p ±1.8 V
Because the common-mode DC voltage of many serial data signals is larger than the signal differential mode voltage, the
relatively small single-ended dynamic range in the 2.5X attenuation setting may not be adequate. As a result, single-ended
measurements will generally be made using the 12.5X attenuation setting.
16 P7313SMA Technical Reference