Differential Measurements - Tektronix TriMode P7700 Series Technical Reference

Reference

Differential measurements

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A single-ended measurement can also be made using a TriMode tip in its
differential input mode. In this case, the single-ended signal to be measured would
be wired to the probe tip A input signal via and the DUT ground reference would
be wired to the probe tip B input signal via. No probe tip ground via connection
needs to be made with this measurement configuration. Selecting differential
input mode enables an A input signal measurement relative to the DUT ground on
the B input. In differential input mode both the A offset and B offset settings will
affect the measurement. Using the example of a 2.5 V CMOS logic signal, the A
offset should again be set to the center of the logic swing at +1.25 V, but the B
offset should be set to 0 V to match the DUT ground reference connection.
The dynamic range of the TekFlex solder-in tips limits their use to CMOS logic
with a signal swing of no more than 2.5 V. The P77BRWSR Browser probe tip has
a larger dynamic range of 6 V
CMOS logic and 5 V CMOS logic signals. Since the P77BRWSR tip operates
only in differential input mode, the browser tip B input connection should be
made to the single-ended DUT ground reference. For a 5 V CMOS logic signal
measurement example, the A offset voltage should be set to +2.5 V, which is the
center of a 5 V CMOS logic swing, and the B offset voltage should again be set to
0 V to match the DUT ground reference.
Differential probes are also ideal for a class of single-ended measurements where
the reference voltage is not ground:
DDR (SSTL_2, STTL_18, SSTL_15): V
PECL: V = V -1.3
REF CC
To measure single-ended signals in this class, connect the B input of the probe
to V .
REF
A differential probe in these applications displays the true signal despite any AC
or DC variation in V from its nominal value. A single-ended probe displays
REF
the signal plus the variation in V
ground referenced single-ended measurements on either single-ended signals or
differential signals like PCI Express or Serial ATA.
A differential probe is optimized to measure high speed differential signals.
Differential signals are formed from two complementary signals with a common
reference voltage. (See Figure 22.)
Devices designed for differential measurements avoid problems presented by
single-ended systems. Differential signal transmission improves signal fidelity
by doubling the effective signal amplitude compared to single-ended signal
transmission. Differential signaling also improves signal fidelity by removing
common mode noise due to the CMRR of the receiver or measurement device.
, which should allow measurements of both 3.3 V
p-p
, V = V
TT REF
. Differential probes can also be used to make
REF
P7700 Series TriMode Probes Technical Reference
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