Chapter 3
Signal Connections
©
National Instruments Corporation
3-19
Crosstalk
Crosstalk mainly occ
u
rs when the capacitance between lines in a cable
ind
u
ces a smaller transition on another line. Fig
u
re 3-10 shows an example
of crosstalk.
Figure 3-10.
Crosstalk Example
In Fig
u
re 3-10, PFI 10 and PFI 11 are config
u
red as inp
u
ts. V
0
drives PFI
10 and V
1
drives PFI 11. When PFI 10 (the offending line) transitions from
one state to another, it ind
u
ces a small transition in PFI 11 (the victim line).
The magnit
u
de of the transition (or crosstalk) ind
u
ced in PFI 11 is
proportional to the following:
•
The speed of the transition on the offending line (PFI 10 in the previo
u
s
example)
•
The length of the cable and the proximity of the victim to the offending
line
•
The so
u
rce impedance of the victim line (V
1
in the previo
u
s example)
and the level of the offending line (V
0
)
Crosstalk is most likely to ca
u
se meas
u
rement errors when the victim line
is at a low voltage. If this crosstalk is 0.5 V or greater, yo
u
may get errors
in meas
u
rement.
Yo
u
sho
u
ld not experience crosstalk if the so
u
rce impedance of the voltage
so
u
rce driving the victim line is less than 100
Ω
. If this so
u
rce impedance
is larger than 100
Ω
and yo
u
see crosstalk problems, yo
u
sho
u
ld
u
se NI-TIO
filters or a voltage follower with a low o
u
tp
u
t impedance to drive the
victim line.
V
1
V
0
Capacitance
Cable
PFI 11
PFI 10
PFI 11
PFI 10
Z
s1
Z
s0
