Chapter 2
Configuration and Installation
© National Instruments Corporation
2-15
SCXI-1100 User Manual
100 k
Ω
V
out
+
–
V
S
PGIA
+
–
+
–
100 k
Ω
W1
(Position 2-3)
Figure 2-5. AC-Coupled Signal Connection
For AC-coupled signals, connect jumper W1 and an external resistor from the positive input
channel to the chassis ground. This connection produces the DC path for the positive input bias
current. Typical resistor values range from 100 k
Ω
to 1 M
Ω
. Although this configuration is
necessary in this case, it lowers the input impedance of the PGIA and introduces an additional
offset voltage proportional to the input offset current and to the resistor value used. The typical
input offset current of the PGIA consists of
±
350 pA and a negligible offset drift current. A
100 k
Ω
resistor results in
±
35
µ
V of offset, which is insignificant in most applications.
However, if you use larger valued bias resistors, significant input offset may result. To determine
the maximum offset introduced by the biasing resistor, use the following equation:
V
ofsbias
=
I
ofsbias
R
bias
(
)
+
R
bias
−
100 k
Ω
(
)
I
bias
Use a 100 k
Ω
value when you use jumper W1 to reduce the amount of offset as well as to
maintain a balanced input and avoid the degradation of the PGIA common-mode rejection ratio
(CMRR). If you need larger valued biasing resistors, do not use jumper W1, and tie an external
biasing resistor to the negative input channels.
For DC-coupled signals, connect jumper W1; no external resistors are required. This
configuration internally connects a 100 k
Ω
value from the negative input of the PGIA to ground,
producing the required biasing. This configuration does not affect the input offset or the
differential input impedance because the voltage developing across the bias resistor appears as a
common-mode voltage.
The SCXI-1100 PGIA can reject any voltage within its common-mode input range that ground
potential differences between the signal source and the module introduce. In addition, the PGIA
can reject common-mode noise pickup in the leads connecting the signal sources to the
SCXI-1100.
The common-mode input range of the SCXI-1100 PGIA is the magnitude of the greatest
common-mode signal that the SCXI-1100 can reject. The PGIA can reject common-mode