National Instruments SCXI-1120 - Manual (Page 31)

Chapter 2 Configuration and Installation
© National Instruments Corporation 2-15 SCXI-1120 User Manual
I
Vs
V out
+
-
+
-
R bias
Vcm
High
CMV +
-
+
Module
Figure 2-7. AC-Coupled Signal Connection with High Common-Mode Voltage
For AC-coupled signals, an external resistor from the positive input channel to the signal
reference should be connected. This is needed to provide the DC path for the positive input bias
current. Typical resistor values range from 100 k
Ω
to 1 M
Ω
. This solution, although necessary
in this case, lowers the input impedance of the input channel amplifier and introduces an
additional offset voltage proportional to the input bias current and to the resistor value used. The
typical input bias current of the amplifier consists of
±
80 pA and a negligible offset drift current.
When a 100 k
Ω
resistor is used, this will result into
±
8
µ
V of offset, which is insignificant in
most applications. However, if larger valued bias resistors are used, significant input offset may
result. To determine the maximum offset introduced by the biasing resistor, use the following
equation:
V
ofsbias
= I
bias
x R
bias
The input signal range of an SCXI-1120 input channel is
±
5 V/ G
total
referenced to its negative
input, where G
total is equal to the product of the first-stage and second-stage gains. In addition,
the input channels are overvoltage protected to 240 Vrms with power on or off at a maximum of
4.5 mArms sink or source.
Warning: Exceeding the input signal range and the common-mode input range results in
distorted signals. Exceeding the maximum input voltage rating (250 Vrms between
terminals, and between any terminal and ground) can result in damage to the
SCXI-1120, the SCXIbus, and the data acquisition board. National Instruments is
not liable for any damages resulting from such signal connections.
Temperature Sensor Connection
Pins C2 and C4 are dedicated for connecting the temperature sensor to the SCXI-1120. The
temperature sensor is not isolated and is referenced to the chassis ground. The connection is
overvoltage protected to
±
25 VDC with power on and
±
15 VDC with power off.
Warning: Exceeding the overvoltage protection on the temperature connections can result in
damage to the SCXI-1120, the SCXIbus, and the data acquisition board. National
Instruments is not liable for any damages resulting from such signal connections.