Chapter 3
Connecting the Signals
©
National Instruments Corporation
3-5
SCXI-1141/1142/1143 User Manual
Figure 3-2 illustrates how to connect a non-referenced (floating) signal
source to an SCXI channel.
Figure 3-2.
Floating Signal Connection
For AC-coupled signals, connect an external resistor from the AC-coupled
input channel to ground. This provides a DC path for the amplifier input
bias current. Typical resistor values range from 100 k
Ω
to 10 M
Ω
. This
solution, although necessary, lowers the input impedance of the channel
and introduces an additional DC offset voltage proportional to the product
of the input bias current and the resistor value used. The inputs of the
SCXI-1141/1142/1143 module have a typical bias current of about
±
200 pA. This bias current is highly dependent on the operating
temperature of the module. Using a 1 M
Ω
resistor results in
±
200 µV of
offset, which is insignificant in most applications. However, if you use
larger-valued bias resistors, significant input offset can result.
Lower-valued bias resistors increase loading of the source, which can result
in gain error.
IN–
IN+
AGND
100 k
Ω
A
SCXI-1141/1142/1143