Chapter 2 Connecting Signals
©
National Instruments Corporation
2-5
The value of the bias resistor should be between 100 k
Ω
and 1 M
Ω .
An
added DC offset voltage results, due to input bias current flowing through
the bias resistor. For example, with a 1 M
Ω
bias resistor and the specified
maximum input bias current of 1 nA, you have a maximum added input
offset voltage of ±1 mV in addition to the initial offset voltage.
Since only the AC signal is of interest when AC-coupling, you can choose
to remove the DC offset in software by using a simple highpass filter.
Caution
Pins A2, A4, A8, C2, C4, C6, and C8 on the front signal connector are not isolated
and do not have the same protection circuitry as the positive and negative analog input pairs
discussed in the
section. Hooking up external signals to these pins can
damage the SCXI-1125 module.
Pin Assignments
The front signal connector is a special 32-pin DIN C male front connector
used for connecting analog input signals, including the CJC, to the analog
circuitry of the SCXI-1125. This connection allows access to the eight
differential analog input signals. The positive terminal is AI
x +
and the
negative terminal AI
x
–. A missing pin exists between two consecutive
input channels to meet the UL spacing requirements for high voltage
signals. CJ TEMP is the signal connection used by the cold-junction
channel on the SCXI-1125. The signals on pins A6, A8, C6, and C8 are
reserved for serial communication. The +5 V signal and CHS GND signals
are used as the power supply and ground signals for the CJC sensor and
other circuitry on the terminal block. The pin assignments for the
SCXI-1125 front signal connection are shown in Table 2-1.
Caution
Do not make signal connections to pins A2, A4, A6, A8, C2, C4, C6, or C8 on the
front signal connector. Connecting external signals to these pins can damage the
SCXI-1125 Module.
