Chapter 3
Signal Connections
©
National Instruments Corporation
3-5
The outer shell of the BNC connectors is not GND (0 V). The outer shell of the
BNC is not physically connected to the metal box of the BNC-2140. The outer shell
is either the inverting differential signal in DIFF mode or is tethered to GND (0 V)
through a 50
Ω,
1 W resistor in SE mode.
Analog Input Signal Connections
The analog input signals for the BNC-2140 device are +ACH<0..3> and
–ACH<0..3>. Connecting of analog input signals to your BNC-2140
accessory depends on the configuration of the input signal sources.
For most signals, you use a DIFF configuration and simply connect the
signal to +ACHx (where x is the BNC-2140 channel) and the signal ground
(or signal minus), as appropriate, to –ACHx. If a signal has a high output
impedance (greater than 1 k
Ω
) and is floating, you may find it useful to use
an SE configuration that tethers the signal minus to AIGND. This reduces
common-mode interference.
Analog Output Signal Connections
The BNC-2140 analog output signals are +DAC0OUT, –DAC0OUT,
+DAC1OUT, and –DAC1OUT.
+DAC0OUT is the voltage output signal for analog output channel 0.
+DAC1OUT is the voltage output signal for analog output channel 1.
Connection of analog output signals from your BNC-2140 accessory
depends on the configuration of the devices receiving the signals. For most
signals, you use a DIFF configuration and simply cDACxOUT
(where x is the BNC-2140 channel) to the signal and –DACxOUT to the
signal ground (or signal minus), as appropriate. When driving some
floating devices, you may sometimes find it helpful to use the SE
configuration and connect the floating ground system of the device to
AOGND to reduce common-mode noise coupled from an interfering
source to the device.
Caution
When you configure an analog output channel in the SE mode, the voltage
between AOGND and –DACxOUT must not exceed ±7.07 V (5 V
rms
). Voltage that
exceeds this rating can damage the BNC-2140, the DSA plug-in device, and the
computer. National Instruments is not responsible for any damages resulting from
connections that exceed this rating.
!
UM.book Page 5 Tuesday, July 14, 1998 10:37 AM
