Chapter 4
Signal Connections
©
National Instruments Corporation
4-15
PCI-4451/4452/4453/4454 User Manual
but connecting AOGND to other earth-connected grounds is not
recommended. AOGND is not directly available if you are using the
BNC-2140 accessory.
The PCI-4451 has two identical analog output channels. One analog output
channel is illustrated in Figure 4-6.
Figure 4-6.
Analog Output Channel Block Diagram for the PCI-4451
The analog output stage is differential and balanced. Each output signal
consists of a plus connection, a minus connection, and a ground (AOGND)
connection. The actual output signal is the difference between the plus and
minus connections. The pair is balanced, meaning that if the impedances
from each of the pair to AOGND is the same (or infinite), then the voltage
at the plus and minus terminals are equal but opposite, so that their
difference is the desired signal and their sum (or average) is zero. If
impedances from each of the pair to AOGND is not the same, the
connection is unbalanced, but the difference between the plus and minus
terminals is still equal to the desired signal. If the minus side is grounded,
the plus voltage is equal to the signal. Conversely, if the plus side is
grounded, the minus voltage is equal to the negative of the signal. In all
cases, the difference is equal to the signal.
Connection of analog output signals from your PCI-4451 device 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 PCI-4451 channel) to the signal and
−
DACxOUT to the signal ground
(or signal minus, as appropriate). When driving some floating devices,
however, 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.You
can make DIFF and SE connections through the BNC-2140 accessory.
Balanced
Differential
Driver
Attenuator
D/A
Converter
Gain = 0 dB
Gain = –20 dB
Gain = –40 dB
Gain = –
∞
dB
11
Ω
100 k
Ω
10 k
Ω
100 k
Ω
+DAC
xOUT
–DAC
xOUT
AOGND
11
Ω
Mute