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Chapter 3 Signal Connections
© National Instruments Corporation 3-7 PC-OPDIO-16 User Manual
Sensing DC Voltages
When a positive or negative DC voltage with a magnitude of at least 2 V is referenced to the
IGND of a channel and is applied to an input of that channel, the PC-OPDIO-16 registers a logic
high for that input. If no voltage is present, the PC-OPDIO-16 will register a logic low for that
input. Thus, you can use the PC-OPDIO-16 to sense a wide range of DC signals—from digital
logic levels to DC power supply levels up to 24 V.
Sensing AC Voltages
The PC-OPDIO-16 senses a wide range of AC signals by registering a constant high while an
AC voltage (referenced to IGND) is present at an input. Signals with low amplitude and low
frequency appear as signals that are alternately turned on and off; therefore, the PC-OPDIO-16
alternately registers logic highs and logic lows for that signal. For sinusoidal signals, a 1 kHz
and higher frequency signal with a voltage of at least 4 Vrms returns a constant logic high level.
Signal Isolation
The VIN and IGND signals of each channel are isolated from the inputs of other channels and
are also isolated from the PC-OPDIO-16 internal power and ground signals. These barriers
provide an isolation for voltages up to +24 V and protect the PC-OPDIO-16. Voltages higher
than the +24 VDC may damage your equipment.
Warning: You must not exceed the voltage limit of the VIN signals referenced to their
respective IGND signals. National Instruments is
NOT liable for any damages
resulting from signal connections that exceed these limits.
Signal Connection Example
Figure 3-4 shows signal connections for the load connected to an isolated input. In this figure,
the PC-OPDIO-16 is being used to sense that a load is being powered. The load is connected to
the power supply by means of a switch. This power supply can be AC or DC and can be any
voltage within the PC-OPDIO-16 range. When the switch is open, no current flows through the
load and no voltage is applied to the load or to the PC-OPDIO-16 input. The digital logic of the
PC-OPDIO-16 then registers a logic low for that channel. When the switch is closed, current
flows through the LED and the PC-OPDIO-16 registers a logic high for that channel.
0.5 W
PC-OPDIO-16
LDA210 (1/2)
Isolation
2.7 kΩ
3.3 kΩ
Load
Digital Logic
+5 V
VIN
IGND
Supply
Isolated Ground
Figure 3-4. Signal Connection Example for Isolated Input