Omega 1002 - Manual (Page 19)

2.6 Analog Input Signal Connection
The OME-PCI-1002 can measure signals in the single-ended or differential mode.
In the differential mode each channel has a unique signal HIGH and signal LOW
connection. In the single-ended mode all channels have a unique signal HIGH connection
but share a common LOW or ground connection. Differential connections are very useful
for low level signals (millivolt), since they better reject electrical noise that can affect the
quality of the measurement. A differential connection is also necessary when a common
ground is unacceptable. The benefit of using a single-ended connection is that twice the
number of channels is available. In general, a single-ended connection is often a good
choice when working with higher level signals (5V or 10V for example), especially if the
signal is coming from an isolated device such as a signal conditioner. Several different
types of wiring diagrams are discussed below.
Figure 2-3A shows a differential connection to a grounded source. If the source is
grounded, making a second connection to the card’s ground could cause a ground loop
resulting in erroneous data. It is important to note that the maximum common mode
voltage between the input source and AGND is 70Vp-p. If the card is connected to a
source with a common mode voltage greater than 70Vp-p, the input multiplexer will
be permanently damaged! When measuring common mode voltage, it is best to use an
oscilloscope rather than a multi-meter.
Figure 2-3B shows a differential connection to a floating source. In such cases a
connection should be made between the low channel input and analog ground.
Figure 2-4 shows connection of multiple sources in single-ended mode. This
connection assumes creating one common ground will not cause a problem. This is
normally the case when connecting to devices that are isolated or floating.
Figure 2-5 demonstrates how to connect bridge transducers. Bridge transducers
include strain gauges, load cells and certain type of pressure transducers. The diagram
assumes that there is a single external power supply providing power to the bridge. Each
bridge is connected to a differential channel. No connection is made between channel
low and analog ground. A connection should be made between analog ground and the
negative of the power supply. An isolated power supply is strongly suggested.
Figure 2-6 demonstrates how to connect a 4-20mA current loop. Since the card
reads voltages, the current is converted to voltage by passing it through a shunt resistor.
By Ohms law (V=IR), when using a 250
Ω resistor, 4 mA will be converted to 1V and
20mA to 5V. If the source is linear, the output voltage range will also be linear.
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