Omega ISA- BUS MULTI-FUNCTIONAL BOARD OME-A822PG - Manual (Page 34)

2.9 Analog Input Signal Connection
The OME-A-822 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 1-A 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 1-B 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 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 3 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 4 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.
OME-A-822PGL/PGH Hardware Manual ---- 32