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 250Ω 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
