DI-148U, DI-148U-SP, and DI–158 Series Hardware Manual
Connecting Input Signals
19
Taking Process Current Measurements with a DI-158
When measuring process current it's important to consider that a potential difference (common mode voltage) may
exist between the loop power supply ground and that of the data acquisition device. To check for such a voltage place
a volt meter between the two grounds.
Ideally 0V will be present between the power supply and data acquisition grounds.
Should a common mode voltage (AC or DC) exist, determine the sum of your input signal and the common mode
voltage (CMV). If this sum exceeds the maximum rating of your device (see “2. Specifications”) you must determine
the source of the CMV and eliminate it before taking any measurements. This may be as simple as plugging the loop
power supply and data acquisition device into the same wall outlet.
Another common mistake when acquiring process current data is to measure across the shunt resistor on the high side
of the current loop. In doing so you're exposing your data acquisition device to the full power supply voltage.
Assuming that the grounds are common, measuring across the shunt resistor on the high side of the circuit exposes
the data acquisition device to the power supply voltage.
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