4.1.1 Possible Ground Loop Problems; 4.1.2 Minimum Supply Voltage; 2-Wire With Data Logger Power; 2-Wire With External Power - Campbell CURS100 Product Manual

4.1.1 Possible Ground Loop Problems

4.1.2 Minimum Supply Voltage

CURS100 100 Ohm Current Shunt Terminal Input Module
FIGURE 4-1. 2-wire with data logger power
FIGURE 4-2. 2-wire with external power
The resistor must be grounded at the data logger to ensure that measurements
are within common mode range. The signal (or low) output on the transmitter
is higher than the data logger ground by the voltage drop across the resistor. A
ground-loop error may occur if the signal output is not electrically isolated but
is connected to the sensor case. If such a sensor is in contact with earth ground
(for example, a pressure transmitter in a well or stream), an alternative path for
current flow is established through earth ground to the data logger earth
ground. This path is in parallel with the path from the signal output through the
resistor; hence, not all the current will pass through the resistor and the
measured voltage will be too low.
When the power supply is in the current loop, as is the case in a 2-wire
transmitter, it is necessary to consider the effect of voltage drop across the
resistor on the voltage applied to the transmitter.
For example, suppose a 4 to 20 mA transmitter requires at least 9 volts to
operate correctly and the system is powered by a 12-volt battery. The voltage
the transmitter sees is the battery voltage minus the voltage drop in the rest of
the current loop. At 20 mA output, the voltage drop across the 100 ohm resistor
is 2 volts. When the battery is at 12 volts, this leaves 10 volts for the
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