Differential Measurement; Completing The Current Loop Circuit - Campbell CURS100 Product Manual

3.1

Differential Measurement

3.2

Completing the Current Loop Circuit

CURS100 100 Ohm Current Shunt Terminal Input Module
Ohm's law describes how a voltage (V) is generated by the signal current (I)
through a completion resistor (R):
V = I (R)
This voltage is measured by the data logger.
The voltage across the completion resistor is measured with the differential
voltage measurement. Use VoltDiff() for the CRBasic data loggers (for
example, CR6, CR1000, CR1000X, CR5000, or CR9000(X)). The differential
voltage measurement measures the difference in voltage between the low and
high terminals. The CURS100 connects the resistor between the high and the
low terminals.
As shown in FIGURE 2-1, the 100 Ω sense resistor in the CURS100 is not
connected to the adjacent ground pin that connects into the data logger signal
ground (⏚). Hence, an additional connection must be made in order to
complete the loop, which is commonly done by connecting the CURS100 L
terminal to a data logger G (power ground) terminal with a jumper wire
(FIGURE 3-1). Connecting the L terminal to the adjacent ground (⏚ or G)
terminal on the CURS100 will result in unwanted return currents flowing into
the data logger signal ground (⏚), which could induce undesirable offset errors
in low-level, single-ended measurements. The ground (⏚ or G) terminal on the
CURS100 can be used to connect cable shields to ground.
Completing the loop by connecting voltages other than ground is possible as
long as the data logger voltage input limits are not exceeded. These input limits
specify the voltage range, relative to data logger ground, which both H and L
input voltages must be within in order to be processed correctly by the data
logger. The input limits are ±5 V for the CR6, CR800, CR850, CR1000,
CR1000X, CR3000, CR5000, and CR9000(X). Hence, when measuring
currents up to 50 mA with the CURS100, a connection to data logger ground is
necessary in order for the resulting (50 mA) • (100 Ω) = 5 V signal to comply
with the ±5 V input limits for the CR6, CR800, CR850, CR1000, CR1000X,
CR3000, CR5000, and CR9000(X) dataloggers. The CR300 is limited to 25 mA
with a –100 to +2500 mV range.
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