Campbell Scientific CR6 CR6 Measurement and Control System - Page 59

Single-Ended Measurements - Overview

Page 59 highlights

Section 5. System Overview Figure 19. Analog Sensor Wired to Differential Channel #1 5.1.2.1.1 Single-Ended Measurements - Overview Reading List: • Single-Ended Measurements - Overview (p. 59) • Single-Ended Measurements - Details (p. 309) A single-ended measurement measures the difference in voltage between the terminal configured for single-ended input and the reference ground. The measurement sequence is illustrated in figure Simplified Voltage Measurement Sequence. While differential measurements are usually preferred, a single-ended measurement is often adequate in applications wherein some types of noise are not a problem and care is taken to avoid problems caused by ground currents. Examples of applications wherein a single-ended measurement may be preferred include: • Not enough differential terminals available. Differential measurements use twice as many U terminals as do single-ended measurements. • Rapid sampling is required. Single-ended measurement time is about half that of differential measurement time. • Sensor is not designed for differential measurements. Many Campbell Scientific sensors are not designed for differential measurement, but the draw backs of a single-ended measurement are usually mitigated by large programmed excitation and/or sensor output voltages. However, be aware that because a single-ended measurement is referenced to CR6 ground, any difference in ground potential between the sensor and the CR6 will result in error, as emphasized in the following examples: • If the measuring junction of a thermocouple used to measure soil temperature is not insulated, and the potential of earth ground is greater at the sensor than 59

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Section 5.
System Overview
Figure 19. Analog Sensor Wired to Differential Channel #1
5.1.2.1.1 Single-Ended Measurements — Overview
Reading List:
Single-Ended Measurements — Overview
(p. 59)
Single-Ended Measurements — Details
(p. 309)
A single-ended measurement measures the difference in voltage between the
terminal configured for single-ended input and the reference ground.
The
measurement sequence is illustrated in figure Simplified Voltage Measurement
Sequence
.
While differential measurements are usually preferred, a single-ended
measurement is often adequate in applications wherein some types of noise are
not a problem and care is taken to avoid problems caused by ground currents.
Examples of applications wherein a single-ended measurement may be preferred
include:
Not enough differential terminals available.
Differential measurements use
twice as many
U
terminals as do single-ended measurements.
Rapid sampling is required.
Single-ended measurement time is about half
that of differential measurement time.
Sensor is not designed for differential measurements.
Many Campbell
Scientific sensors are not designed for differential measurement, but the draw
backs of a single-ended measurement are usually mitigated by large
programmed excitation and/or
sensor output voltages.
However, be aware that because a single-ended measurement is referenced to CR6
ground, any difference in ground potential between the sensor and the CR6 will
result in error, as emphasized in the following examples:
If the measuring junction of a thermocouple used to measure soil temperature
is not insulated, and the potential of earth ground is greater at the sensor than
59