Campbell Scientific CR1000KD CR800 and CR850 Measurement and Control Systems - Page 90

Section 7. Installation 7.5.2 Single-Ended Measurement Reference Low-level, single-ended voltage measurements are sensitive to ground potential fluctuations. The grounding scheme in the CR800 has been designed to eliminate ground potential fluctuations due to changing return currents from 12V, SW12, 5V, and C1 - C4 terminals. This is accomplished by utilizing separate signal grounds ( ) and power grounds (G). To take advantage of this design, observe the following grounding rule: Note Always connect a device ground next to the active terminal associated with that ground. Several ground wires can be connected to the same ground terminal. Examples: • Connect grounds associated with 5V, 12V, and C1 - C4 terminals to G terminals. • Connect excitation grounds to the closest ( ) terminal on the excitation terminal block. • Connect the low side of single-ended sensors to the nearest ( ) terminal on the analog input terminal blocks. • Connect shield wires to the nearest ( ) terminal on the analog input terminal blocks. If offset problems occur because of shield or ground leads with large current flow, tying the problem leads into the ( ) terminals next to the excitation and pulsecounter channels should help. Problem leads can also be tied directly to the ground lug to minimize induced single-ended offset voltages. 7.5.3 Ground Potential Differences Because a single-ended measurement is referenced to CR800 ground, any difference in ground potential between the sensor and the CR800 will result in a measurement error. Differential measurements MUST be used when the input ground is known to be at a different ground potential from CR800 ground. Ground potential differences are a common problem when measuring full-bridge sensors (strain gages, pressure transducers, etc), and when measuring thermocouples in soil. 7.5.3.1 Soil Temperature Thermocouple If the measuring junction of a copper-constantan thermocouple is not insulated when in soil or water, and the potential of earth ground is, for example, 1 mV greater at the sensor than at the point where the CR800 is grounded, the measured voltage is 1 mV greater than the thermocouple output, which equates to approximately 25°C higher than actual. 7.5.3.2 External Signal Conditioner External signal conditioners, an infrared gas analyzer (IRGA) is an example, are frequently used to make measurements and send analog information to the CR800. These instruments are often powered by the same ac line source as the CR800. Despite being tied to the same ground, differences in current drain and 90