Specifications

Section 7. Grounding
7-4
7.3 Single-Ended Measurement Reference
Low-level single-ended voltage measurements are sensitive to ground potential
fluctuations. The grounding scheme in the CR1000 has been designed to
eliminate ground potential fluctuations due to changing return currents from 12
V, SW-12, 5 V, and the control ports. This is accomplished by utilizing
separate signal grounds (
) and power grounds (G). To take advantage of
this design, observe the following grounding rule:
Always connect a device’s ground next to the active terminal
associated with that ground. Several ground wires can be
connected to the same ground terminal.
Examples:
1. Connect 5 Volt, 12 Volt, and control grounds to G terminals.
2. Connect excitation grounds to the closest
terminal on the excitation
terminal block.
3. Connect the low side of single-ended sensors to the nearest
terminal on
the analog input terminal blocks.
4. 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
pulse-counter channels should help. Problem leads can also be tied directly to
the ground lug to minimize induced single-ended offset voltages.
7.4 Ground Potential Differences
Because a single-ended measurement is referenced to CR1000 ground, any
difference in ground potential between the sensor and the CR1000 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 CR1000
ground.
Ground potential differences are a common problem in application measuring
full bridge sensors (strain gages, pressure transducers, etc), and thermocouples
when used to measure soil temperature.
7.4.1 Soil Temperature Thermocouple
If the measuring junction of a copper-constantan thermocouple being used to
measure soil temperature is not insulated, and the potential of earth ground is 1
mV greater at the sensor than at the point where the CR1000 is grounded, the
measured voltage will be 1 mV greater than the thermocouple output, or
approximately 25
o
C high.
NOTE