Campbell Scientific CR5000 Measurement and Control Module - Manual (Page 33)

Section 1. Installation and Maintenance
1-9
1.7.3 Effect of Grounding on Single-Ended Measurements
Low-level single-ended voltage measurements can be problematic because of
ground potential fluctuations. The grounding scheme in the CR5000 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.
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, CAO,
and pulse-counter channels should help. Problem leads can also be tied
directly to the ground lug to minimize induced single-ended offset voltages.
1.8 Powering Sensors and Peripherals
The CR5000 is a convenient source of power for sensors and peripherals
requiring a continuous or semi-continuous 5 VDC or 12 VDC source. The
CR5000 has 2 continuous 12 Volt (12V) supply terminals, 2 switched 12 Volt
(SW-12) supply terminals, and 2 continuous 5 Volt (5V) supply terminals.
Voltage on the 12V and SW-12 terminals will change with the CR5000 supply
voltage. The 5V terminal is regulated and will always remain near 5 Volts
(
±
4%)so long as the CR5000 supply voltage remains above 11 Volts. The 5V
terminal is not suitable for resistive bridge sensor excitation. Table 1.8-1
shows the current limits of the 12 Volt and 5 Volt ports. Table 1.8-2 shows
current requirements for several CSI peripherals. Other devices normally have
current requirements listed in their specifications. Current drain of all
peripherals and sensors combined should not exceed current sourcing limits of
the CR5000.
NOTE