Specifications

Section 1. Installation and Maintenance
1-14
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 CR3000 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 (VX and CAO) 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.
1.8 Powering Sensors and Peripherals
The CR3000 is a convenient source of power for sensors and peripherals
requiring a continuous or semi-continuous 5 VDC or 12 VDC source. The
CR3000 has two continuous 12-Volt (12V) supply terminals, two switched 12
Volt (SW-12) supply terminals, and one continuous 5 Volt (5V) supply
terminals. Voltage on the 12V and SW-12 terminals will change with the
CR3000 supply voltage. The 5V terminal is regulated and will always remain
near 5 Volts (±4%) so long as the CR3000 supply voltage remains above 10
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 CR3000.
NOTE