Measurement Computing CIO-DAS48-I - Manual (Page 13)

13
Chapter 3
Functional Details
Current loop measurements
The CIO-DAS48-I is configured for current loop measurements in the 4 to 20 mA range. Ranges of 2 to 10 mA,
1 to 5 mA and 0.5 to 2.5 mA are also possible but are less commonly used. A typical current loop system uses
24V to power the current loop. Current output devices are available in two-wire and three-wire configurations.
Use the figures below as guides in connecting devices to the CIO-DAS48-I.
Two-wire devices
The "floating device" configuration is required for most systems using two-wire devices due to the restriction
on common mode range for the CIO-DAS48-I. If the power supply were connected to the CIO-DAS48-I input,
as in a "grounded device" system, the common mode input range spec would be violated and inaccurate
measurements would result.
Figure 7 shows a "floating device" configuration for a two-wire, 4 to 20 mA device. This is the correct
configuration that ensures that the common mode range input specification is not violated.
CH 0 IN HI
CH 0 IN LO
100
2-wire, 4 - 20 mA
Device
CH 0
LLGND
CIO-DAS48-I
+ 24V
Figure 7. Floating device configuration
Figure 8 below shows a "grounded device" configuration for a two-wire, 4 to 20 mA device. DO NOT USE this
configuration — the 24 V supply connection, direct to the input, violates the common mode range input
specification of the CIO-DAS48-I.
CH 0 IN HI
CH 0 IN LO
100
2-wire,
4 - 20 mA
Device
CH 0
LLGND
+ 24V
CIO-DAS48-I
Figure 8. Grounded device configuration – Caution – DO NOT USE!