CSE Kingfisher PLUS+, Hardware Reference Manual

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Superseded Hardware Manual 7.12 Page 64
1.22 AO-2: An a lo g Cu rre n t Ou tp u t, 4 Ch a n n e l
FEATURES
12-Bit Resolution
0-20 or 4-20 mA Outputs
1.5 kV Field to Logic Isolation
1.5 kV Channel Group to Channel
Group Isolation
The AO-2 provides four current output channels that use 12-bit digital to analog converter (DAC). The
sign bit is not used in the conversion process. The placement of the 12 bits within the data word is
shown below.
MSB
LSB
0 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 0 0 0
If the module is sent data that is out of range (less than 0 or greater than 32767), the software does
not accept the value.
This module provides two output ranges. The default range is 4 to 20 mA with user data scaled so that
a count of 0 corresponds to 4 mA and a count of 32760 corresponds to 20 mA. A range link (on the
rear of the module) selects either 4-20 mA or 0-20 mA for all four channels. The default condition is 4-
20 mA with the link on. The module provides 12 bits of resolution in either range. Scaling of the output
is as shown below.
32760
0
0 4 20
Current (mA)
(4 to 20 mA range)
Raw
Value
(decimal)
32760
0
0
20
Current (mA)
(0-20 mA range)
Raw
Value
(decimal)
Figure: AO-2 Scaling for Current Output
Analog outputs 1 & 2 and analog outputs 3 & 4 form two groups that are isolated from each other and
from the backplane logic. Each group is powered by a separate DC-DC converter module that
produces an isolated voltage of +24 V to drive the current loop outputs and analog front-end.
The current loop drivers on the module are source type drivers. To minimize the capacitive loading
and noise, all field connections to the module should be wired using a good grade of twisted, shielded
instrumentation cable. The shields should be connected to E (Shield) on the user terminal connector
block. The E connection provides path to the backplane (frame Earth / Ground) resulting in attenuation
of noise caused by the shield drain currents. To further improve noise immunity, direct connection of
the analog cable shields to the Earth / Ground rails with metal clamps or similar method is
recommended, especially in environments with high electromagnetic noise levels.
This module can be installed in any I/O slot of a 4, 6 or 12 slot backplane in a system.