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Linearized Thermocouple Input Modules
FEATURES
● INTERFACES TO TYPES J, K, T, E, R, S, N, AND B
THERMOCOUPLES
● LINEARIZES THERMOCOUPLE SIGNAL
● HIGH LEVEL VOLTAGE OUTPUTS
● 1500Vrms TRANSFORMER ISOLATION
● ANSI/IEEE C37.90.1-1989 TRANSIENT PROTECTION
● INPUT PROTECTED TO 240VAC CONTINUOUS
● 160dB CMR
● 95dB NMR AT 60Hz, 90dB at 50Hz
● ±1µV/°C DRIFT
● CSA CERTIFIED, FM APPROVED, CE COMPLIANT
● MIX AND MATCH OM5 TYPES ON BACKPANEL
DESCRIPTION
Each OM5-LTC thermocouple input module provides a single channel of
thermocouple input which is filtered, isolated, amplified, linearized and
converted to a high level analog voltage output (Figure 1). This voltage
output is logic-switch controlled, allowing these modules to share a
common analog bus without the requirement of external multiplexers.
+EXC
4
+In
Thermocouple
3
–In
2
–EXC
1
Backpanel
Term Block
Temp
Sensor
FIGURE 1. OM5-LTC Block Diagram.
OM5-LTC
+2.5V
100MΩ
4
NC
LPF
Surge
6
Suppression
and
5
Protection
3
CJC
1
2
+V
Isolated
–V
Field-Side
Power
30
The OM5-LTC modules are designed with a completely isolated computer
side circuit which can be floated to ±50V from Power Common, pin 16.
This complete isolation means that no connection is required between I/O
Common and Power Common for proper operation of the output switch. If
desired, the output switch can be turned on continuously by simply
connecting pin 22, the Read-Enable pin to I/O Common, pin 19.
The OM5-LTC can interface to eight industry standard thermocouple types:
J, K, T, E, R, S, N, and B. Its corresponding output signal operates over a
0V to +5V range. Each module is cold-junction compensated to correct for
parasitic thermocouples formed by the thermocouple wire and screw
terminals on the mounting backpanel. Upscale open thermocouple detect
is provided by an internal pull-up resistor. Downscale indication can be
implemented by installing an external 47MΩ resistor, ±20% tolerance,
between screw terminals 1 and 3 on the OM5-BP backpanels.
Signal filtering is accomplished with a six-pole filter which provides 95dB
of normal-mode-rejection at 60Hz and 90dB at 50Hz. Two poles of this filter
are on the field side of the isolation barrier, and the other four are on the
computer side.
After the initial field-side filtering, the input signal is chopped by a
proprietary chopper circuit. Isolation is provided by transformer coupling,
again using a proprietary technique to suppress transmission of common
mode spikes or surges. The module is powered from +5VDC, ±5%.
A special input circuit provides protection against accidental connection of
power-line voltages up to 240VAC.
Linearizer
Isochopper™
Amplifier
LPF
Pat Pending
+V
–V
Isolated
Precision
Computer-Side
Reference
Power
Power
Oscillator
20
V
OUT
19
I/O Common
22
RD EN\
17
+5V
16
Power Common
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