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Linearized 4-Wire RTD Input Modules
FEATURES
● INTERFACES TO 100 PLATINUM, 10 COPPER,
OR 120 NICKEL RTDs
● TRUE 4-WIRE INPUT
● LINEARIZES RTD SIGNAL
● HIGH LEVEL VOLTAGE OUTPUT
● 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
● CSA CERTIFIED, FM APPROVED, CE COMPLIANT
● MIX AND MATCH OM5 TYPES ON BACKPANEL
DESCRIPTION
In RTD temperature measurement applications requiring a very high level
of accuracy, the OM5 4-Wire RTD input module offers a significant
advantage over 3-wire measurement techniques (Figure 1). The OM5
measures only the voltage dropped across the RTD and almost completely
ignores the resistance or length of the RTD lead wires. The OM5
3-Wire RTD module provides lead resistance compensation, but requires
equal lead resistances, while the 4-wire modules do not require matched
lead resistances.
+EXC
4
4
+In
6
3
RTD
5
–In
4 Wire
2
3
RTD
–EXC
1
Backpanel
Term Block
FIGURE 1. OM5-IP4/IC4/IN4 Block Diagram.
OM5-IP4/IC4/IN4
LPF
Surge
Suppression
and
Protection
1
–V
NC
2
NC
+V
Isolated
–V
Field-Side
Power
12
Each OM5 4-wire RTD input module provides a single channel of RTD input
which is filtered, isolated, amplified, linearized, and converted to a high
level analog voltage output. This voltage output is logic switch controlled,
which allows these modules to share a common analog bus without the
requirement of external multiplexers.
The OM5 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.
RTD excitation is provided from the module by a precision current source.
The excitation current is available on two leads which are separate from the
two input signal measuring leads. The excitation current does not flow in
the input signal leads, which allows RTD measurement to be totally
independent of lead resistance. The excitation current is very small (0.25mA
for 100
Pt and 120
Ni and 1.0 mA for 10 Cu) which minimizes self-
heating of the RTD.
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 trans-
former coupling, again using a proprietary technique to suppress transmis-
sion 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™
LPF
Amplifier
Pat Pending
+V
–V
Isolated
Computer-Side
Power
Power
Oscillator
20
V
OUT
19
I/O Common
22
RD EN\
17
+5V
16
Power Common
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