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Linearized 2- or 3-Wire RTD Input Modules
FEATURES
● INTERFACES TO 100
PLATINUM, 10
OR 120
NICKEL RTDs
● LINEARIZES RTD 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
● CSA CERTIFIED, FM APPROVED, CE COMPLIANT
● MIX AND MATCH OM5 TYPES ON BACKPANEL
DESCRIPTION
Each OM5 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 (Figure 1). 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
(1)
3 and 4 Wire
2 Wire
+EXC
4
4
+In
6
3
RTD
5
–In
2
3
NC
–EXC
1
(1)
4 Wire
Backpanel
Term Block
NOTE: (1) 4-wire sensors may be
used with this module. However,
for true 4-wire measurement,
use OM5-IP4/IC4/IN4.
FIGURE 1. OM5-IP/IC/IN Block Diagram.
OM5-IP/IC/IN
COPPER,
LPF
Surge
Suppression
R
and
Z
Protection
1
–V
–V
NC
2
+V
Isolated
–V
Field-Side
Power
10
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 two matched current
sources. When using a three-wire RTD, this method allows an equal current
to flow in each RTD lead, which cancels the effects of lead resistances. The
excitation currents are very small (0.25mA for 100 Pt and 120 Ni, and
1.0mA 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 on the OM5 modules provides protection against
accidental connection of power-line voltages up to 240VAC.
Linearizer
Isochopper™
Amplifier
LPF
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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