Impulse SCM5B34 - Manual

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Isolated Analog Signal Conditioning Products SCM5B
SCM5B34
Linearized 2- or 3-Wire RTD Input Modules
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Features
• Interfaces to 100 Ω Platinum, 10 Ω Copper,
or 120 Ω Nickel RTDs
• Linearizes RTD Signal
• High Level Voltage Outputs
• 1500Vrms Transformer Isolation
• ANSI/IEEE C37.90.1 Transient Protection
• Input Protected to 240VAC Continuous
• 160dB CMR
• 95dB NMR at 60Hz, 90dB at 50Hz
• CSA Certified, FM Approved, CE and ATEX
Compliant
• Mix and Match SCM5B Types on Backpanel
Figure 1: SCM5B34 Block Diagram
Each SCM5B34 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 SCM5B 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 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 Ω Pt and 120 Ω Ni, and 1.0mA for 10 Ω
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 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 on the SCM5B34 modules provides protection against
accidental connection of power-line voltages up to 240VAC.
Description