Impulse SCM5B35 - Manual

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Isolated Analog Signal Conditioning Products SCM5B
In RTD temperature measurement applications requiring a very high level of
accuracy, the SCM5B35 4-Wire RTD input module offers a significant advan-
tage over 3-wire measurement techniques (Figure 1). The SCM5B35 measures
only the voltage dropped across the RTD and almost completely ignores
the resistance or length of the RTD lead wires. The SCM5B34 3-Wire RTD
module provides lead resistance compensation, but requires equal lead
resistances, while the SCM5B35 does not require matched lead resistances.
Each SCM5B35 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 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 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 Ω Pt and
120 Ω Ni and 1.0 mA 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 SCM5B35 modules provides protection against
accidental connection of power-line voltages up to 240VAC.
SCM5B35
Linearized 4-Wire RTD Input Modules
Description
▲
Features
• Interfaces to 100 Ω Platinum, 10 Ω Copper,
or 120 Ω Nickel RTDs
• True 4-Wire Input
• Linearizes RTD Signal
• High Level Voltage Output
• 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: SCM5B35 Block Diagram