Dataq DI-720-EN, User Manual

Page: 91 / 108

DI–720/DI–722/DI–730 Series User Manual
Accessories
81
DI-5B34 Linearized 2- or 3-Wire RTD Input Modules
Each DI-5B34 RTD input module provides a single channel of RTD input which is filtered, isolated, amplified, lin-
earized, and converted to a high level analog voltage output (see block diagram). This voltage output is logic switch
controlled, which allows these modules to share a common analog bus without the requirement of external multiplex-
ers.
The DI-5B 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 com-
puter 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 DI-5B34 modules provides protection against accidental connection of power-line volt-
ages up to 240VAC.