AD588BQ Просмотр технического описания (PDF) - Analog Devices
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AD588BQ Datasheet PDF : 12 Pages
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RTD EXCITATION
The Resistance Temperature Detector (RTD) is a circuit ele-
ment whose resistance is characterized by a positive temperature
coefficient. A measurement of resistance indicates the measured
temperature. Unfortunately, the resistance of the wires leading
to the RTD often adds error to this measurement. The 4-wire
ohms measurement overcomes this problem. This method uses
two wires to bring an excitation current to the RTD and two
additional wires to tap off the resulting RTD voltage. If these
additional two wires go to a high input impedance measurement
circuit, the effect of their resistance is negligible. Therefore, they
transmit the true RTD voltage.
AD588
Figure 18. 4-Wire Ohms Measurement
A practical consideration when using the 4-wire ohms technique
with an RTD is the self-heating effect that the excitation current
has on the temperature of the RTD. The designer must choose
the smallest practical excitation current that still gives the de-
sired resolution. RTD manufactures usually specify the
self-heating, effect of each of their models or types of RTDs.
Figure 19 shows an AD588 providing the precision excitation
current for a 100 Ω RTD. The small excitation current of 1 mA
dissipates a mere 0.1 mW of power in the RTD.
Figure 20. Boosted Precision Current Source
BRIDGE DRIVER CIRCUITS
The Wheatstone bridge is a common transducer. In its simplest
form, a bridge consists of 4 two terminal elements connected to
form a quadrilateral, a source of excitation connected along one
of the diagonals and a detector comprising the other diagonal.
Figure 21a shows a simple bridge driven from a unipolar excita-
tion supply. EO, a differential voltage, is proportional to the de-
viation of the element from the initial bridge values. Unfortunately,
this bridge output voltage is riding on a common-mode voltage
equal to approximately VIN/2. Further processing of this signal
may necessarily be limited to high common-mode rejection
techniques such as instrumentation or isolation amplifiers.
Figure 21b shows the same bridge transducer, but this time it is
driven from pair of bipolar supplies. This configuration ideally
eliminates the common-mode voltage and relaxes the restric-
tions on any processing elements that follow.
Figure 19. Precision Current Source for RTD
BOOSTED PRECISION CURRENT SOURCE
In the RTD current-source application the load current is lim-
ited to ± 10 mA by the output drive capability of amplifier A3.
In the event that more drive current is needed, a series pass
transistor can be inserted inside the feedback loop to provide
higher current. Accuracy and drift performance are unaffected
by the pass transistor.
a. Unipolar Drive
b. Bipolar Drive
Figure 21. Bridge Transducer Excitation
REV. B
–11–
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