Data Sheet
AD588
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 desired
resolution. RTD manufacturers usually specify the self-heating
effect of each of their models or types of RTDs.
7
6
4
3
R
C
VISHAY S102C
OR SIMILAR
A3
A4
1
R
B
A1
R1
AD588
14
15
R
= 10kΩ
C
R4
R5
1.0mA
0.01%
Figure 36 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.
+
R2
100Ω
V
OUT
+V
2
R6
S
–
R3
A2
BOOSTED PRECISION CURRENT SOURCE
16
–15V
OR
–V
S
In the RTD current-source application, the load current is limited
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.
GROUND
12
11 13
5
9
10
8
RTD = Ω K4515
0.24°C/mW SELF-HEATING
Figure 36. Precision Current Source for RTD
BRIDGE DRIVER CIRCUITS
7
6
4
3
V
CC
The Wheatstone bridge is a common transducer. In its simplest
form, a bridge consists of four 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.
220Ω
A3
A4
1
Q
1
R
B
A1
R1
AD588
14
15
R4
Figure 38 shows a simple bridge driven from a unipolar excitation
supply. EO, a differential voltage, is proportional to the deviation
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.
R2
R5
10V
R
C
I
=
L
+V
2
S
R6
R3
A2
16
–V
S
12
13
5
9
10
8
11
LIMITED BY
AND R
Q
1
C
POWER
DISSIPATION
Figure 39 shows the same bridge transducer, this time driven
from a pair of bipolar supplies. This configuration ideally
eliminates the common-mode voltage and relaxes the
restrictions on any processing elements that follow.
LOAD
Figure 37. Boosted Precision Current Source
R4
R1
R3
R2
+
–
–
+
V
IN
E
O
Figure 38. Bridge Transducer Excitation—Unipolar Drive
+
R4
R1
R3
R2
V
V
1
–
–
+
E
O
+
–
2
Figure 39. Bridge Transducer Excitation—Bipolar Drive
Rev. M | Page 17 of 20
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