Datasheet

AD588
Rev. L | Page 17 of 20
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.
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.
BOOSTED PRECISION CURRENT SOURCE
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.
BRIDGE DRIVER CIRCUITS
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.
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 V
IN
/2. Further
processing of this signal may necessarily be limited to high
common-mode rejection techniques, such as instrumentation
or isolation amplifiers.
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.
00531-036
R3
R
B
R1
R2
R4
R5
R6
–V
S
+V
S
A1
AD588
A3
100Ω
1.0mA
0.01%
+
–
V
OUT
R
C
= 10kΩ
R
C
VISHAY S102C
OR SIMILAR
RTD = Ω K4515
0.24°C/mW SELF-HEATING
–15V
OR
GROUND
A4
59108
12
11 13
7643
16
2
15
14
1
A2
00531-037
R3
R
B
R1
R2
R4
R5
R6
–V
S
+V
S
A1
A4
AD588
A3
LOAD
V
CC
Q
1
I
L
=
220Ω
LIMITED BY
Q
1
AND R
C
POWER
DISSIPATION
10V
R
C
13
11
12
81095
3467
1
14
15
2
16
A2
Figure 36. Precision Current Source for RTD Figure 37. Boosted Precision Current Source
00531-038
V
IN
–
+
R4
R3
R2
R1
E
O
+–
V
1
R4
R3
R2
R1
E
O
+–
V
2
–
+
–
+
00531-039
Figure 38. Bridge Transducer Excitation—Unipolar Drive Figure 39. Bridge Transducer Excitation—Bipolar Drive