AD588TQ Analog Devices, AD588TQ Datasheet - Page 13

• Manufacturer Part Number: AD588TQ
• Description: High Precision Voltage Reference
• Manufacturer: Analog Devices

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 diago-
nal. Figure 26a 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. Unfor-
tunately, this bridge output voltage is riding on a common-mode
REV. D
R3
R3
Figure 25. Boosted Precision Current Source
R
Figure 24. Precision Current Source for RTD
R
B
B
R2
R2
A2
A2
A1
A1
R1
R1
R4
R5
R4
R5
AD588
R6
R6
AD588
A4
A3
A3
A4
RTD = OMEGA K4515
0.24 C/mW SELF-HEATING
+V
–V
220
+V
–V
S
S
100
S
S
LOAD
R
V
GROUND
C
CC
Q
R
VISHAY S102C
OR SIMILAR
= 10k
–15V
C
1
LIMITED BY
Q
POWER
DISSIPATION
OR
I
1.0mA
L
0.01%
1
=
AND R
10V
R
C
V
OUT
C
+
–
–13–
voltage equal to approximately V
signal may necessarily be limited to high common-mode rejec-
tion techniques such as instrumentation or isolation amplifiers.
Figure 26b shows the same bridge transducer, this time driven
from a pair of bipolar supplies. This configuration ideally elimi-
nates the common-mode voltage and relaxes the restrictions on
any processing elements that follow.
As shown in Figure 27, the AD588 is an excellent choice for the
control element in a bipolar bridge driver scheme. Transistors
Q1 and Q2 serve as series-pass elements to boost the current
drive capability to the 28 mA required by a typical 350 Ω bridge.
A differential gain stage may still be required if the bridge balance
is not perfect. Such gain stages can be expensive.
R3
Figure 26a. Bridge Transducer Excitation—
Unipolar Drive
Figure 26b. Bridge Transducer Excitation—
Bipolar Drive
R
B
R2
A2
Figure 27. Bipolar Bridge Drive
A1
R1
V
IN
V
V
1
2
R4
R5
AD588
+
–
+
–
+
–
R6
IN
R4
R1
/2. Further processing of this
R4
R1
–
–
E
O
E
+
O
R2
+
A3
A4
R3
R2
R3
–V
+V
AD588
220
220
S
S
–
–15V
+15V
E
O
Q
2N3904
+
Q
2N3904
1
2
=
=