ADR430/ADR431/ADR433/ADR434/ADR435/ADR439
V IN
2
R LW
V IN
V OUT
SENSE
PROGRAMMABLE CURRENT SOURCE
Together with a digital potentiometer and a Howland current
ADR43x
V OUT 6
A1
OP191
+
R LW
V OUT
FORCE
pump, the ADR435 forms the reference source for a programmable
current as
? R2 A + R 2 B
I L = ?
?
?
?
? × V
?
?
R L
GND
4
Figure 37. Advantage of Kelvin Connection
DUAL POLARITY REFERENCES
and
?
?
R1
R 2 B
?
?
W
(4)
V W =
× V REF
Dual polarity references can easily be made with an operational
amplifier and a pair of resistors. To avoid defeating the accuracy
obtained by the ADR43x, it is imperative to match the resistance
tolerance as well as the temperature coefficient of all the components.
V IN
1μF 0.1μF 2
D
2 N
where:
D is the decimal equivalent of the input code.
N is the number of bits.
(5)
V IN V OUT 6
ADR435
U1
GND TRIM 5
4
R1
10k ?
R3
5k ?
R2
10k ?
+10V
V+
OP1177
U2
V–
–10V
+5V
–5V
In addition, R1' and R2' must be equal to R1 and (R2 A + R2 B ),
respectively. In theory, R2 B can be made as small as needed to
achieve the necessary current within the A2 output current
driving capability. In this example, the OP2177 can deliver a
maximum output current of 10 mA. Because the current pump
employs both positive and negative feedback, C1 and C2
capacitors are needed to ensure that the negative feedback
prevails and, therefore, avoids oscillation. This circuit also
Figure 38. +5 V and ?5 V References Using ADR435
+2.5V
+10V
allows bidirectional current flow if the V A and V B inputs of
the digital potentiometer are supplied with the dual polarity
references, as shown in Figure 40.
2
V IN
V OUT 6
C1
10pF
ADR435
U1
R1
5.6k ?
V DD
R1'
50k ?
R2'
1k ?
R2
V OUT 6
GND TRIM 5
4
5.6k ? V+
OP1177
U2
V–
–2.5V
–10V
Figure 39. +2.5 V and ?2.5 V References Using ADR435
2
V IN TRIM 5
ADR435
U1
GND
4
U2
AD5232
A
B W
V DD
V+
OP2177
A1
V–
C2
10pF
R1
50k ?
V DD
V+
OP2177
A2
V–
V SS
R2 A
1k ?
R2 B
10 ?
V SS
+
VL
I L
I L
Figure 40. Programmable Current Source
Rev. J | Page 19 of 24
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