Datasheet

ADR430/ADR431/ADR433/ADR434/ADR435/ADR439
Rev. J | Page 19 of 24
V
IN
V
OUT
GND
R
LW
R
L
V
OUT
SENSE
V
OUT
FORCE
R
LW
V
IN
2
6
4
ADR43x
A1
OP191
+
04500-008
Figure 37. Advantage of Kelvin Connection
DUAL POLARITY REFERENCES
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.
6
2
4
5
– 10V
V
IN
V
IN
V
OUT
GND
TRIM
R1
R2
U2
R3
V+
V–
+10V
–5V
+5V
10kΩ
1µF 0.1µF
U1
ADR435
OP1177
5kΩ
10kΩ
04500-009
Figure 38. +5 V and −5 V References Using ADR435
6
2
4
5
V
IN
V
OUT
GND
TRIM
R1
5.6kΩ
U2
V+
V–
+10V
U1
ADR435
OP1177
+2.5
V
–2.5V
R2
5.6kΩ
–10V
04500-010
Figure 39. +2.5 V and −2.5 V References Using ADR435
PROGRAMMABLE CURRENT SOURCE
Together with a digital potentiometer and a Howland current
pump, the ADR435 forms the reference source for a programmable
current as
W
B
B
A
L
V
R
R1
RR2
I ×
⎟
⎟
⎟
⎟
⎠
⎞
⎜
⎜
⎜
⎜
⎝
⎛
+
=
2
2
(4)
and
REF
N
W
V
D
V ×=
2
(5)
where:
D is the decimal equivalent of the input code.
N is the number of bits.
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
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.
6
2
4
5
V
IN
V
DD
V
OUT
GND
TRIM
C2
10pF
U1
V+
V–
I
L
ADR435
OP2177
R1
50kΩ
OP2177
V–
V+
A2
A1
I
L
V
DD
U2
AD5232
W
A
B
V
SS
R1'
50kΩ
R2'
1kΩ
R2
A
1kΩ
R2
B
10Ω
V
DD
V
SS
C1
10pF
+
VL
–
04500-011
Figure 40. Programmable Current Source