Datasheet

ADR420/ADR421/ADR423/ADR425 Data Sheet
Rev. J | Page 20 of 24
PROGRAMMABLE CURRENT SOURCE
Together with a digital potentiometer and a Howland current
pump, the ADR425 forms the reference source for a program-
mable current as
W
B
B
A
L
V
R2
R1
R2
R2
I
×






+
=
(3)
and
REF
N
V
D
VW ×=
2
(4)
where:
D is the decimal equivalent of the input code.
N is the number of bits.
02432-048
U1
ADR425
V
OUT
V
IN
V
DD
V
DD
V
SS
TRIM
GND
A
W
B
U2
AD5232
U2
DIGITAL POT
IL
V+
V–
2
5
6
4
A1
OP2177
V
DD
V
SS
V+
V–
A2
OP2177
LOAD
VL
R1
50kΩ
R2
A
1kΩ
R2
B
10Ω
C2
10pF
R2'
1kΩ
R1'
50kΩ
C1
10pF
Figure 46. Programmable Current Source
R1' and R2' must be equal to R1 and R2
A
+ R2
B
, respectively.
Theoretically, R2
B
can be made as small as needed to achieve
the current needed within A2 output current driving capability.
In the example shown in Figure 46, OP2177 is able to deliver
a maximum of 10 mA. Because the current pump uses both
positive and negative feedback, capacitors C1 and C2 are needed
to ensure that negative feedback prevails and, therefore, avoiding
oscillation. This circuit also allows bidirectional current flow if
the inputs V
A
and V
B
of the digital potentiometer are supplied
with the dual-polarity references as previously shown.
PROGRAMMABLE DAC REFERENCE VOLTAGE
With a multichannel DAC, such as the quad, 12-bit voltage
output AD7398, one of its internal DACs, and an ADR42x
voltage reference can be used as a common programmable
V
REF
x for the rest of the DACs. The circuit configuration is
shown in Figure 47. The relationship of V
REF
x to V
REF
depends
on the digital code and the ratio of R1 and R, and is given by






×+






+×
=
R1
R2D
R1
R2
V
xV
N
REF
REF
2
1
1
(5)
where:
D is the decimal equivalent of input code.
N is the number of bits.
V
REF
is the applied external reference.
V
REF
x is the reference voltage for DACs A to D.
Table 9. V
REF
x vs. R1 and R2
R1, R2 Digital Code V
REF
R1 = R2 0000 0000 0000 2 V
REF
R1 = R2 1000 0000 0000 1.3 V
REF
R1 = R2 1111 1111 1111 V
REF
R1 = 3R2
0000 0000 0000
4 V
REF
R1 = 3R2 1000 0000 0000 1.6 V
REF
R1 = 3R2 1111 1111 1111 V
REF
02432-049
V
IN
DACA
DACB
DACC
DACD
AD7398
ADR425
V
REF
V
REF
A
V
OUT
A
R1
±0.1%
R2
±0.1%
V
OB
= V
REF
x (D
B
)
V
OC
= V
REF
x (D
C
)
V
OD
= V
REF
x (D
D
)
V
OUT
B
V
OUT
C
V
OUT
D
V
REF
B
V
REF
C
V
REF
D
Figure 47. Programmable DAC Reference