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
⎜⎛ R2A + R2B ⎟⎞
IL = ⎝
R1
R2 B
⎠ × VW
(3)
and
VW
=
D
2N
× VREF
(4)
where:
D is the decimal equivalent of the input code.
N is the number of bits.
C1
10pF
VDD
2
VIN TRIM 5
U1
ADR425
GND VOUT 6
4
AD5232
U2
DIGITAL POT
A
U2
BW
VDD
V+
A1
OP2177
V–
R1'
50kΩ
C2
10pF
R1
50kΩ
R2'
1kΩ
VDD
V+
A2
OP2177
V–
R2B
10Ω
VSS
R2A
1kΩ
VSS
VL
LOAD
IL
Figure 47. Programmable Current Source
R1' and R2' must be equal to R1 and R2A + R2B,B respectively.
Theoretically, R2BB can be made as small as needed to achieve
the current needed within A2 output current driving capability.
In the example shown in Figure 47, 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 VA and VBB of the digital potentiometer are supplied
with the dual-polarity references as previously shown.
ADR420/ADR421/ADR423/ADR425
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
VREFx for the rest of the DACs. The circuit configuration is
shown in Figure 48. The relationship of VREFx to VREF depends
on the digital code and the ratio of R1 and R, and is given by
VREF x
=
VREF
×
⎜⎛1 +
⎝
R2 ⎟⎞
R1 ⎠
⎜⎛1 + D × R2 ⎟⎞
(5)
⎝ 2N R1 ⎠
where:
D is the decimal equivalent of input code.
N is the number of bits.
VREF is the applied external reference.
VREFx is the reference voltage for DACs A to D.
Table 9. VREFx vs. R1 and R2
R1, R2
Digital Code
R1 = R2
0000 0000 0000
R1 = R2
1000 0000 0000
R1 = R2
1111 1111 1111
R1 = 3R2
0000 0000 0000
R1 = 3R2
1000 0000 0000
R1 = 3R2
1111 1111 1111
VREF
2 VREF
1.3 VREF
VREF
4 VREF
1.6 VREF
VREF
VREFA
DACA
VREFB
DACB
VOUTA
R1
±0.1%
R2
±0.1%
VREF
VIN ADR425
VOUTB
VOB = VREFx (DB)
VREFC
DACC
VOUTC
VOC = VREFx (DC)
VREFD
DACD
VOUTD
AD7398
VOD = VREFx (DD)
Figure 48. Programmable DAC Reference
Rev. H | Page 19 of 24