ADR431B AD [Analog Devices], ADR431B Datasheet - Page 18

• Manufacturer Part Number: ADR431B
• Description: Ultralow Noise XFET Voltage References with Current Sink and Source Capability
• Manufacturer: AD [Analog Devices]

ADR430/ADR431/ADR433/ADR434/ADR435/ADR439
PROGRAMMABLE CURRENT SOURCE
Together with a digital potentiometer and a Howland current
pump, ADR435 forms the reference source for a programmable
current as
and
where:
D is the decimal equivalent of the input code.
N is the number of bits.
In addition,
respectively. R2
achieve the necessary current within the A2 output current
driving capability. In this example, OP2177 can deliver a maxi-
mum of 10 mA. Because the current pump employs both positive
and negative feedback, capacitors C1 and C2 are needed to
ensure that the negative feedback prevails and, therefore, avoids
oscillation. This circuit also allows bidirectional current flow if
the inputs V
with the dual polarity references, as shown previously.
V
GND
V
ADR435
DD
2
IN
4
U1
V
I
V
L
TRIM
W
OUT
=
=
⎛
⎜
⎜
⎜
⎜
⎝
5
6
R2
2
D
POTENTIOMETER
A
N
R ′
A
and V
R
1
U2
×
R1
Figure 38. Programmable Current Source
DIGITAL
B
+
2
AD5232
V
A
B
B
and
in theory can be made as small as needed to
R
REF
U2
2
W
B
B
of the digital potentiometer are supplied
R ′
⎞
⎟
⎟
⎟
⎟
⎠
×
2
V
V
V
OP2177
V+
V–
DD
W
SS
must be equal to R1 and R2
A1
50kΩ
10pF
50kΩ
R1'
C2
R1
VL
V
OP2177
V
+
–
V+
V–
DD
SS
10pF
1kΩ
R2'
A2
C1
R2
1kΩ
A
LOAD
A
GND
+ R2
R2
10Ω
B
IL
B
Rev. B | Page 18 of 24
,
(3)
(4)
PROGRAMMABLE DAC REFERENCE VOLTAGE
With a multichannel DAC such as a quad 12-bit voltage output
DAC AD7398, one of its internal DACs and an ADR43x voltage
reference can be used as a common programmable V
rest of the DACs. The circuit configuration is shown in Figure 39.
The relationship of V
and the ratio of R1 and R2, and is given by
where:
D is the decimal equivalent of input code.
N is the number of bits.
V
V
Table 10. V
R1, R2
R1 = R2
R1 = R2
R1 = R2
R1 = 3R2
R1 = 3R2
R1 = 3R2
REF
REFX
is the applied external reference.
V
is the reference voltage for DAC A to DAC D.
REFX
DAC A
DAC B
DAC C
DAC D
V
V
V
V
REFA
REFB
REFC
REFD
=
REFX
V
⎛
⎜
⎝
REF
Figure 39. Programmable DAC Reference
1
vs. R1 and R2
+
AD7398
2
×
Digital Code
0000 0000 0000
1000 0000 0000
1111 1111 1111
0000 0000 0000
1000 0000 0000
1111 1111 1111
D
N
⎛ +
⎜
⎝
REFX
1
×
R2
R1
R2
R1
to V
⎞
⎟
⎠
⎞
⎟
⎠
REF
V
V
V
V
OUTA
OUTB
OUTC
OUTD
depends on the digital code
V
R1 ± 0.1%
IN
V
V
V
ADR436
OC
OD
OB
= V
= V
= V
R2
± 0.1%
V
REFX
REFX
REFX
REF
V
2 V
1.3 V
V
4 V
1.6 V
V
REF
REF
REF
REF
REF
(D
(D
(D
REFX
REF
REF
C
D
B
)
)
)
for the
(5)