ADR430 Analog Devices, Inc., ADR430 Datasheet - Page 18

• Manufacturer Part Number: ADR430
• Description: Ultralow Noise Xfet Voltage References With Current Sink And Source Capability
• Manufacturer: Analog Devices, Inc.

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, R1' and R2' must be equal to R1 and (R2
respectively. In theory, R2
achieve the necessary current within the A2 output current
driving capability. In this example, 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
digital potentiometer are supplied with the dual-polarity
references, as shown in Figure 38.
V
GND
V
ADR435
DD
2
IN
4
U1
V
I
L
V
TRIM
W
OUT
=
=
⎛
⎜
⎜
⎜
⎜
⎝
5
R2
2
6
D
N
A
R
×
R1
Figure 38. Programmable Current Source
AD5232
+
2
V
B
A
B
R
REF
U2
2
B
W
⎞
⎟
⎟
⎟
⎟
⎠
×
V
V
V
OP2177
B
W
V+
V–
DD
SS
can be made as small as needed to
A1
50kΩ
10pF
50kΩ
R1'
C2
R1
A
and V
VL
V
V
OP2177
+
–
V+
V–
DD
SS
10pF
1kΩ
B
R2'
A2
C1
inputs of the
R2
1kΩ
A
A
+ R2
I
L
R2
10Ω
B
B
),
Rev. D | Page 18 of 24
I
L
(3)
(4)
PROGRAMMABLE DAC REFERENCE VOLTAGE
By employing 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
configuration is shown in Figure 39.
The relationship of V
and the ratio of R1 and R2, given by
where:
D is the decimal equivalent of the 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
⎛
⎜
⎝
Figure 39. Programmable DAC Reference
REF
1
vs. R1 and R2
+
REFX
AD7398
2
×
D
Digital Code
0000 0000 0000
1000 0000 0000
1111 1111 1111
0000 0000 0000
1000 0000 0000
1111 1111 1111
N
⎛ +
⎜
⎝
for the rest of the DACs. The circuit
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
ADR43x
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
(D
(D
(D
REF
REF
C
D
B
)
)
REF
REF
)
(5)