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參數資料
| 型號： | ADR439BR-REEL7 |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 基準電壓源/電流源 |
| 英文描述： | Ultralow Noise XFET Voltage References with Current Sink and Source Capability |
| 中文描述： | 1-OUTPUT THREE TERM VOLTAGE REFERENCE, 4.5 V, PDSO8 |
| 封裝： | MS-012AA, SOIC-8 |
| 文件頁數： | 18/24頁 |
| 文件大小： | 868K |
| 代理商： | ADR439BR-REEL7 |
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
Rev. B | Page 18 of 24
W
B
B
A
L
V
R
R1
2
R
R2
I
×
+
=
2
(3)
and
REF
N
W
V
D
V
×
=
2
(4)
where:
D
is the decimal equivalent of the input code.
N
is the number of bits.
In addition,
and
1
R
′
R
′
respectively.
R2
B
in theory can be made as small as needed to
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
A
and V
B
of the digital potentiometer are supplied
with the dual polarity references, as shown previously.
must be equal to
R1
and
R2
2
A
+
R2
B
,
GND
6
2
4
5
V
IN
V
DD
V
OUT
GND
TRIM
C2
10pF
U1
V+
V–
IL
U2
ADR435
OP2177
R1
50k
OP2177
V–
V+
LOAD
V
DD
AD5232
U2
DIGITAL
POTENTIOMETER
W
A
B
V
SS
R1'
50k
R2'
1k
R2
A
1k
R2
B
10
V
DD
V
SS
C1
10pF
+
VL
–
0
Figure 38. Programmable Current Source
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
REFX
for the
rest of the DACs. The circuit configuration is shown in Figure 39.
V
REFA
DAC A
V
REFB
DAC B
V
REFC
DAC C
V
REFD
DAC D
V
OUTA
V
OUTB
V
OUTC
V
OUTD
V
OB
= V
REFX
(D
B
)
V
OC
= V
REFX
(D
C
)
V
OD
= V
REFX
(D
D
)
ADR436
AD7398
V
IN
V
REF
R1
±
0.1%
R2
±
0.1%
0
Figure 39. Programmable DAC Reference
The relationship of V
REFX
to V
REF
depends on the digital code
and the ratio of R1 and R2, and is given by
×
+
1
×
=
R1
R2
D
N
2
R1
R2
V
V
REF
REFX
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
REFX
is the reference voltage for DAC A to DAC D.
Table 10. V
REFX
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
V
REF
2 V
REF
1.3 V
REF
V
REF
4 V
REF
1.6 V
REF
V
REF
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