ADR423 Analog Devices, ADR423 Datasheet - Page 14

• Manufacturer Part Number: ADR423
• Description: Ultraprecision Low Noise/ 2.048 V/2.500 V/ 3.00 V/5.00 V XFET Voltage References
• Manufacturer: Analog Devices

Dual Polarity References
Dual polarity references can easily be made with an op amp and
a pair of resistors. In order not to defeat the accuracy obtained
by ADR42x, it is imperative to match the resistance tolerance as
well as the temperature coefficient of all the components.
Programmable Current Source
ADR420/ADR421/ADR423/ADR425
V
GND
V
ADR425
DD
2
IN
4
V
IN
U1
TRIM
V
OUT 6
1 F
+10V
5
DIGITAL POT
AD5232
U2
0.1 F
A
B
U2
GND
V
W
2
IN
4
ADR425
GND
V
2
IN
4
U1
ADR425
OP2177
V
V
–
V+
V–
TRIM
V
DD
SS
A1
U1
OUT
V
TRIM
OUT
6
5
5.6k
5.6k
6
5
R1
R2
–2.5V
5k
10k
+2.5V
R3
50k
R1
R1’
10pF
50k
C2
R1
OP1177
OP1177
+10V
–10V
V+
V–
U2
–10V
U2
V+
V–
OP2177
VL
V
V
V+
V–
10k
DD
SS
10pF
A2
1k
R2
R2’
C1
1k
R2
A
LOAD
+5V
–5V
R2
10
B
IL
Together with a digital potentiometer and a Howland current
pump, ADR425 forms the reference source for a programmable
current as
and
where
In addition, R1' and R2' must be equal to R1 and R2
respectively. R2
achieve the current needed within A
capability. In this example, OP2177 is able to deliver a maxi-
mum of 10 mA. Since the current pump employs both positive and
negative feedback, capacitors C1 and C2 are needed to ensure the
negative feedback prevails and, therefore, avoids oscillation.
This circuit also allows bidirectional current flow if the inputs
V
dual polarity references as shown previously.
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 ADR42x voltage
reference can be served as a common programmable V
rest of the DACs. The circuit configuration is shown in Figure 11.
The relationship of V
and the ratio of R1 and R2 and is given by:
where
N = Number of Bits
A
D = Decimal Equivalent of Input Code and
N = Number of Bits
V
V
D = Decimal Equivalent of the Input Code
REF
REFX
V
and V
REFX
= Applied External Reference
= Reference Voltage for DAC A to D
B
=
of the digital potentiometer are supplied with the
V



REF
1
B
+
in theory can be made as small as needed to
×
2
D
N



1
I
REFX
×
L
+
=
V
R
R
R
R
W
2
1
R
2
1
to V



2
=



A
R
2
R
D
REF
+
N
2
1
B
R
×
depends upon the digital code
2
V
B
REF
2
output current driving
×
V
W
REFX
A
+ R2
for the
(3)
(4)
(5)
B
,