ADR420/ADR421/ADR423/ADR425
Dual Polarity References
VIN
1F 0.1F 2
VIN
VOUT 6
+5V
ADR425
U1
R1
10k⍀
R2
10k⍀
+10V
GND
TRIM 5
4
V+
OP1177
–5V
U2
R3
5k⍀
V–
–10V
Figure 8. +5 V and –5 V Reference Using ADR425
+2.5V
+10V
2
VIN
VOUT 6
ADR425
U1
R1
5.6k⍀
GND
4
TRIM 5
R2
5.6k⍀
–2.5V
V+
OP1177
U2
V–
–10V
Figure 9. +2.5 V and –2.5 V Reference Using ADR425
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
C1
10pF
VDD
2
VIN TRIM 5
ADR425
U1
GND VOUT 6
4
AD5232
U2
DIGITAL POT
A
U2
BW
VDD
V+
OP2177
A1
V–
R1’
50k⍀
C2
10pF
R1
50k⍀
R2’
1k⍀
VDD
V+
OP2177
A2
V–
VSS
R2A
1k⍀
R2B
10⍀
VSS
VL
LOAD
IL
Figure 10. Programmable Current Source
Together with a digital potentiometer and a Howland current
pump, ADR425 forms the reference source for a programmable
current as
R2A + R2B
IL =
R1
R2B
× VW
(3)
and
D
V
W
=
2N
×V
REF
(4)
where
D = Decimal Equivalent of the Input Code
N = Number of Bits
In addition, R1' and R2' must be equal to R1 and R2A + R2B,
respectively. R2B in theory can be made as small as needed to
achieve the current needed within A2 output current driving
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
VA and VB of the digital potentiometer are supplied with the
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 VREFX for the
rest of the DACs. The circuit configuration is shown in Figure 11.
The relationship of VREFX to VREF depends upon the digital code
and the ratio of R1 and R2 and is given by:
VREFX
VREF
=
×
1
+
R2
R1
D R2
1 + 2N × R1
(5)
where
D = Decimal Equivalent of Input Code and
N = Number of Bits
VREF = Applied External Reference
VREFX = Reference Voltage for DAC A to D
–14–
REV. B