REV. B

–14–

ADR420/ADR421/ADR423/ADR425

Dual Polarity References

GND

6

2

4

ADR425

U1

5

TRIM

1 F

0.1 F

R1

10k

+10V

–10V

OP1177

U2

V+

V–

–5V

R2

10k

+5V

R3

5k

Figure 8. +5 V and –5 V Reference Using ADR425

GND

6

2

4

ADR425

U1

5

TRIM

+10V

R1

5.6k

+2.5V

R2

5.6k

–2.5V

–10V

OP1177

U2

V+

V–

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

AD5232

U2

DIGITAL POT

A

BW

U2

R1

50k

C2

10pF

1k

10

R2’

1k

C1

10pF

R1’

50k

GND

2

4

ADR425

U1

5

TRIM

LOAD

IL

VL

OP2177

A1

V+

V–

OP2177

A2

V+

V–

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

I

RR

R

R

V

L

AB

B

W

=

+

×

22

1

2

(3)

and

V

D

V

W

N

REF

=

×

2

(4)

where

D = Decimal Equivalent of the Input Code

N = Number of Bits

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

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

rest of the DACs. The circuit configuration is shown in Figure 11.

and the ratio of R1 and R2 and is given by:

V

V

R

R

DR

R

REFX

REF

N

=

×+









+×









1

2

1

1

2

2

1

where

D = Decimal Equivalent of Input Code and

N = Number of Bits

(5)