ADR430/ADR431/ADR433/ADR434/ADR435/ADR439

Rev. B | Page 18 of 24

PROGRAMMABLE CURRENT SOURCE

Together with a digital potentiometer and a Howland current

pump, ADR435 forms the reference source for a programmable

current as

W

B

B

A

L

V

R

R1

R

R2

I

×

⎟

⎟

⎟

⎟

⎠

⎞

⎜

⎜

⎜

⎜

⎝

⎛

+

=

2

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

must be equal to R1 and R2

1

R ′

2

R ′

A

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

with the dual polarity references, as shown previously.

GND

6

2

4

5

GND

TRIM

C2

10pF

U1

V+

V–

IL

U2

ADR435

OP2177

R1

50k

Ω

OP2177

V–

V+

A2

A1

LOAD

AD5232

U2

DIGITAL

POTENTIOMETER

W

A

B

R1'

50k

Ω

R2'

1k

Ω

1k

Ω

10

Ω

C1

10pF

+

VL

–

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

rest of the DACs. The circuit configuration is shown in Figure 39.

DAC A

DAC B

DAC C

DAC D

ADR436

AD7398

R1

± 0.1%

R2

± 0.1%

Figure 39. Programmable DAC Reference

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

⎟

⎠

⎞

⎜

⎝

⎛

×

+

⎟

⎠

⎞

⎜

⎝

⎛ +

×

=

R1

R2

D

R1

R2

V

V

N

REF

REFX

2

1

1

(5)

where:

D is the decimal equivalent of input code.

N is the number of bits.

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