ADR421ARMZ datasheet(19/24 Pages) AD

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ADR420/ADR421/ADR423/ADR425

Rev. I | Page 19 of 24

PROGRAMMABLE CURRENT SOURCE

Together with a digital potentiometer and a Howland current

pump, the ADR425 forms the reference source for a program-

mable current as

⎟

⎠

⎞

⎜

⎝

⎛

(3)

and

REF

(4)

where:

D is the decimal equivalent of the input code.

N is the number of bits.

ADR425

VOUT

VIN

VDD

VSS

TRIM

GND

AD5232

DIGITAL POT

V+

V–

OP2177

VDD

VSS

V+

V–

OP2177

LOAD

50kΩ

R2A

1kΩ

R2B

10Ω

10pF

R2'

1kΩ

R1'

50kΩ

10pF

Figure 46. Programmable Current Source

R1' and R2' must be equal to R1 and R2A + R2B, respectively.

Theoretically, R2B can be made as small as needed to achieve

the current needed within A2 output current driving capability.

In the example shown in Figure 46, OP2177 is able to deliver

a maximum of 10 mA. Because the current pump uses both

positive and negative feedback, capacitors C1 and C2 are needed

to ensure that negative feedback prevails and, therefore, avoiding

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 previously shown.

PROGRAMMABLE DAC REFERENCE VOLTAGE

With a multichannel DAC, such as the quad, 12-bit voltage

output AD7398, one of its internal DACs, and an ADR42x

voltage reference can be used as a common programmable

VREFx for the rest of the DACs. The circuit configuration is

shown in Figure 47. The relationship of VREFx to VREF depends

on the digital code and the ratio of R1 and R, and is given by

⎟

⎠

⎞

⎜

⎝

⎛

⎟

⎠

⎞

⎜

⎝

⎛ +

REF

(5)

where:

D is the decimal equivalent of input code.

N is the number of bits.

VREF is the applied external reference.

VREFx is the reference voltage for DACs A to D.

Table 9. VREFx vs. R1 and R2

R1, R2

Digital Code

VREF

R1 = R2

0000 0000 0000

2 VREF

R1 = R2

1000 0000 0000

1.3 VREF

R1 = R2

1111 1111 1111

VREF

R1 = 3R2

0000 0000 0000

4 VREF

R1 = 3R2

1000 0000 0000

1.6 VREF

R1 = 3R2

1111 1111 1111

VREF

VIN

DACA

DACB

DACC

DACD

AD7398

ADR425

VREF

VREFA

VOUTA

±0.1%

VOB = VREFx (DB)

VOC = VREFx (DC)

VOD = VREFx (DD)

VOUTB

VOUTC

VOUTD

VREFB

VREFC

VREFD

Figure 47. Programmable DAC Reference

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