ADR445ARMZ datasheet(16/20 Pages) AD

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ADR440/ADR441/ADR443/ADR444/ADR445

Rev. A | Page 16 of 20

PROGRAMMABLE VOLTAGE SOURCE

To obtain different voltages than those offered by the ADR44x,

some extra components are needed. In Figure 38, two

potentiometers are used to set the desired voltage, while the

buffering amplifier provides current drive. The potentiometer

connected between VOUT and GND, with its wiper connected to

the noninverting input of the operational amplifier, takes care of

coarse trim. The second potentiometer, with its wiper

connected to the trim terminal of the ADR44x, is used for fine

adjustment. Resolution depends on the end-to-end resistance

value and the resolution of the selected potentiometer.

ADR440/

ADR441/

ADR443/

ADR444/

ADR445

VIN

VOUT

GND

10kΩ

ADJ VREF

+VDD

10kΩ

Figure 38. Programmable Voltage Source

For a completely programmable solution, replace the two

potentiometers in Figure 38 with one Analog Devices dual

digital potentiometer, offered with either an SPI® or an I2C®

interface. These interfaces set the position of the wiper on both

potentiometers and allow the output voltage to be set. Table 9

lists compatible Analog Devices digital potentiometers.

Table 9. Digital Potentiometer Parts

Part No.

No. of

Channels

No. of

Positions

ITF

R (kΩ)

VDD1

AD5251

2.00

64.00

I2C

1, 10, 50, 100

5.5

AD5207

2.00

256.00

SPI

10, 50, 100

5.5

AD5242

2.00

256.00

I2C

10, 100, 1M

5.5

AD5262

2.00

256.00

SPI

20, 50, 200

AD5282

2.00

256.00

I2C

20, 50, 100

AD5252

2.00

256.00

I2C

1, 10, 50, 100

5.5

AD5232

2.00

256.00

SPI

10, 50, 100

5.5

AD5235

2.00

1024.00

SPI

25, 250

5.5

ADN2850

2.00

1024.00

SPI

25, 250

5.5

1 Can also use a negative supply.

Adding a negative supply to the operational amplifier allows

the user also to produce a negative programmable reference,

by connecting the reference output to the inverting terminal

of the operational amplifier. Choose feedback resistors to

minimize errors over temperature.

PROGRAMMABLE CURRENT SOURCE

It is possible to build a programmable current source using a

setup similar to the programmable voltage source, as shown in

Figure 39. The constant voltage on the gate of the transistor sets

the current through the load. Varying the voltage on the gate

changes the current. This circuit does not require a dual digital

potentiometer.

ADR440/

ADR441/

ADR443/

ADR444/

ADR445

VIN

VOUT

GND

ILOAD

VCC

RSENSE

AD5259

0.1µF

Figure 39. Programmable Current Source

HIGH VOLTAGE FLOATING CURRENT SOURCE

Use the circuit in Figure 40 to generate a floating current source

with minimal self heating. This particular configuration can

operate on high supply voltages, determined by the breakdown

voltage of the N-channel JFET.

ADR440/

ADR441/

ADR443/

ADR444/

ADR445

VIN

VOUT

GND

OP90

+VS

SST111

VISHAY

2N3904

–VS

Figure 40. Floating Current Source

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