AD5262 datasheet(15/20 Pages) AD

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REV. 0

–15–

AD5260/AD5262

VIN

GND

VOUT

AD1582

AD8601

AD5260

Figure 15. Programmable Voltage Reference

8-Bit Bipolar DAC

Figure 16 shows a low cost 8-bit bipolar DAC. It offers the same

number of adjustable steps but not the precision of conventional

DACs. The linearity and temperature coefficients, especially at low

values codes, are skewed by the effects of the digital potentiometer

wiper resistance. The output of this circuit is:

REF

ËÁ

¯˜

256

(4)

5VREF

OP2177

–5V

OP2177

+5V

–5V

+5V

+5VREF

VIN

GND

VOUT

TRIM

AD5260

ADR425

Figure 16. 8-Bit Bipolar DAC

Bipolar Programmable Gain Amplifier

For applications that require bipolar gain, Figure 17 shows one

implementation. Digital potentiometer U1 sets the adjustment

range. The wiper voltage at W2 can therefore be programmed

between Vi and –KVi at a given U2 setting. Configuring A2 in

the noninverting mode allows linear gain and attenuation. The

transfer function is:

ËÁ

¯˜

¥¥ +

() -

ËÁ

¯˜

256

(5)

where K is the ratio of RWB1/RWA1 set by U1.

–KVi

OP2177

VDD

VSS

VDD

VSS

OP2177

AD5262

Figure 17. Bipolar Programmable Gain Amplifier

Similar to the previous example, in the simpler (and much more

usual) case, where K = 1, a single digital pot AD5260, and U1

is replaced by a matched pair of resistors to apply Vi and – Vi at

the ends of the digital pot. The relationship becomes:

ËÁ

¯˜

ËÁ

¯˜

256

(6)

If R2 is large, a few picofarad compensation capacitors may be

needed to avoid any gain peaking.

Table VIII shows the result of adjusting D, with A2 configured as a

unity gain, a gain of 2, and a gain of 10. The result is a bipolar

amplifier with linearly programmable gain and 256-step resolution.

Table VIII. Result of Bipolar Gain Amplifier

DR1 =

•, R2 = 0

R1 = R2

R2 = 9R1

0–1

–2

–10

–0.5

–1

–5

128

192

0.5

255

0.968

1.937

9.680

Programmable Voltage Source with Boosted Output

For applications that require high current adjustment such as a

laser diode driver or turnable laser, a boosted voltage source can

be considered (see Figure 18).

SIGNAL LO

10k

RBIAS

U1= AD5260

A1= AD8601, AD8605, AD8541

P1= FDP360P, NDS9430

N1= FDV301N, 2N7002

Figure 18. Programmable Boosted Voltage Source

In this circuit, the inverting input of the op amp forces the VO to be

equal to the wiper voltage set by the digital potentiometer. The

load current is then delivered by the supply via the P-Ch FET P1.

The N-Ch FET N1 simplifies the op amp driving requirement.

A1 needs to be the rail-to-rail input type. Resistor R1 is needed to

prevent P1 from not turning off once it is on. The choice of R1 is a

balance between the power loss of this resistor and the output turn-

off time. N1 can be any general-purpose signal FET; on the other

hand, P1 is driven in the saturation state, and therefore its power

handling must be adequate to dissipate (Vi – VO)

IL power. This

circuit can source a maximum of 100 mA at 5 V supply. Higher

current can be achieved with P1 in a larger package. Note, a single

N-Ch FET can replace P1, N1, and R1 altogether. However, the out-

put swing will be limited unless separate power supplies are used.

For precision application, a voltage reference such as ADR423,

ADR292, and AD1584 can be applied at the input of the digital

potentiometer.

Programmable 4-to-20 mA Current Source

A programmable 4-to-20 mA current source can be implemented

with the circuit shown in Figure 19. REF191 is a unique low

supply headroom and high current handling precision reference

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