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AD5280/AD5282

Rev. C | Page 21 of

8-BIT BIPOLAR DAC

VIN

VOUT

+5VREF

U2 –

–15V

–5VREF

–15V

–

OP2177

–

OP2177

TRIM

ADR425

GND

AD5280

+15V

Figure 58. 8-Bit Bipolar DAC

Figure 58 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 value codes, are skewed by the effects of the digital potenti-

ometer wiper resistance. The output of this circuit is

REF

⎟

⎠

⎜

⎝

−

256

⎞

⎛ 2

(6)

BIPOLAR PROGRAMMABLE GAIN AMPLIFIER

VDD

VS8

–kVI

V+

V–

–

OP2177

V+

V–

–

OP2177

AD5282

Figure 59. Bipolar Programmable Gain Amplifier

For applications that require bipolar gain, Figure 59 shows one

implementation similar to the previous circuit. The 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 noninverting mode allows

linear gain and attenuation. The transfer function is

()

⎟

⎠

⎜

⎝

−

⎟

⎠

⎜

⎝

256

⎞

⎛

⎞

⎛

(7)

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

As in the previous example, in the simpler and more common

case where K = 1, a single digital AD5280 potentiometer is

used. U1 is replaced by a matched pair of resistors to apply

Vi and −Vi at the ends of the digital potentiometer. The

relationship becomes

⎟

⎠

⎞

⎜

⎝

⎛

−

⎟

⎠

⎞

⎜

⎝

⎛ +

256

(7)

If R2 is large, a compensation capacitor having a few pF may be

needed to avoid any gain peaking.

Table 7 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 a 256-step

resolution.

Table 7. Result of Bipolar Gain Amplifier

R1 = ∞, R2 = 0

R1 = R2

R2 = 9R1

−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 adjustments, such as a

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

be considered (see Figure 60).

U1 = AD5280

A1 = AD8501, AD8605, AD8541

N1 = FDV301N, 2N7002

RBIAS

V+

V–

–

SIGNAL

Figure 60. Programmable Booster Voltage Source

In this circuit, the inverting input of the op amp forces the

VBIAS to be equal to the wiper voltage set by the digital potenti-

ometer. The load current is then delivered by the supply via the

N-channel FET N1. The N1 power handling must be adequate

to dissipate (Vi – VO) × IL power. This circuit can source a

maximum of 100 mA with a 5 V supply. A1 needs to be a rail-

to-rail input type. For precision applications, a voltage reference

such as ADR423, ADR292, or AD1584 can be applied at the

input of the digital potentiometer.

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