AD5231 datasheet(19/24 Pages) AD

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REV. 0

AD5231

–19–

V+

V–

OP2177

AD5231

V+

V–

OP2177

AD5231

–kVi

VDD

VSS

VDD

VSS

Figure 23. Bipolar Programmable Gain Amplifier

In the simpler (and much more usual) case where K = 1, a pair

of matched resistors can replace U1. Equation 4 simplifies to:







 ×









1024

2 –

(5)

Table XVI 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 1024-step resolution.

Table XVI. Result of Bipolar Gain Amplifier

R1 =

, R2 = 0

R1 = R2 R2 = 9 R1

–1

–2

–10

256

–0.5

–1

–5

512

768

0.5

1023

0.992

1.984

9.92

10-Bit Bipolar DAC

If the circuit is changed in Figure 23 with the input taking from a

voltage reference and configure A2 as a buffer, a 10-bit bipolar DAC

can be realized. Compared to the conventional DAC, this circuit

offers comparable resolution but not the precision because of the wiper

resistance effects. Degradation of the nonlinearity and temperature

coefficient are prominent near both ends of the adjustment range.

On the other hand, this circuit offers a unique nonvolatile memory

feature which in some cases outweigh the shortfall of nonprecision.

The output of this circuit is:

REF









1024

–

(6)

V+

V–

AD8552

AD5231

V+

V–

AD8552

–2.5VREF

+2.5VREF

VIN VOUT

TRIM

GND

ADR421

+5V

–5V

+5V

Figure 24. 10-Bit Bipolar DAC

Programmable Voltage Reference

For programmable voltage divider mode operation (Figure 25)

it is common to buffer the output of the digital potentiometer

unless the load is much larger than the source resistance RWB. In

addition, the current handling of the digital potentiometer is

limited by its maximum operating voltage, power dissipation, and

the maximum current handling of the internal switches at a

given resistance (see TPC 20). As a result, the added buffer can

be used to deliver the current needed to the load as long as it is

within its current handling capability.

AD5231

V+

V–

AD8601

VIN VOUT

GND

AD1582

Figure 25. Programmable Voltage Reference

Programmable Voltage Source with Boosted Output

For applications such as laser diode driver or turnable laser,

requiring high current adjustment a boosted voltage source can

be considered (see Figure 26).

AD5231

V+

V–

VBIAS

RBIAS

SIGNAL CC

IBIAS

10k

A1 = AD8601, AD8605, AD8541

P1 = FDP360P, NDS9430

N1 = FDV301N, 2N7002

Figure 26. Boosted 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

P-Ch FET P1. The N-Ch FET N1 simplifies the op amp driving

requirement. Resistor R1 is needed to prevent P1 for 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 (VS – VBIAS)

IBIAS power. This circuit

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

can be achieved with P1 in larger package. Note a single N-Ch

FET can replace P1, N1, and R1 altogether. However, the output

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.

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