AD5263BRUZ200-R7 datasheet(23/28 Pages) AD

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Data Sheet

AD5263

Rev. F | Page 23 of 28

APPLICATIONS INFORMATION

BIPOLAR DC OR AC OPERATION FROM DUAL

SUPPLIES

The AD5263 can be operated from dual supplies, enabling

control of ground referenced ac signals or bipolar operation.

The ac signal, as high as VDD/VSS, can be applied directly across

Terminal A to Terminal B, with the output taken from Terminal W.

GND

SCLK

VDD

AD5263

µC

GND

MCSI

SDA

SCL

VDD

VSS

+5.0V

±5V p-p

±2.5V p-p

5.0V

D = 0x90

Figure 57. Bipolar Operation from Dual Supplies

GAIN CONTROL COMPENSATION

A digital potentiometer is commonly used in gain control such

as the noninverting gain amplifier shown in Figure 58.

47kΩ

25pF

200kΩ

4.7pF

Figure 58. Typical Noninverting Gain Amplifier

Notice the RDAC B terminal parasitic capacitance is connected to

the op amp noninverting node. It introduces a zero for the 1/βo

term with +20 dB/dec, whereas a typical op amp GBP has

−20 dB/dec characteristics. A large R2 and finite C1 can cause

this zero’s frequency to fall well below the crossover frequency.

Thus, the rate of closure becomes 40 dB/dec and the system has

0° phase margin at the crossover frequency. The output may ring or

oscillate if the input is a rectangular pulse or step function.

Similarly, it is also likely to ring when switching between two gain

values, because this is equivalent to a step change at the input.

Depending on the op amp GBP, reducing the feedback resistor

may extend the zero’s frequency far enough to overcome the

problem. A better approach is to include a compensation capacitor

C2 to cancel the effect caused by C1. Optimum compensation

occurs when R1 × C1 = R2 × C2. This is not an option, because

of the variation of R2. As a result, one may use the relationship

described and scale C2 as if R2 is at its maximum value. Doing

so may overcompensate and compromise the performance slightly

when R2 is set at low values. However, it avoids the gain peaking,

ringing, or oscillation in the worst case. For critical applications,

C2 should be found empirically to suit the need. In general, C2

in the range of a few pF to no more than a few tenths of pF is

usually adequate for the compensation.

Similarly, there are W and A terminal capacitances connected to

the output (not shown); fortunately, their effect at this node is less

significant and the compensation can be disregarded in most cases.

PROGRAMMABLE VOLTAGE REFERENCE

For voltage divider mode operation (Figure 59), it is common to

buffer the output of the digital potentiometer unless the load is

much larger than RWB. Not only does the buffer serve the purpose

of impedance conversion, but it also allows a heavier load to be

driven.

VIN

GND

AD8601

VOUT

AD5263

AD1582

Figure 59. Programmable Voltage Reference

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