AD5280 datasheet(20/28 Pages) AD

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AD5280/AD5282

Rev. C | Page 20 of

APPLICATIONS INFORMATION

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 unless C2 is scaled as if R2 were 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 at the worst case.

For critical applications, C2 should be found empirically to suit

the need. In general, C2 in the range of a few picofarads (pF) to

no more than a few tenths of a picofarad is usually adequate for

the compensation.

BIPOLAR DC OR AC OPERATION FROM DUAL

SUPPLIES

The AD5280/AD5282 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. See Figure 55 for a typical circuit connection.

AD5282

VDD

VSS

SCLK

SCL

SDA

±2.5V p-p

±5V p-p

D–80H

+5.0V

–5.0V

GND

MOSI

GND

MICROCONTROLLER

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 avoided in most cases.

15 V, 8-BIT I2C DAC

Figure 55. Bipolar Operation from Dual Supplies

GAIN CONTROL COMPENSATION

The digital potentiometer is commonly used in gain control

applications such as the noninverting gain amplifier shown in

Figure 56.

4.7pF

R1 C

25pF

47kΩ

200kΩ

Figure 56. Typical Noninverting Gain Amplifier

Notice that the RDAC B terminal parasitic capacitance is

connected to the op amp noninverting node. It introduces a 0

for the 1/βO term with 20 dB/decade (dec), whereas a typical op

amp GBP has −20 dB/dec characteristics. A large R2 and finite

C1 can cause the 0 frequency to fall well below the crossover

frequency. Thus the rate of closure becomes 40 dB/dec, and the

system has a 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.

AD5280

AD8512

V+

V–

AD8512

VDD

RBIAS

VDD

U1B

ADR512

200kΩ

U1A

Figure 57. 8-Bit I2C DAC

AD5280/AD5282 can be configured as a high voltage DAC, as

high as 15 V. The output is

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