AD5227BUJZ50-R2 datasheet(10/16 Pages) AD

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AD5227

Rev. B | Page 10 of 1

THEORY OF OPERATION

The AD5227 is a 64-position 3-terminal digitally controlled

potentiometer device. It presets to a midscale at system power-

on. When CS is enabled, changing the resistance settings is

achieved by clocking the CLK pin. It is negative-edge triggered,

and the direction of stepping is determined by the state of the

U/D input. When the wiper reaches the maximum or the

minimum setting, additional CLK pulses do not change the

wiper setting.

U/D

CLK

GND

VDD

6-BIT UP/DOWN

CONTROL

LOGIC

POR

MIDSCALE

WIPER

AD5227

Figure 24. Functional Block Diagram

RS = RAB/64

RDAC

UP/DOWN

CTRL AND

DECODE

Figure 25. AD5227 Equivalent RDAC Circuit

PROGRAMMING THE DIGITAL POTENTIOMETERS

Rheostat Operation

If only the W-to-B or W-to-A terminals are used as variable

resistors, the unused terminal can be opened or shorted with W.

This operation is called rheostat mode and is shown in Figure 26.

Figure 26. Rheostat Mode Configuration

The end-to-end resistance, RAB, has 64 contact points accessed

by the wiper terminal, plus the B terminal contact, assuming

that RWB is used (see Figure 25). Clocking the CLK input steps,

RWB by one step. The direction is determined by the state of

U/D pin. The change of RWB can be determined by the number

of clock pulses, provided that the AD5227 has not reached its

maximum or minimum scale. ΔRWB can, therefore, be

approximated as

⎟

⎠

⎞

⎜

⎝

⎛

(1)

where:

CP is the number of clock pulses.

RAB is the end-to-end resistance.

RW is the wiper resistance contributed by the on-resistance of

the internal switch.

Since in the lowest end of the resistor string a finite wiper

resistance is present, care should be taken to limit the current

flow between W and B in this state to a maximum pulse current

of no more than 20 mA. Otherwise, degradation or possible

destruction of the internal switches can occur.

Similar to the mechanical potentiometer, the resistance of the

RDAC between the Wiper W and Terminal A also produces a

digitally controlled complementary resistance, RWA. When these

terminals are used, the B terminal can be opened or shorted to

W. Similarly, ΔRWA can be approximated as

()

⎟

⎠

⎞

⎜

⎝

⎛

−

(2)

Equations 1 and 2 do not apply when CP = 0.

The typical distribution of the resistance tolerance from device

to device is process lot dependent. It is possible to have ±20%

tolerance.

Potentiometer Mode Operation

If all three terminals are used, the operation is called

potentiometer mode. The most common configuration is the

voltage divider operation as shown in Figure 27.

Figure 27. Potentiometer Mode Configuration

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