AD5253 datasheet(24/32 Pages) AD

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AD5253/AD5254

Data Sheet

Rev. C | Page 24 of 32

DIGITAL POTENTIOMETER OPERATION

The structure of the RDAC is designed to emulate the

performance of a mechanical potentiometer. The RDAC

contains a string of resistor segments with an array of analog

switches that act as the wiper connection to the resistor array.

The number of points is the resolution of the device. For

example, the AD5253/AD5254 emulate 64/256 connection

points with 64/256 equal resistance, RS, allowing them to

provide better than 1.5%/0.4% resolution.

Figure 41 provides an equivalent diagram of the connections

between the three terminals that make up one channel of the

RDAC. Switches SWA and SWB are always on, but only one of

switches SW(0) to SW(2N–1) can be on at a time (determined by

the setting decoded from the data bit). Because the switches are

nonideal, there is a 75 Ω wiper resistance, RW. Wiper resistance

is a function of supply voltage and temperature: Lower supply

voltages and higher temperatures result in higher wiper

resistances. Consideration of wiper resistance dynamics is

important in applications in which accurate prediction of

output resistance is required.

SWA

SW (2N – 1)

SW (2N – 2)

SW(1)

DIGITAL

CIRCUITRY

OMIITTED FOR

CLARITY

RDAC

WIPER

AND

DECODER

SW(0)

SWB

RS = RAB/2N

Figure 41. Equivalent RDAC Structure

PROGRAMMABLE RHEOSTAT OPERATION

If either the W-to-B or W-to-A terminal is used as a variable

resistor, the unused terminal can be opened or shorted with W;

such operation is called rheostat mode (see Figure 42). The

resistance tolerance can range ±20%.

Figure 42. Rheostat Mode Configuration

The nominal resistance of the AD5253/AD5254 has 64/256

contact points accessed by the wiper terminal, plus the B

terminal contact. The 6-/8-bit data-word in the RDAC register

is decoded to select one of the 64/256 settings. The wiper’s first

connection starts at the B terminal for Data 0x00. This B termi-

nal connection has a wiper contact resistance, RW, of 75 Ω,

regardless of the nominal resistance. The second connection

(the AD5253 10 kΩ part) is the first tap point where RWB = 231 Ω

(RWB = RAB/64 + RW = 156 Ω + 75 Ω) for Data 0x01, and so on.

Each LSB data value increase moves the wiper up the resistor

ladder until the last tap point is reached at RWB = 9893 Ω. See

Figure 41 for a simplified diagram of the equivalent RDAC circuit.

The general equation that determines the digitally programmed

output resistance between W and B is

AD5253: RWB(D) = (D/64) × RAB + 75 Ω

(1)

AD5254: RWB(D) = (D/256) × RAB + 75 Ω

(2)

where:

D is the decimal equivalent of the data contained in the

RDAC latch.

RAB is the nominal end-to-end resistance.

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