AD5259

Data Sheet

Rev. C | Page 14 of 24

THEORY OF OPERATION

The AD5259 is a 256-position digitally-controlled variable

resistor (VR) device. EEPROM is pre-loaded at midscale from

the factory, and initial power-up is, accordingly, at midscale.

PROGRAMMING THE VARIABLE RESISTOR

Rheostat Operation

and Terminal B is available in 5 kΩ, 10 kΩ, 50 kΩ, and 100 kΩ.

The nominal resistance of the VR has 256 contact points accessed

by the wiper terminal. The 8-bit data in the RDAC latch is

decoded to select one of 256 possible settings.

A

W

B

A

W

B

A

W

B

Figure 38. Rheostat Mode Configuration

The general equation determining the digitally programmed

output resistance between Wiper W and Terminal B is

W

AB

WB

R

R

D

D

R

×

+

×

=

2

256

)

(

(1)

where:

D is the decimal equivalent of the binary code loaded in the

8-bit RDAC register.

each internal switch.

D5

D4

D3

D7

D6

D2

D1

D0

RDAC

LATCH

AND

DECODER

A

W

B

Figure 39. AD5259 Equivalent RDAC Circuit

In the zero-scale condition, there is a relatively low value finite

wiper resistance. Care should be taken to limit the current flow

between Wiper W and Terminal B in this state to a maximum

pulse current of no more than 20 mA. Otherwise, degradation

or destruction of the internal switch contact can occur.

Similar to the mechanical potentiometer, the resistance of the

RDAC between Wiper W and Terminal A produces a digitally

decreases as the data loaded in the latch increases in value.

The general equation for this operation is

W

AB

WA

R

R

D

D

R

×

+

×

−

=

2

256

256

)

(

(2)

Typical device-to-device matching is process lot dependent and

may vary by up to ±30%. For this reason, resistance tolerance is

stored in the EEPROM, enabling the user to know the actual

PROGRAMMING THE POTENTIOMETER DIVIDER

Voltage Output Operation

The digital potentiometer easily generates a voltage divider at

Wiper W to Terminal B and Wiper W to Terminal A propor-

tional to the input voltage at Terminal A to Terminal B. Unlike

across Terminal A to Terminal B, Wiper W to Terminal A, and

Wiper W to Terminal B can be at either polarity.

A

W

B

Figure 40. Potentiometer Mode Configuration

If ignoring the effect of the wiper resistance for approximation,

connecting the A terminal to 5 V and the B terminal to ground

produces an output voltage at Wiper W to Terminal B starting

at 0 V up to 1 LSB less than 5 V. The general equation defining

input voltage applied to Terminal A and Terminal B is

B

A

W

V

D

V

D

D

V

256

256

256

)

(

−

+

=

(3)

A more accurate calculation, which includes the effect of wiper

B

AB

WA

A

AB

WB

W

V

R

D

R

V

R

D

R

D

V

)

(

)

(

)

(

+

=

(4)

Operation of the digital potentiometer in the divider mode

results in a more accurate operation over temperature. Unlike

the rheostat mode, the output voltage is dependent mainly

the absolute values.