REV. B

–9–

AD5241/AD5242

OPERATION

The AD5241/AD5242 provide a single-/dual-channel, 256-

position digitally controlled variable resistor (VR) device. The

terms VR, RDAC, and programmable resistor are commonly

used interchangeably to refer to digital potentiometer.

To program the VR settings, refer to the Digital Interface sec-

tion. Both parts have an internal power ON preset that places

the wiper in midscale during power-on, which simplifies the

fault condition recovery at power-up. In addition, the shutdown

SHDN Pin of AD5241/AD5242 places the RDAC in an almost

zero power consumption state where Terminal A is open circuited

and Wiper W is connected to Terminal B, resulting in only

leakage current being consumed in the VR structure. During

shutdown, the VR latch contents are maintained when the RDAC

is inactive. When the part is returned from shutdown, the stored

VR setting will be applied to the RDAC.

SW

N

2–1

R

R

SW

N

2–2

RDAC

LATCH

AND

DECODER

R

N

B

W

DIGITAL CIRCUITRY

OMITTED FOR CLARITY

A

R

R

D7

D6

D5

D4

D3

D2

D1

D0

SHDN

Figure 4. Equivalent RDAC Circuit

PROGRAMMING THE VARIABLE RESISTOR

Rheostat Operation

The nominal resistance of the RDAC between Terminals A and

B is available in 10 k

Ω, 100 kΩ, and 1 MΩ. The final two or

three digits of the part number determine the nominal resistance

value, e.g., 10 k

Ω = 10; 100 kΩ = 100; 1 MΩ = 1 M. The

accessed by the Wiper Terminal, plus the B Terminal con-

tact. The 8-bit data in the RDAC latch is decoded to select

one of the 256 possible settings. Assume a 10 k

Ω part is used;

the wiper’s first connection starts at the B Terminal for data

Ω wiper contact resistance, such con-

nection yields a minimum of 60

Ω resistance between Terminals

W and B. The second connection is the first tap point that cor-

responds to 99

138

Ω (39 × 2 + 60) for data 02

value increase moves the wiper up the resistor ladder until the

last tap point is reached at 10021

Ω [R

Figure 4 shows a simplified diagram of the equivalent RDAC

circuit where the last resistor string will not be accessed; there-

fore, there is 1 LSB less of the nominal resistance at full scale in

addition to the wiper resistance.

The general equation determining the digitally programmed

resistance between W and B is:

RD

D

RR

WB

AB

W

+

256

(1)

where:

D

is the decimal equivalent of the binary code between 0

and 255, which is loaded in the 8-bit RDAC register.

of the internal switch.

Ω and the A Terminal can be either open

be set for the following RDAC latch codes.

FREQUENCY – Hz

6

–36

–42

–48

–54

100k

10k

1k

100

–30

–24

–18

–12

–6

0

TPC 13. AD5242 100 k

Ω Gain vs. Frequency vs. Code

FREQUENCY – Hz

6

–36

–42

–48

–54

100k

10k

1k

100

–30

–24

–18

–12

–6

0

TPC 14. AD5242 1 M

Ω Gain vs. Frequency vs. Code