AD5241BRUZ1M-R7 datasheet(12/20 Pages) AD

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AD5241/AD5242

Rev. C | Page 12 of 20

THEORY OF 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 section.

Both parts have an internal power-on preset that places the wiper

in midscale during power-on that simplifies the fault condition

recovery at power-up. In addition, the shutdown pin (SHDN)

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 returns from shutdown, the stored VR setting is applied

to the RDAC.

SWSHDN

SW N

2–1

SW N

2–2

RDAC

LATCH

AND

DECODER

RRAB/2N

DIGITAL CIRCUITRY

OMITTED FOR CLARITY

SW1

SW0

SHDN

Figure 31. Equivalent RDAC Circuit

PROGRAMMING THE VARIABLE RESISTOR

Rheostat Operation

The nominal resistance of the RDAC between Terminal A and

Terminal B is available in 10 kΩ, 100 kΩ, and 1 MΩ. The final two

or three digits of the part number determine the nominal resistance

value, for example, 10 kΩ = 10, 100 kΩ = 100, and 1 MΩ = 1 M.

The nominal resistance (RAB) of the VR has 256 contact points

accessed by the wiper terminal, plus the B terminal contact. The

8-bit data in the RDAC latch is decoded to select one of the 256

possible settings. Assume a 10 kΩ part is used; the first connection

of the wiper starts at the B terminal for Data 0x00. Because there is

a 60 Ω wiper contact resistance, such connection yields a minimum

of 60 Ω resistance between Terminal W and Terminal B. The

second connection is the first tap point that corresponds to 99 Ω

(RWB = RAB/256 + RW = 39 + 60) for Data 0x01. The third connection

is the next tap point representing 138 Ω (39 × 2 + 60) for Data 0x02,

and so on. Each LSB data value increase moves the wiper up the

resistor ladder until the last tap point is reached at 10,021 Ω

[RAB – 1 LSB + RW].

Figure 31 shows a simplified diagram of the equivalent RDAC

circuit where the last resistor string is not accessed; therefore,

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

RWB(D) =

256

× RAB + RW

(1)

where:

D is the decimal equivalent of the binary code between 0 and 255,

which is loaded in the 8-bit RDAC register.

RAB is the nominal end-to-end resistance.

RW is the wiper resistance contributed by the on resistance of

the internal switch.

Again, if RAB = 10 kΩ, Terminal A can be either open circuit or

tied to W. Table 6 shows the RWB resistance based on the code

set in the RDAC latch.

Table 6. RWB (D) at Selected Codes for RAB = 10 kΩ

D (DEC)

RWB (Ω)

Output State

255

10021

Full-scale (RWB – 1 LSB + RW)

128

5060

Midscale

1 LSB

Zero-scale (wiper contact resistance)

Note that in the zero-scale condition, a finite wiper resistance of

60 Ω is present. Care should be taken to limit the current flow

between W and B in this state to a maximum current of no more

than 20 mA. Otherwise, degradation or possible destruction of

the internal switch contact can occur.

Similar to the mechanical potentiometer, the resistance of the

RDAC between Wiper W and Terminal A also produces a

digitally controlled resistance, RWA. When these terminals are

used, Terminal B can be opened or tied to the wiper terminal.

The minimum RWA resistance is for Data 0xFF and increases as

the data loaded in the latch decreases in value. The general

equation for this operation is

RWA(D) =

256

256 D

−

× RAB + RW

(2)

For RAB = 10 kΩ, Terminal B can be either open circuit or tied

to W. Table 7 shows the RWA resistance based on the code set in

the RDAC latch.

Table 7. RWA (D) at Selected Codes for RAB = 10 kΩ

D (DEC)

RWA (Ω)

Output State

255

Full-scale

128

5060

Midscale

10021

1 LSB

10060

Zero-scale

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