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AD5280

15 V, I2C Compatible 256-Position Digital Potentiometers

Analog Devices 

AD5280/AD5282

OPERATION

The AD5280/AD5282 provides a single-/dual-channel, 256-

position, digitally controlled variable resistor (VR) device.

To program the VR settings, refer to the Digital Interface section.

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

at midscale during power-on, which simplifies the fault condition

recovery at power-up. Operation of the power-on preset function

also depends on the state of the VL pin. In addition, the shutdown

SHDN pin of the AD5280/AD5282 places the RDAC in an

almost zero power consumption state where terminal A is open

circuited and the wiper W is connected to terminal B, resulting

in only leakage currents being consumed in the VR structure.

During shutdown, the VR latch settings are maintained or new

settings can be programmed. When the part is returned from

shutdown, the corresponding VR setting will be applied to the

RDAC.

Ax

SHDN

RS

D7

D6

RS

D5

D4

D3

RS

D2

Wx

D1

D0

RDAC

LATCH

AND

DECODER RS

Bx

Figure 4. AD5280/AD5282 Equivalent RDAC Circuit

PROGRAMMING THE VARIABLE RESISTOR

Rheostat Operation

The nominal resistance of the RDAC between terminals A and

B is available in 20 kΩ, 50 kΩ, and 200 kΩ. The final two or

three digits of the part number determine the nominal resistance

value, e.g., 20 kΩ = 20; 50 kΩ = 50; 200 kΩ = 200. 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. Assuming a 20 kΩ part is used, the wiper’s first con-

nection starts at the B terminal for data 00H. Since there is a 60 Ω

wiper contact resistance, such a connection yields a minimum

of 60 Ω resistance between terminals W and B. The second

connection is the first tap point that corresponds to 138 Ω

(RWB = RAB/256 + RW = 78 Ω + 60 Ω) for data 01H. The third

connection is the next tap point representing 216 Ω (78 ϫ 2 + 60)

for data 02H, and so on. Each LSB data value increase moves

the wiper up the resistor ladder until the last tap point is reached

at 19982 Ω [RAB – 1 LSB + RW]. Figure 4 shows a simplified

diagram of the equivalent RDAC circuit where the last resistor

string will not be 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

output resistance between W and B is:

( ) RWB

D

=

D

256

× RAB

+

RW

(1)

where:

D is the decimal equivalent of the binary code 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 = 20 kΩ and the A terminal is open circuited, the

following output resistance values, RWB, will be set for the fol-

lowing RDAC latch codes.

Table I. Codes and Corresponding Resistances

D (DEC)

255

128

1

0

RWB (⍀)

19982

10060

138

60

Output State

Full Scale (RAB – 1 LSB + RW)

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 pulse current of no more than 20 mA. Otherwise, degrada-

tion or possible destruction of the internal switch contact 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. Setting the

resistance value for RWA starts at a maximum value of resistance

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

The general equation for this operation is:

( ) RWA

D

=

256 – D

256

× RAB

+ RW

(2)

For RAB = 20 kΩ and B terminal open circuited, the following

output resistance, RWA, will be set for the following RDAC

latch codes.

Table II. Codes and Corresponding Resistances

D (DEC)

RWA (⍀)

Output State

255

138

Full Scale

128

10060

Midscale

1

19982

1 LSB

0

20060

Zero Scale

The typical distribution of the nominal resistance, RAB, from

channel-to-channel matches within ± 1%. Device-to-device

matching is process lot dependent and is possible to have ± 30%

variation. Since the resistance element is processed in thin film

technology, the change in RAB with temperature has a very low

30 ppm/°C temperature coefficient.

REV. 0

–11–

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