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AD5280/AD5282

Rev. C | Page 14 of

THEORY OF OPERATION

The AD5280/AD5282 are single-channel and dual-channel,

256-position, digitally controlled variable resistors (VRs). To

program the VR settings, see 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.

RDAC

LATCH

AND

DECODER

SHDN

0xFF

0x01

SWB

SWA

0x00

()

Figure 44. AD5280/AD5282 Equivalent RDAC Circuit

RHEOSTAT OPERATION

The nominal resistance of the RDAC between Terminal A and

Terminal 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, for example, 20 kΩ = 20, 50 kΩ = 50, and

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 eight-bit data in the RDAC latch is

decoded to select one of the 256 possible settings. Assuming

that a 20 kΩ part is used, the wiper’s first connection starts at

the B terminal for data 0x00. Because there is a 60 Ω wiper

contact resistance, such a 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

138 Ω (RWB = RAB/256 + RW = 78 Ω + 60 Ω) for data 0x01. The

third connection is the next tap point representing 216 Ω (78 ×

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 19,982 Ω (RAB – 1 LSB + RW). Figure 46 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

output resistance between W and B is

256

()

(1)

where:

is the decimal equivalent of the binary code loaded in the 8-

bit RDAC register.

RAB

is the nominal end-to-end resistance.

is the wiper resistance contributed by the on resistance of

the internal switch.

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, degradation or possible

destruction of the internal switch contact can occur.

As in the mechanical potentiometer, the resistance of the RDAC

between 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

−

256

(2)

The typical distribution of the nominal resistance, RAB, from

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

matching is process lot dependent, and it is possible to have a

±30% variation. Because the resistance element is processed in

thin film technology, the change in RAB with temperature is very

small (30 ppm/°C).

POTENTIOMETER OPERATION

The digital potentiometer easily generates a voltage divider at

wiper to B and wiper to A to be proportional to the input voltage

at A to B. Unlike the polarity of VDD – VSS, which must be

positive, voltage across A to B, W to A, and W to B can be at

either polarity, provided that VSS is powered by a negative supply.

If the effect of the wiper resistance for approximation is ignored,

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

produces an output voltage at the wiper to B starting at 0 V up

to 1 LSB less than 5 V. Each LSB of voltage is equal to the

voltage applied across A to B divided by the 256 positions of the

potentiometer divider. Because the AD5280/AD5282 can be

supplied by dual supplies, the general equation defining the

output voltage at VW with respect to ground for any valid

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