AD5253 datasheet(25/32 Pages) AD

25 / 32 page

Data Sheet

AD5253/AD5254

Rev. C | Page 25 of 32

D (Code in Decimal)

100

RWA

RWB

Figure 43. AD5253 RWA(D) and RWB(D) vs. Decimal Code

Since the digital potentiometer is not ideal, a 75 Ω finite wiper

resistance is present that can easily be seen when the device is

programmed at zero scale. Because of the fine geometric and

interconnects employed by the device, care should be taken to

limit the current conduction between W and B to no more than

±5 mA continuous for a total resistance of 1 kΩ or a pulse of

±20 mA to avoid degradation or possible destruction of the

device. The maximum dc current for AD5253 and AD5254 are

shown in Figure 21 and Figure 22, respectively.

Similar to 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. The RWA

starts at a maximum value and decreases as the data loaded into

the latch increases in value (see Figure 43. The general equation

for this operation is

AD5253: RWA(D) = [(64 – D)/64] × RAB + 75 Ω

(3)

AD5254: RWA(D) = [(256 – D)/256] × RAB + 75 Ω

(4)

The typical distribution of RAB from channel-to-channel

matches is about ±0.15% within a given device. On the other

hand, device-to-device matching is process-lot dependent with

a ±20% tolerance.

PROGRAMMABLE POTENTIOMETER OPERATION

If all three terminals are used, the operation is called potenti-

ometer mode (see Figure 44); the most common configuration

is the voltage divider operation.

Figure 44. Potentiometer Mode Configuration

If the wiper resistance is ignored, the transfer function is simply

AD5253:

(5)

AD5254:

256

(6)

A more accurate calculation that includes the wiper resistance

effect is

)

(

(7)

where 2N is the number of steps.

Unlike in rheostat mode operation, where the tolerance is high,

potentiometer mode operation yields an almost ratiometric

function of D/2N with a relatively small error contributed by the

RW terms. Therefore, the tolerance effect is almost cancelled.

Similarly, the ratiometric adjustment also reduces the

temperature coefficient effect to 50 ppm/°C, except at low value

codes where RW dominates.

Potentiometer mode operations include other applications such

as op amp input, feedback-resistor networks, and other voltage-

scaling applications. The A, W, and B terminals can, in fact, be

input or output terminals, provided that |VA|, |VW|, and |VB| do

not exceed VDD to VSS.

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