AD5251 datasheet(23/28 Pages) AD

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AD5251/AD5252

Rev. 0 | Page 23 of 28

D (Code in Decimal)

100

RWA

RWB

Figure 42. AD5251 RWA(D) and RWB(D) vs. Decimal Code

Table 13. RWB vs. Codes; RAB = 10 kΩ, A Terminal = Open

D (DEC)

RWB (Ω)

Output State

9918

Full scale

5075

Midscale

231

1 LSB

Zero scale (wiper resistance)

Note that in the zero-scale condition, a 75 Ω finite wiper

resistance is present. Care should be taken to limit the current

conduction between W and B in this state to no more than

±5 mA continuous for a total resistance of 1 kΩ, or a ±20 mA

pulse, to avoid degradation or possible destruction of the

internal switch contact.

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. 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

(see Figure 40). The general equation for this operation is

AD5251: RWA(D) = [(64 – D)/64] × RAB + 75 Ω

(3)

AD5252: RWA(D) = [(256 – D)/256] × RAB + 75 Ω

(4)

Table 14. RWA vs. Codes; AD5251, RAB=10 kΩ, B Terminal

Open

D (DEC)

RWA (Ω)

Output State

231

Full scale

5075

Midscale

9918

1 LSB

10075

Zero scale

The typical distribution of RAB from channel-to-channel

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

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

±20% tolerance.

PROGRAMMABLE POTENTIOMETER OPERATION

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

ometer mode and the most common configuration is the

voltage divider operation (see Figure 43).

Figure 43. Potentiometer Mode Configuration

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

AD5251:

(5)

AD5252:

256

(6)

A more accurate calculation, which includes the wiper

resistance effect, yields

)

(

(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 |VA|, |VW|, and |VB| do not

exceed VDD to VSS.

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