AD5253 datasheet(24/28 Pages) AD

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AD5253/AD5254

Rev. 0 | Page 24 of 28

For example, the RWB values shown in Table 13 can be found on

AD5253 10 kΩ parts.

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 41). The general equation for this operation is

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

(3)

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

(4)

Table 14. RWA vs. Codes; AD5253, 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 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 and the most common configuration is the

voltage divider operation (see Figure 44).

Figure 44. Potentiometer Mode Configuration

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

(AD5253)

(5)

256

(AD5254)

(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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