AD5255 datasheet(18/20 Pages) AD

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AD5255

Rev. 0 | Page 18 of 20

For RDAC0 and RDAC1:

()

512

(1)

For RDAC2:

()

128

(2)

where D is the decimal equivalent of the data contained in the

RDAC register and RW is the wiper resistance.

The output resistance values in Table 11 are set for the given

RDAC latch codes with VDD = 5 V, which applies to RAB = 25 kΩ

digital potentiometers.

Table 11. RWB at Selected Codes for RWB_FS = 25 kΩ

D (DEC)

RWB(d) (Ω)

Output State

511

25051

Full scale

256

12600

Midscale

148.8

1 LSB

100

Zero scale (wiper contact resistance)

Note that in the zero-scale condition, a finite wiper resistance of

100 Ω is present. To avoid degradation or possible destruction

of the internal switches, care should be taken to limit the current

flow between W and B to no more than 20 mA intermittently or

2 mA continuously.

Channel-to-channel RWB matching is better than 0.1%. The

change in RWB with temperature has a 35 ppm/°C temperature

coefficient.

Like the mechanical potentiometer that the RDAC replaces, the

AD5255 parts are totally symmetrical. The resistance between

the W wiper and the A terminal also produces a digitally

controlled complementary resistance, RWA. When RWA is used,

the B terminal can be floating or tied to the wiper. Setting the

resistance value for RWA starts at a maximum value of resistance

and decreases as the data loaded in the latch is increased in

value. The general transfer equations for this operation are as

follows.

For RDAC0 and RDAC1:

()

−

512

(3)

For RDAC2:

()

−

128

(4)

For example, the following RDAC latch codes set the

corresponding output resistance values, which apply to

RAB = 25 kΩ digital potentiometers.

Table 12. RWA(d) at Selected Codes for RAB = 25 kΩ

D (DEC)

RWA(d) (Ω)

Output State

511

148.8

Full scale

256

12600

Midscale

25051

1 LSB

25100

Zero scale

The typical distribution of RAB from channel-to-channel is

±0.1% within the same package. Device-to-device matching is

process lot-dependent, with a worst-case variation of ±15%. RAB

temperature coefficient is 35 ppm/°C.

PROGRAMMING THE POTENTIOMETER DIVIDER

Voltage Output Operation

The digital potentiometer can be configured to generate an

output voltage at the wiper terminal that is proportional to the

input voltages applied to the A and B terminals. Connecting the

A terminal to 5 V and the B terminal to ground produces an

output voltage at the wiper that can vary between 0 V to 5 V.

Each LSB of voltage is equal to the voltage applied across the A

and B terminals divided by the 2N position resolution of the

potentiometer divider.

Since the AD5255 can operate from dual supplies, the general

equations defining the output voltage at VW with respect to

ground for any given input voltages applied to the A and B

terminals are as follows.

For RDAC0 and RDAC1:

()

512

(5)

For RDAC2:

()

128

(6)

Equation 5 assumes that VW is buffered so that the effect of

wiper resistance is nulled. Operation of the digital potentiometer

in the divider mode results in more accurate operation over

temperature. In this mode, the output voltage is dependent on

the ratio of the internal resistors, not on the absolute value;

therefore, the drift improves to 15 ppm/°C. There is no voltage

polarity restriction between the A, B, and W terminals as long as

the terminal voltage (VTERM) stays within VSS < VTERM < VDD.

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