AD5231 datasheet(20/28 Pages) AD

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AD5231

Data Sheet

Rev. D | Page 20 of 28

CODE (Decimal)

100

1023

256

512

768

RWB

RWA

Figure 44. RWA(D) and RWB(D) vs. Decimal Code

The general equation that determines the programmed output

resistance between W and B is

1024

)

(

(1)

where:

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

RAB is the nominal resistance between Terminal A and

Terminal B.

RW is the wiper resistance.

For example, the output resistance values in Table 11 are set

for the given RDAC latch codes with VDD = 5 V (applies to

RAB = 10 kΩ digital potentiometers).

Table 11. RWB (D) at Selected Codes for RAB = 10 kΩ

D (DEC)

RWB(D) (Ω)

Output State

1023

10,005

Full scale

512

50,015

Midscale

24.7

1 LSB

Zero scale (wiper contact resistor)

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

of 15 Ω is present. Care should be taken to limit the current

flow between W and B in this state to no more than 20 mA to

avoid degradation or possible destruction of the internal switches.

Like the mechanical potentiometer that the RDAC replaces, the

AD5231 part is totally symmetrical. The resistance between

Wiper W and Terminal A also produces a digitally controlled

complementary resistance, RWA. Figure 44 shows the symmetrical

programmability of the various terminal connections. When

RWA is used, Terminal B can be left 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 equation for this operation is

−

1024

)

(

(2)

For example, the output resistance values in Table 12 are set for

the RDAC latch codes with VDD = 5 V (applies to RAB = 10 kΩ

digital potentiometers).

Table 12. RWA(D) at Selected Codes for RAB= 10 kΩ

D (DEC)

RWA(D) (Ω)

Output State

1023

24.7

Full scale

512

5015

Midscale

10005

1 LSB

10,015

Zero scale

The typical distribution of RAB from device to device matches

tightly when they are processed in the same batch. When

devices are processed at a different time, device-to-device

matching becomes process-lot dependent and exhibits a −40%

to +20% variation. The change in RAB with temperature has a

600 ppm/°C temperature coefficient.

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 Terminal A and Terminal B. For

example, connecting Terminal A to 5 V and Terminal B to

ground produces an output voltage at the wiper that can be any

value from 0 V to 5 V. Each LSB of voltage is equal to the

voltage applied across Terminals A–B divided by the 2N position

resolution of the potentiometer divider.

Because AD5231 can also be supplied by dual supplies, the

general equation defining the output voltage at VW with respect

to ground for any given input voltages applied to Terminal A

and Terminal B is

1024

)

(

(3)

Equation 3 assumes that VW is buffered so that the effect of

wiper resistance is minimized. Operation of the digital

potentiometer in divider mode results in more accurate

operation over temperature. Here, the output voltage is

dependent on the ratio of the internal resistors and not the

absolute value; therefore, the drift improves to 15 ppm/°C.

There is no voltage polarity restriction between Terminal A,

Terminal B, and Terminal W as long as the terminal voltage

(VTERM) stays within VSS < VTERM < VDD.

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