AD5172 datasheet(13/24 Pages) AD

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AD5172/AD5173

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The general equation that determines the digitally programmed

output resistance between W and B is

128

)

(

(1)

where D is the decimal equivalent of the binary code loaded in

the 8-bit RDAC register, RAB is the end-to-end resistance, and

is the wiper resistance contributed by the on resistance of

the internal switch.

In summary, if RAB = 10 kΩ and the A terminal is open-

circuited, the output resistance RWB is set for the RDAC latch

codes, as shown in Table 5.

Table 5. Codes and Corresponding RWB Resistance

D (Dec.)

RWB (Ω)

Output State

255

9,961

Full-Scale (RAB – 1 LSB + RW)

128

5,060

Midscale

139

1 LSB

100

Zero-Scale (Wiper Contact Resistance)

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

100 Ω is present. Care should be taken to limit the current flow

between W and B in this state to a maximum pulse current of

no more than 20 mA. Otherwise, degradation or possible

destruction of the internal switch contact can occur.

Similar to the mechanical potentiometer, the resistance of the

RDAC between the 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.

The general equation for this operation is

128

–

256

)

(

(2)

For RAB = 10 kΩ and the B terminal open-circuited, the

following output resistance RWA is set for the RDAC latch codes,

as shown in Table 6.

Table 6. Codes and Corresponding RWA Resistance

D (Dec.)

RWA (Ω)

Output State

255

139

Full-Scale

128

5,060

Midscale

9,961

1 LSB

10,060

Zero-Scale

Typical device-to-device matching is process lot dependent and

may vary by up to ±30%. Because the resistance element is pro-

cessed using thin film technology, the change in RAB with

temperature has a very low 35 ppm/°C temperature coefficient.

PROGRAMMING THE POTENTIOMETER DIVIDER

Voltage Output Operation

The digital potentiometer easily generates a voltage divider at

wiper-to-B and wiper-to-A proportional to the input voltage at

A-to-B. Unlike the polarity of VDD to GND, which must be posi-

tive, voltage across A-B, W-A, and W-B can be at either polarity.

Figure 38. Potentiometer Mode Configuration

If ignoring the effect of the wiper resistance for approximation,

connecting the A terminal to 5 V and the B terminal to ground

produces an output voltage at the wiper-to-B starting at 0 V up

to 1 LSB less than 5 V. Each LSB of voltage is equal to the

voltage applied across terminal AB divided by the 256 positions

of the potentiometer divider. The general equation defining the

output voltage at VW with respect to ground for any valid input

voltage applied to terminals A and B is

256

)

(

−

(3)

For a more accurate calculation, which includes the effect of

wiper resistance, VW can be found as

)

(

)

(

)

(

(4)

Operation of the digital potentiometer in the divider mode

results in a more accurate operation over temperature. Unlike

the rheostat mode, the output voltage is dependent mainly on

the ratio of the internal resistors RWA and RWB and not the abso-

lute values. Thus, the temperature drift reduces to 15 ppm/°C.

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