AD5170 datasheet(13/24 Pages) AD

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AD5170

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RDAC

LATCH

AND

DECODER

SD BIT

Figure 32. AD5170 Equivalent RDAC Circuit

The general equation that determines the digitally programmed

output resistance between Terminal W and Terminal 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

RW 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 Terminal W and Terminal 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, Terminal 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%. Since 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

positive, voltage across A–B, W–A, and W–B can be at either

polarity.

Figure 33. 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 Terminal A and Terminal 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 ab-

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

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