AD5121 datasheet(23/25 Pages) AD

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Data Sheet

AD5123/AD5143

THEORY OF OPERATION

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Rev. C | 23 of 25

PROGRAMMING THE VARIABLE RESISTOR

Rheostat Operation—±8% Resistor Tolerance

The AD5123/AD5143 operate in rheostat mode when only two

terminals are used as a variable resistor. The unused terminal can

be floating, or it can be tied to Terminal W, as shown in Figure 37.

Figure 37. Rheostat Mode Configuration

The nominal resistance between Terminal A and Terminal B, RAB,

is 10 kΩ or 100 kΩ, and has 128/256 tap points accessed by the

wiper terminal. The 7-bit/8-bit data in the RDAC latch is decoded

to select one of the 128/256 possible wiper settings. The general

equations for determining the digitally programmed output resist-

ance between Terminal W and Terminal B are

AD5123:

RWBD = D128×RAB+RW From 0x00 to 0x7F (1)

AD5143:

RWBD = D256×RAB+RW From 0x00 to 0xFF (2)

where:

D is the decimal equivalent of the binary code in the 7-bit/8-bit

RDAC register.

RAB is the end to end resistance.

RW is the wiper resistance.

In potentiometer mode, similar to the mechanical potentiometer, the

resistance between Terminal W and Terminal A also produces a

digitally controlled complementary resistance, RWA. RWA also gives

a maximum of 8% absolute. RWA starts at the maximum resistance

value and decreases as the data loaded into the latch increases.

The general equations for this operation are

AD5123:

RAWD =128−D128×RAB+RW From 0x00 to 0x7F (3)

AD5143:

RAWD =256−D256×RAB+RW From 0x00 to 0xFF (4)

where:

D is the decimal equivalent of the binary code in the 7-bit/8-bit

RDAC register.

RAB is the end to end resistance.

RW is the wiper resistance.

If the device is configured in linear gain setting mode, the resist-

ance between Terminal W and Terminal A is directly proportional

to the code loaded in the associate RDAC register. The general

equations for this operation are

AD5123:

RAWD = D128×RAB+RW From 0x00 to 0x7F (5)

AD5123:

RAWD = D256×RAB+RW From 0x00 to 0xFF (6)

where:

D is the decimal equivalent of the binary code in the 7-bit/8-bit

RDAC register.

RAB is the end to end resistance.

RW is the wiper resistance.

In the bottom scale condition or top scale condition, a finite total

wiper resistance of 40 Ω is present. Regardless of which setting

the device is operating in, limit the current between Terminal A to

Terminal B, Terminal W to Terminal A, and Terminal W to Terminal

B, to the maximum continuous current of ±6 mA or to the pulse

current specified in Table 5. Otherwise, degradation or possible

destruction of the internal switch contact can occur.

PROGRAMMING THE POTENTIOMETER

DIVIDER

Voltage Output Operation

The digital potentiometer easily generates a voltage divider at

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

A to B, as shown in Figure 38.

Figure 38. Potentiometer Mode Configuration

Connecting Terminal A to 5 V and Terminal B to ground produces

an output voltage at the Wiper W to Terminal B ranging from 0 V

to 5 V. 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

VW(D)=RWB(D)RAB×VA+RAW(D)RAB×VB

(7)

where:

RWB(D) can be obtained from Equation 1 and Equation 2.

RAW(D) can be obtained from Equation 3 and Equation 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, RAW and RWB, and not the absolute values.

Therefore, the temperature drift reduces to 5 ppm/°C.

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