AD5124 datasheet(28/30 Pages) AD

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Data Sheet

AD5121/AD5141

THEORY OF OPERATION

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Rev. E | 28 of 30

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 of the RDAC between Terminal W and Terminal A also

produces a digitally controlled complementary resistance, RWA. RWA

also gives a maximum of 8% absolute resistance error. RWA starts

at the maximum resistance value and decreases as the data loaded

into the latch increases. The general equations for this operation

are

AD5121:

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

AD5141:

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

AD5121:

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

AD5141:

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 7. Otherwise, degradation or possible

destruction of the internal switch contact can occur.

Calculate the Actual End-to-End Resistance

The resistance tolerance is stored in the internal memory during

factory testing. Therefore, the actual end-to-end resistance can be

calculated (which is valuable for calibration, tolerance matching,

and precision applications).

The resistance tolerance (in percentage) is stored in fixed point

format, using a 16-bit sign magnitude binary. The sign bit (0 =

negative and 1 = positive) and the integer part are located in

Address 0x02, as shown in Table 20. Address 0x03 contains the

fractional part, as shown in Table 20.

That is, if the data readback from Address 0x02 is 00000010, and

the data readback from Address 0x03 is 10110000, the end-to-end

resistance can be calculated as follows.

For Memory Map Address 0x02, DB[7] = 0 = negative, and DB[6:0]

= 0000010 = 2.

For Memory Map Address 0x03, DB[7:0] = 10110000 = 176 × 2−8 =

0.6875, and therefore, tolerance = −2.6875%, and RAB = 9.731 kΩ.

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 47.

Figure 47. 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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