AD5116 datasheet(25/28 Pages) AD

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Data Sheet

AD5110/AD5112/AD5114

Rev. 0 | Page 25 of 28

Similar to the mechanical potentiometer, the resistance of

the RDAC between the W terminal and the A terminal 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

AD5110:

Bottom scale (0xFF) (7)

−

128

)

(

From 0x00 to 0x7F (8)

Top scale (0x80) (9)

AD5112:

Bottom scale (0xFF) (10)

−

)

(

From 0x00 to 0x3F (11)

Top scale (0x40) (12)

AD5114:

Bottom scale (0xFF) (13)

−

)

(

From 0x00 to 0x1F (14)

Top scale (0x20) (15)

where:

D is the decimal equivalent of the binary code in the 5-/6-/7-bit

RDAC register.

RAB is the end-to-end resistance.

RW is the wiper resistance.

RTS is the wiper resistance at top scale.

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

total wiper resistance of 45 Ω is present. Regardless of which

setting the part is operating in, take care to 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 6.

Otherwise, degradation or possible destruction of the internal

switch contact can occur.

Calculating the Actual End-to-End Resistance

The resistance tolerance is stored in the internal memory

during factory testing. The actual end-to-end resistance can,

therefore, be calculated, which is valuable for calibration,

tolerance matching, and precision applications.

The resistance tolerance in percentage is stored in fixed-point

format, using an 8-bit sign magnitude binary. The data can be

read back by executing Command 6 and setting Bit DB0 (A0).

The MSB is the sign bit (0 = − and 1 = +) and the next four bits

are the integer part, the fractional part is represented by the

three LSBs, as shown in Table 11.

Table 11. Tolerance Format

Data Byte

DB7

DB6

DB5

DB4

DB3

DB2

DB1

DB0

Sign

2-1

2-2

2-3

For example, if RAB = 10 kΩ and the data readback shows

01010010, the end-to-end resistance can be calculated as,

if,

DB[7] is 0 = negative

DB[6:3] is 1010 = 10

DB[2:0] is 010 = 2 × 2−3 = 0.25

then,

tolerance = −10.25% and, therefore, RAB = 8.975 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 48. Unlike the polarity of

VDD to GND, which must be positive, voltage across A-to-B, W-

to-A, and W-to-B can be at either polarity.

Figure 48. 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:

)

(

)

(

)

(

(16)

where:

RWB(D) can be obtained from Equation 1 to Equation 6.

RAW(D) can be obtained from Equation 7 to Equation 15.

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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