AD5273

Data Sheet

Rev. J | Page 14 of 20

VARIABLE RESISTANCE AND VOLTAGE FOR

RHEOSTAT MODE

If only the W-to-B or W-to-A terminals are used as variable

resistors, the unused A or B terminal can be opened or shorted

with W. This operation is called rheostat mode (see Figure 33).

Figure 33. Rheostat Mode Configuration

The nominal resistance, RAB, of the RDAC has 64 contact points

accessed by the wiper terminal, plus the B terminal contact if RWB is

considered. The 6-bit data in the RDAC latch is decoded to select

one of the 64 settings. Assuming that a 10 kΩ part is used, the

wiper’s first connection starts at Terminal B for Data Register 0x00.

This connection yields a minimum of 60 Ω resistance between

Terminal W and Terminal B because of the 60 Ω wiper contact

resistance. The second connection is the first tap point, which

corresponds to 219 Ω (RW = 1 × RAB/63 + RW) for Data Register

0x01, and so on. Each LSB data value increase moves the wiper

up the resistor ladder until the last tap point is reached at 10,060 Ω

(63 × RAB/63 + RW). Figure 34 shows a simplified diagram of the

equivalent RDAC circuit. The general equation determining RWB is



W

AB

WB

R

R

D

D

R







63

(1)

where:

D is the decimal equivalent of the 6-bit binary code.

RAB is the end-to-end resistance.

RW is the wiper resistance contributed by the on resistance of

the internal switch.

Table 4. RWB vs. Codes; RAB = 10 kΩ; Terminal A Opened

D (Dec)

RWB (Ω)

Output State

63

10,060

Full scale (RAB + RW)

32

5139

Midscale

1

219

1 LSB

0

60

Zero scale (wiper contact resistance)

Because a finite wiper resistance of 60 Ω is present in the zero-

scale condition, care should be taken to limit the current flow

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

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

complementary resistance, RWA. When these terminals are used,

Terminal B can be opened or shorted to W. 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



W

AB

WA

R

R

D

D

R









63

63

(2)

Table 5. RWA vs. Codes; RAB =10 kΩ; Terminal B Opened

D (Dec)

RWA (Ω)

Output State

63

60

Full scale

32

4980

Midscale

1

9901

1 LSB

0

10,060

Zero scale

The typical distribution of the resistance tolerance from device

to device is process-lot dependent, and it is possible to have

±30% tolerance.

Figure 34. AD5273 Equivalent RDAC Circuit

VARIABLE RESISTANCE AND VOLTAGE FOR

POTENTIOMETER MODE

If all three terminals are used, the operation is called the

potentiometer mode. The most common configuration is

the voltage divider operation (see Figure 35).

Figure 35. Potentiometer Mode Configuration

Ignoring the effect of the wiper resistance, the transfer function

is simply



A

W

V

D

D

V

63



(3)

A more accurate calculation, which includes the wiper

resistance effect, yields



A

W

AB

W

AB

W

V

R

R

R

R

D

D

V

2

63







(4)

Unlike rheostat mode where the absolute tolerance is high,

potentiometer mode yields an almost ratiometric function of

D/63 with a relatively small error contributed by the RW terms.

Therefore, the tolerance effect is almost cancelled. Although the

A

W

B

A

W

B

A

W

B

D5

D4

D3

D2

D1

D0

RDAC

LATCH

AND

DECODER

RS

RS

RS

A

W

B

A

VI

W

B

VO