AD5243 datasheet(13/20 Pages) AD

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Data Sheet

AD5243/AD5248

Rev. C | Page 13 of 20

THEORY OF OPERATION

The AD5243/AD5248 are 256-position, digitally controlled

variable resistor (VR) devices.

An internal power-on preset places the wiper at midscale

during power-on, which simplifies the fault condition recovery

at power-up.

PROGRAMMING THE VARIABLE RESISTOR AND

VOLTAGE

Rheostat Operation

The nominal resistance of the RDAC between Terminal A and

Terminal B is available in 2.5 kΩ, 10 kΩ, 50 kΩ, and 100 kΩ.

The nominal resistance (RAB) of the VR has 256 contact points

accessed by the wiper terminal and the B terminal contact. The

8-bit data in the RDAC latch is decoded to select one of the 256

possible settings.

Figure 37. Rheostat Mode Configuration

Assuming that a 10 kΩ part is used, the first connection of the

wiper starts at the B terminal for Data 0x00. Because there is

a 160 Ω wiper contact resistance, such a connection yields a

minimum of 320 Ω (2 × 160 Ω) resistance between Terminal W

and Terminal B. The second connection is the first tap point,

which corresponds to 359 Ω (RWB = RAB/256 + 2 × RW = 39 Ω +

2 × 160 Ω) for Data 0x01. The third connection is the next tap

point, representing 398 Ω (2 × 39 Ω + 2 × 160 Ω) for Data 0x02,

and so on. Each LSB data value increase moves the wiper up the

resistor ladder until the last tap point is reached at 10,281 Ω

(RAB + 2 × RW).

RDAC

LATCH

AND

DECODER

Figure 38. AD5243 Equivalent RDAC Circuit

The general equation determining the digitally programmed

output resistance between W and B is









256

)

(

(1)

where:

is the decimal equivalent of the binary code loaded in the

8-bit RDAC register.

RAB

is the end-to-end resistance.

is the wiper resistance contributed by the on resistance of

the internal switch.

In summary, if RAB is 10 kΩ and the A terminal is open circuited,

the following output resistance, RWB, is set for the indicated

RDAC latch codes.

Table 7. Codes and Corresponding RWB Resistance

D (Dec)

RWB (Ω)

Output State

255

10,281

Full scale (RAB − 1 LSB + 2 × RW)

128

5380

Midscale

359

1 LSB + 2 × RW

320

Zero scale (wiper contact resistance)

Note that in the zero-scale condition, a finite wiper resistance of

320 Ω is present. Care should be taken to limit the current flow

between W and 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 may occur.

Similar to the mechanical potentiometer, the resistance of the

RDAC between Wiper 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











256

)

(

(2)

When RAB is 10 kΩ and the B terminal is open circuited, the

output resistance, RWA, is set according to the RDAC latch

codes, as listed in Table 8.

Table 8. Codes and Corresponding RWA Resistance

D (Dec)

RWA (Ω)

Output State

255

359

Full scale

128

5320

Midscale

10,280

1 LSB + 2 × RW

10,320

Zero scale

Typical device-to-device matching is process-lot dependent and

may vary by up to ±30%. Because the resistance element is pro-

cessed in thin-film technology, the change in RAB with temperature

has a very low temperature coefficient of 35 ppm/°C.

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