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AD5247

OPERATION

The AD5247 is a 128-position, digitally controlled variable

resistor (VR) device. An internal power-on preset places the

wiper at midscale during power-on, which simplifies the default

condition recovery at power-up.

PROGRAMMING THE VARIABLE RESISTOR

Rheostat Operation

The nominal resistance of the RDAC between terminals A and

B is available in 5 kΩ, 10 kΩ, 50 kΩ, and 100 kΩ. The final two

or three digits of the part number determine the nominal

resistance value, e.g., 10 kΩ = 10, 50 kΩ = 50. The nominal

resistance (RAB) of the VR has 128 contact points accessed by

the wiper terminal, plus the B terminal contact. The 7-bit data

in the RDAC latch is decoded to select one of the 128 possible

settings.

Assuming a 10 kΩ part is used, the wiper’s first connection

starts at the B terminal for data 0x00. Since there is a 50 Ω wiper

contact resistance, such a connection yields a minimum of

100 Ω (2 × 50 Ω) resistance between terminals W and B. The

second connection is the first tap point, which corresponds to

178 Ω (RWB = RAB/128+ RW = 78 Ω + 2 × 50 Ω) for data 0x01.

The third connection is the next tap point, representing 256 Ω

(2 × 78 Ω + 2 × 50 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 10,100 Ω (RAB + 2 × RW).

Figure 34

Figure 34. AD5247 Equivalent RDAC Circuit

shows a simplified diagram of the equivalent RDAC

circuit where the last resistor string will not be accessed.

RDAC

LATCH

AND

DECODER

The general equation determining the digitally programmed

output resistance between W and B is

128

)

(

(1)

where D is the decimal equivalent of the binary code loaded in

the 7-bit RDAC register, RAB is the end-to-end resistance, and

is the wiper resistance contributed by the on resistance of

the internal switch. In summary, if RAB = 10 kΩ and the A

terminal is open-circuited, the output resistance RWB shown in

will be set for the indicated RDAC latch codes.

Table 7

Table 7. Codes and Corresponding RWB Resistance

D (Dec.)

RWB (Ω)

Output State

127

10,100

Full Scale (RAB + 2 × RW)

5,100

Midscale

178

1 LSB

100

Zero Scale (Wiper Contact Resistance)

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

100 Ω between terminals W and B 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 can occur.

Similar to the mechanical potentiometer, the resistance of the

RDAC between the 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

128

–

128

)

(

(2)

For RAB = 10 kΩ and the B terminal open circuited, the output

resistance RWA shown in

will be set for the indicated

RDAC latch codes.

Table 8

Table 8. Codes and Corresponding RWA Resistance

D (Dec.)

RWA (Ω)

Output State

127

178

Full Scale

5,100

Midscale

9,961

1 LSB

10,100

Zero Scale

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

may vary by up to ±30%. Since the resistance element is

processed in thin film technology, the change in RAB with

temperature has a very low 45 ppm/°C temperature coefficient.

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