AD5246BKS5-R2 datasheet(12/20 Pages) AD

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AD5246

Rev. 0 | Page 12 of 20

OPERATION

The AD5246 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 28 shows a simplified diagram of the equivalent RDAC

circuit.

RDAC

LATCH

AND

DECODER

Figure 28. AD5246 Equivalent RDAC Circuit

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

Table 7 will be set for the indicated RDAC latch codes.

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 wiper resistance of

100 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 can occur.

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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