AD5245BRJZ5-R2 datasheet(13/20 Pages) AD

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AD5245

Rev. B | Page 13 of 20

THEORY OF OPERATION

The AD5245 is a 256-position digitally controlled variable

resistor (VR) device.

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

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 nominal

resistance (RAB) of the VR has 256 contact points accessed by

the wiper terminal, plus the B terminal contact. The 8-bit data

in the RDAC latch is decoded to select one of the 256 possible

settings.

Figure 35. Rheostat Mode Configuration

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

starts at the B terminal for Data 0x00. Because 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

139 Ω (RWB = RAB/256 + 2 × RW = 39 Ω + 2 × 50 Ω) for Data 0x01.

The third connection is the next tap point, representing 178 Ω

(2 × 39 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).

RDAC

LATCH

AND

DECODER

Figure 36. AD5245 Equivalent RDAC Circuit

The general equation determining the digitally programmed

output resistance between W and B is

256

)

(

(1)

where:

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

8-bit RDAC register.

RAB is the end-to-end resistance.

RW 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, then the following output resistance RWB is set for the

indicated RDAC latch codes.

Table 6. Codes and Corresponding RWB Resistance

D (Dec.)

RWB (Ω)

Output State

255

9,961

Full Scale (RAB – 1 LSB + RW)

128

5,060

Midscale

139

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.

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

−

256

)

(

(2)

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

following output resistance RWA is set for the indicated RDAC

latch codes.

Table 7. Codes and Corresponding RWA Resistance

D (Dec.)

RWA (Ω)

Output State

255

139

Full Scale

128

5,060

Midscale

9,961

1 LSB

10,060

Zero Scale

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

can vary by up to ±30%. Because 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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