AD5263BRUZ200-R7 datasheet(17/28 Pages) AD

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Data Sheet

AD5263

Rev. F | Page 17 of 28

OPERATION

The AD5263 is a quad-channel, 256-position, digitally

controlled, variable resistor (VR) device.

To program the VR settings, refer to the SPI-Compatible Digital

Interface (DIS = 0) section and the I2C-Compatible Digital

Interface (DIS = 1) section. The part has an internal power-on

preset that places the wiper at midscale during power-on,

simplifying the fault condition recovery at power-up. In addition,

the shutdown (SHDN) pin of AD5263 places the RDAC in an

almost zero-power consumption state where Terminal A is

open circuited and the wiper W is connected to Terminal B,

resulting in only leakage current consumption in the VR structure.

During shutdown, the VR latch settings are maintained or new

settings can be programmed. When the part is returned from

shutdown, the corresponding VR setting is applied to the RDAC.

SD BIT

RDAC

LATCH

AND

DECODER RS

Figure 45. AD5263 Equivalent RDAC Circuit

PROGRAMMING THE VARIABLE RESISTOR

Rheostat Operation

The nominal resistance of the RDAC between Terminal A and

Terminal B is available in 20 kΩ, 50 kΩ, and 200 kΩ. The final

two or three digits of the part number determine the nominal

resistance value, for example, 20 kΩ = 20; 50 kΩ = 50;

200 kΩ = 200. 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. Assuming a 20 kΩ part is

used, the wiper’s first connection starts at the B terminal for data

0x00. Because there is a 60 Ω wiper contact resistance, such a

connection yields a minimum of 2 × 60 Ω resistance between the

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

and corresponds to 198 Ω (RWB = RAB/256 + RW = 78 Ω + 2 ×

60 Ω) for Data 0x01. The third connection is the next tap point

representing 276 Ω (RWB = 78 Ω × 2 + 2 × 60 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 20,042 Ω

(RAB – 1 LSB + 2 × RW). Figure 45 shows a simplified diagram of

the equivalent RDAC circuit, where the last resistor string is not

accessed; therefore, there is 1 LSB less of the nominal resistance at

full scale in addition to the wiper resistance.

The general equation determining the digitally programmed

output resistance between the W and B terminals 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 one

internal switch.

In summary, if RAB= 20 kΩ and the A terminal is open circuited,

the RDAC latch codes in Table 5 result in the corresponding output

resistance, RWB.

Table 5. Codes and Corresponding RWB Resistances

D (Dec)

RWB(Ω)

Output State

255

20,042

Full-scale (R

AB − 1 LSB + 2 × RW)

128

10,120

Midscale

198

1 LSB + 2 × R

120

Zero-scale (wiper contact resistance)

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

120 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 W wiper 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 = 20 kΩ and the B terminal is open circuited, the RDAC

latch codes in Table 6 result in the corresponding output

resistance RWA.

Table 6. Codes and Corresponding RWA Resistances

D (Dec)

WA (Ω)

Output State

255

198

Full scale

128

10,120

Midscale

20,042

1 LSB + 2 × R

20,120

Zero scale

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