AD5258BRMZ10-R7 datasheet(14/24 Pages) AD

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AD5258

Data Sheet

Rev. D | Page 14 of 24

THEORY OF OPERATION

The AD5258 is a 64-position digitally controlled variable

resistor (VR) device. The wipers default value prior to pro-

gramming the EEPROM is midscale.

PROGRAMMING THE VARIABLE RESISTOR

Rheostat Operation

The nominal resistance (RAB) of the RDAC between Terminal A

and Terminal B is available in 1 kΩ, 10 kΩ, 50 kΩ, and 100 kΩ.

The nominal resistance of the VR has 64 contact points accessed

by the wiper terminal. The 6-bit data in the RDAC latch is

decoded to select one of 64 possible settings.

Figure 36. Rheostat Mode Configuration

The general equation determining the digitally programmed

output resistance between Wiper W and Terminal B is

( )

(1)

where:

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

6-bit RDAC register.

RAB is the end-to-end resistance.

RW is the wiper resistance contributed by the on resistance of

each internal switch.

RDAC

LATCH

AND

DECODER

Figure 37. AD5258 Equivalent RDAC Circuit

Note that in the zero-scale condition, there is a relatively

low value finite wiper resistance. Care should be taken to

limit the current flow between Wiper W and Terminal B in

this state to a maximum pulse current of no more than 20 mA.

Otherwise, degradation or destruction of the internal switch

contact may occur.

Similar to the mechanical potentiometer, the resistance of the

RDAC between Wiper W and Terminal A produces a digitally

controlled complementary resistance, RWA. The resistance value

setting 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

( )

−

(2)

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

may vary by up to ±30%. For this reason, resistance tolerance is

stored in the EEPROM such that the user will know the actual

RAB within 0.1%.

PROGRAMMING THE POTENTIOMETER DIVIDER

Voltage Output Operation

The digital potentiometer easily generates a voltage divider at

Wiper W-to-Terminal B and Wiper W-to-Terminal A propor-

tional to the input voltage at Terminal A-to-Terminal B. Unlike

the polarity of VDD-to-GND, which must be positive, voltage

across Terminal A-to-Terminal B, Wiper W-to-Terminal A,

and Wiper W-to-Terminal B can be at either polarity.

Figure 38. Potentiometer Mode Configuration

If ignoring the effect of the wiper resistance for approximation,

connecting the A terminal to 5 V and the B terminal to ground

produces an output voltage at Wiper W-to-Terminal B starting

at 0 V up to 1 LSB less than 5 V. The general equation defining

the output voltage at VW with respect to ground for any valid

input voltage applied to Terminal A and Terminal B is

( )

−

(3)

A more accurate calculation, which includes the effect of wiper

resistance (VW) is

)

(

)

(

)

(

(4)

Operation of the digital potentiometer in the divider mode

results in a more accurate operation over temperature. Unlike

the rheostat mode, the output voltage is dependent mainly on

the ratio of internal resistors (RWA and RWB) and not the abso-

lute values.

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