AD5232 datasheet(19/24 Pages) AD

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Data Sheet

AD5232

Rev. C | Page 19 of 24

Table 11. Nominal Individual Segment Resistor Values (Ω)

Device

Resolution

Segmented Resistor Size

for RAB End-to-End Values

10 kΩ

Version

50 kΩ

Version

100 kΩ

Version

8-Bit

78.10

390.5

781.0

PROGRAMMING THE VARIABLE RESISTOR

Rheostat Operation

The nominal resistances of the RDACx between Terminal A and

Terminal B are available with values of 10 kΩ, 50 kΩ, and 100 kΩ.

The final digits of the part number determine the nominal

resistance value; for example, 10 kΩ = 10; 100 kΩ = 100. The

nominal resistance (RAB) of the AD5232 VR has 256 contact

points accessed by Wiper Terminal W, plus the Terminal B contact.

The 8-bit data-word in the RDACx latch is decoded to select

one of the 256 possible settings.

The general transfer equation, which determines the digitally

programmed output resistance between Wx and Bx, is







256

)

(

(1)

where:

is the decimal equivalent of the data contained in the RDACx

RAB

is the nominal resistance between Terminal A and Terminal B.

is the wiper resistance.

Table 12 lists the output resistance values that are set for the

RDACx latch codes shown for 8-bit, 10 kΩ potentiometers.

Table 12. Nominal Resistance Value at Selected Codes for

RAB = 10 kΩ

D (Dec)

RWB (D) (Ω)

Output State

255

10011

Full scale

128

5050

Midscale

1 LSB

Zero scale1 (wiper contact resistance)

1 Note that in the zero-scale condition, a finite wiper resistance of 50 Ω is

present. Care should be taken to limit the current flow between Wx and Bx

in this state to a maximum continuous value of 2 mA to avoid degradation

or possible destruction of the internal switch metallization. Intermittent

current operation to 20 mA is allowed.

Like the mechanical potentiometer that the RDACx replaces,

the AD5232 parts are totally symmetrical. The resistance between

the Wiper Terminal W and Terminal A also produces a digitally

controlled resistance, RWA. Figure 41 shows the symmetrical

programmability of the various terminal connections.

100

128

192

258

CODE (Decimal)

RWB

RWA

Figure 41. Symmetrical RDAC Operation

When these terminals are used, Terminal B should be tied to

the wiper. Setting the resistance value for RWA starts at a maximum

value of resistance and decreases as the data loaded in the latch

is increased in value. The general transfer equation for this

operation is









256

)

(

(2)

where:

D is the decimal equivalent of the data contained in the RDAC

RAB is the nominal resistance between Terminal A and Terminal B.

RW is the wiper resistance.

Table 13 lists the output resistance values that are set for the

RDACx latch codes shown for 8-bit, 10 kΩ potentiometers.

Table 13. Nominal Resistance Value at Selected Codes for

RAB = 10 kΩ

D (Dec)

RWA (D) (Ω)

Output State

255

Full scale

128

5050

Midscale

10011

1 LSB

10050

Zero scale

The multichannel AD5232 has a ±0.2% typical distribution of

internal channel-to-channel RBA match. Device-to-device matching

is dependent on process lot and exhibits a −40% to +20% variation.

The change in RBA with temperature has a 600 ppm/°C temperature

coefficient.

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