AD5231 datasheet(15/24 Pages) AD

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REV. 0

AD5231

–15–

part. For VDD = 5 V, the wiper first connection starts at the B

terminal for data 000H. RWB(0) is 15

Ω because of the wiper

resistance and it is independent of the nominal resistance. The

second connection is the first tap point where RWB(1) becomes

9.7

Ω + 15 Ω = 27.4 Ω for data 001

H. The third connection is

the next tap point representing RWB(2) = 19.4 + 15 = 34.4

Ω for

data 002H and so on. Each LSB data value increase moves the

wiper up the resistor ladder until the last tap point is reached at

RWB(1023) = 10005

Ω. See Figure 9 for a simplified diagram of

the equivalent RDAC circuit. When RWB is used, the A–terminal

can be left floating or tied to the wiper.

CODE – Decimal

100

1023

256

512

768

RWB

RWA

Figure 10. RWA(D) and RWB(D) vs. Decimal Code

The general equation, which determines the programmed output

resistance between W and B, is:

()

=×

1024

(1)

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

RDAC register, RAB is the Nominal Resistance between terminals

A-and-B, and RW is the wiper resistance.

For example, the following output resistance values will be set

for the following RDAC latch codes with VDD = 5 V (applies to

RAB = 10 k

Ω Digital Potentiometers):

Table VII. RWB at Selected Codes for RAB = 10 k

Ω

D(DEC) RWB(D) (

Ω) Output State

1023

10,005

Full-Scale

512

50015

MidScale

24.7

1 LSB

Zero-Scale (Wiper Contact Resistor)

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

Ω is present. Care should be taken to limit the current flow

between W and B in this state to no more than 20 mA to avoid

degradation or possible destruction of the internal switches.

Like the mechanical potentiometer the RDAC replaces, the

AD5231 parts are totally symmetrical. The resistance between the

wiper W and terminal A also produces a digitally controlled

complementary resistance RWA. Figure 10 shows the symmetrical

programmability of the various terminal connections. When RWA

is used, the B-terminal can be let floating or 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:

NOTES

1RDAC data stored in EEMEM location is transferred to the RDAC Register at

Power ON, or when instructions #1, #8, and

PR are executed.

2Execution of instruction #1 leaves the device in the Read Mode power con-

sumption state. After the last instruction #1 is executed, the user should perform

a NOP, instruction #0 to return the device to the low power idling state.

3O1 and O2 data stored in EEMEM locations are transferred to their corresponding

Digital Register at Power ON, or when instructions #1 and #8 are executed.

4USER <data> are internal nonvolatile EEMEM registers available to store and

retrieve constants and other 16-bit information using #3 and #9, respectively.

RDAC STRUCTURE

The patent pending RDAC contains multiple strings of equal

resistor segments, with an array of analog switches, that act as

the wiper connection. The number of positions is the resolution

of the device. The AD5231 has 1024 connection points allowing

it to provide better than 0.1% set-ability resolution. Figure 9

shows an equivalent structure of the connections between the

three terminals of the RDAC. The SWA and SWB will always be

ON, while one of the switches SW(0) to SW(2

N–1) will be ON

one at a time depending on the resistance position decoded

from the data bits. Since the switch is not ideal, there is a 15

Ω

wiper resistance, RW. Wiper resistance is a function of supply

voltage and temperature. The lower the supply voltage, or the

higher the temperature, the higher the resulting wiper resistance.

Users should be aware of the wiper resistance dynamics if accu-

rate prediction of the output resistance is needed.

SW(1)

SW(0)

SWB

SWA

SW(2N–1)

SW(2N– 2)

RDAC

WIPER

AND

DECODER

RS = RAB /2

DIGITAL

CIRCUITRY

OMITTED FOR

CLARITY

Figure 9. Equivalent RDAC Structure (Patent Pending)

Table VI. Nominal Individual Segment Resistor (RS)

Device

10 k

50 k

100 k

Resolution

Version

10-Bit

9.8

Ω

48.8

Ω

97.6

Ω

PROGRAMMING THE VARIABLE RESISTOR

Rheostat Operation

The nominal resistance of the RDAC between terminals A-and-

B, RAB is available with 10 k

Ω, 50 kΩ, and 100 kΩ with 1024

positions (10-bit resolution). The final digit(s) of the part number

determine the nominal resistance value, e.g., 10 k

Ω = 10; 50 kΩ = 50;

100 k

Ω = C.

The 10-bit data word in the RDAC latch is decoded to select one

of the 1024 possible settings. The following discussion describes

the calculation of resistance RWB at different codes of a 10 k

Ω

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