AD5235BRUZ250 datasheet(22/32 Pages) AD

Part #	AD5235BRUZ250

Description	Nonvolatile Memory, Dual 1024-Position Digital Potentiometer

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AD5235

Data Sheet

Rev. F | Page 22 of 32

PROGRAMMING THE VARIABLE RESISTOR

Rheostat Operation

The nominal resistance of the RDAC between Terminal A

and Terminal B, RAB, is available with 25 kΩ and 250 kΩ with

1024 positions (10-bit resolution). The final digits of the part

number determine the nominal resistance value, for example,

25 kΩ = 24.4 Ω; 250 kΩ = 244 Ω.

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

of the 1024 possible settings. The following description provides

the calculation of resistance, RWB, at different codes of a 25 kΩ

part. The first connection of the wiper starts at Terminal B for

Data 0x000. RWB(0) is 30 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 24.4 Ω + 30 Ω = 54.4 Ω

for Data 0x001. The third connection is the next tap point

representing RWB(2) = 48.8 Ω + 30 Ω = 78.8 Ω for Data 0x002,

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) =

25006 Ω. See Figure 45 for a simplified diagram of the equivalent

RDAC circuit. When RWB is used, Terminal A can be left

floating or tied to the wiper.

CODE (Decimal)

100

75

0

1023

256

512

768

50

25

RWA

RWB

Figure 46. RWA(D) and RWB(D) vs. Decimal Code

The general equation that determines the programmed output

resistance between Terminal Bx and Terminal Wx is

W

AB

WB

R

D

R

+

×

=

1024

)

(

(1)

where:

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

register.

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

RW is the wiper resistance.

For example, the output resistance values in Table 12 are set for

the given RDAC latch codes (applies to RAB = 25 kΩ digital

potentiometers).

Table 12. RWB (D) at Selected Codes for RAB = 25 kΩ

D (Dec)

RWB(D) (Ω)

Output State

1023

25,006

Full scale

512

12,530

Midscale

1

54.4

1 LSB

0

30

Zero scale (wiper contact resistor)

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 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 that the RDAC replaces, the

AD5235 part is symmetrical. The resistance between Wiper W

and Terminal A also produces a digitally controlled complementary

resistance, RWA. Figure 46 shows the symmetrical

programmability of the various terminal connections. When RWA

is used, Terminal B can be left 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

W

AB

WA

R

D

R

+

×

−

=

1024

)

(

(2)

For example, the output resistance values in Table 13 are set for

the given RDAC latch codes (applies to RAB = 25 kΩ digital

potentiometers).

Table 13. RWA(D) at Selected Codes for RAB= 25 kΩ

D (Dec)

RWA(D) (Ω)

Output State

1023

54.4

Full scale

512

12,530

Midscale

1

25,006

1 LSB

0

25,030

Zero scale (wiper contact resistance)

The typical distribution of RAB from channel to channel is

±0.2% within the same package. Device-to-device matching is

process lot dependent upon the worst case of ±30% variation.

However, the change in RAB with temperature has a 35 ppm/°C

temperature coefficient.

PROGRAMMING THE POTENTIOMETER DIVIDER

Voltage Output Operation

The digital potentiometer can be configured to generate an output

voltage at the wiper terminal that is proportional to the input

voltages applied to Terminal A and Terminal B. For example,

connecting Terminal A to 5 V and Terminal B to ground

produces an output voltage at the wiper that can be any value

from 0 V to 5 V. Each LSB of voltage is equal to the voltage

applied across Terminal A to Terminal B divided by the 2N

position resolution of the potentiometer divider.

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