Data Sheet

AD5290

Rev. C | Page 15 of 20

THEORY OF OPERATION

PROGRAMMING THE VARIABLE RESISTOR

Rheostat Operation

The part operates in the rheostat mode when only two termi-

nals are used as a variable resistor. The unused terminal can

be floating or tied to the W terminal as shown in Figure 26.

A

W

B

A

W

B

A

W

B

Figure 26. Rheostat Mode Configuration

The nominal resistance between Terminal A and Terminal B,

ance and has 256 tap points accessed by the wiper terminal. The

8-bit data in the RDAC latch is decoded to select one of the 256

possible settings. Figure 27 shows a simplified RDAC structure.

W

A

B

8-BIT ADDRESS

DECODER

Figure 27. AD5290 Simplified RDAC Circuit.

In order to achieve optimum cost performance, Analog Devices

has patented the RDAC segmentation architecture for all the

digital potentiometers. In particular, the AD5290 employs a

3-stage segmentation approach as shown in Figure 27. As

a result, the general equation determining the digitally

programmed output resistance between the W terminal

and B terminal is

W

AB

WB

R

R

D

D

R

×

+

×

=

3

256

)

(

(1)

where:

D is the decimal equivalent of the binary code loaded in

the 8-bit RDAC register from 0 to 255.

resistance of an internal switch.

The AD5290 wiper switch is designed with the transmission

gate CMOS topology and with the gate voltage derived from

which is only 35 ppm/°C, the temperature coefficient of the wiper

resistance is significantly higher because the wiper resistance

doubles from 25°C to 125°C. As a result, the user must take into

consideration the contribution of RW on the desirable

resistance. On the other hand, the wiper resistance is insensitive

to the tap point potential. As a result, RW remains relatively flat

Assuming that an ideal 10 kΩ part is used, the wiper’s first

connection starts at the B terminal for the programming code

of 0x00 where SWB is closed. The minimum resistance between

Terminal W and Terminal B is, therefore, generally 150 Ω. The

second connection is the first tap point, which corresponds to

and so on. Each LSB data value increase moves the wiper up the

resistor ladder until the last tap point is reached at 10,110 Ω.

In the zero-scale condition, a finite total wiper resistance of

150 Ω is present. Regardless of which setting the part is oper-

ating in, care should be taken to limit the current between

the A terminal to B terminal, W terminal to A terminal, and

W terminal to B terminal, to the maximum dc current of 5 mA

or pulse current of 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 terminal and the A terminal also

the data loaded into the latch increases. The general equation

for this operation is

W

AB

WA

R

R

D

D

R

×

+

×

−

=

3

256

256

)

(

(2)