AD5273

–17–

2.5V

CONTROLLER

2.7V–5.5V

AD5273

Rp

Rp

Rp

Rp

G

G

S

D

M1

S

D

M2

SDA1

SCL1

SDA2

SCL2

Figure 18. Level Shift for Different Voltage Operation

Resistance Scaling

The AD5273 offers 1 k , 10 k , 50 k , and 100 k

nominal

resistances. For users who need to optimize the resolution with

an arbitrary full-range resistance, the following techniques can

be the solutions. Applicable only to the voltage divider mode, by

paralleling a discrete resistor as shown in Figure 19, a propor-

tionately lower voltage appears at terminal A–B. This translates

into a finer degree of precision because the step size at terminal

W will be smaller. The voltage can be found as:

VD

RR

RR

R

D

V

W

AB

AB

DD

+

¥¥

//

//

2

32

256

(9)

R1

R2

B

A

R3

W

Figure 19. Lowering the Nominal Resistance

Figure 19 shows that the digital potentiometer changes steps lin-

early. On the other hand, log taper adjustment is usually preferred in

applications like volume control. Figure 20 shows another way of

resistance scaling. In this circuit, the smaller the R2 with respect

The wiper voltage is simply:

VD

RD

R

RD

RD

R

V

W

WB

WA

WB

I

()

¥

//

//

2

2

(10)

R1

B

A

R2

W

Figure 20. Resistor Scaling with Log Adjustment

Characteristics

Resolution Enhancement

Borrowed from ADI’s patented RDAC segmentation technique,

users can configure three AD5273s to double the resolution. (See

Figure 21.) First, U3 must be paralleled with a discrete resistor

may see that adjusting U1 and U2 together forms the coarse 6-bit

adjustment and adjusting U3 alone forms the finer 6-bit adjust-

ment. As a result, the effective resolution becomes 12-bit.

U1

A1

B1

W1

U2

A2

B2

W3

W2

U3

A3

B3

COARSE

ADJUSTMENT

FINE

ADJUSTMENT

Figure 21. Double the Resolution in Rheostat Mode Operation

RDAC CIRCUIT SIMULATION MODEL

The internal parasitic capacitances and the external capacitive

loads dominate the ac characteristics of the digital potentiometers.

Configured as a potentiometer divider, the –3 dB bandwidth of

the AD5273 (1 k

resistor) measures 6 MHz at half scale. TPCs

15–18 provide the large signal BODE plot characteristics of the

four available resistor versions 1 k , 10 k , 50 k , and 100 k .

Figure 22 shows a parasitic simulation model. The code following

Figure 22 provides a macro model net list for the 1 k

device:

55pF

25pF

25pF

A

B

1k

W

Figure 22. Circuit Simulation Model for RDAC = 1 k

Macro Model Net List for RDAC

.PARAM D = 63, RDAC = 1E3

*

.SUBCKT DPOT (A,W,B)

*

CA

A

0

25E-12

RWA

A

W

{(1-D/63)*RDAC+60}

CW

W

0

55E-12

RWB

W

B

{D/63*RDAC+60}

CB

B

0

25E-12

*

.ENDS DPOT

REV. 0