Data Sheet

AD5263

Rev. F | Page 27 of 28

RESISTANCE SCALING

The AD5263 offers 20 kΩ, 50 kΩ, and 200 kΩ nominal resistances.

Users who need a lower resistance and the same number of step

adjustments can place multiple devices in parallel. For example,

Figure 67 shows a simple scheme of using two channels in parallel.

To adjust half of the resistance linearly per step, users need to

program both channels to the same settings.

W2

A1

B1

A2

B2

LED

W1

Figure 67. Reduce Resistance by Half with Linear Adjustment Characteristics

Applicable only to the voltage divider mode, by connecting a

discrete resistor in parallel as shown in Figure 68, a proportionately

lower voltage appears at Terminal A. This translates into a finer

degree of precision because the step size at Terminal W is smaller.

The voltage can be found as

(

) (

)

R1

R

R1

R

R2

V

D

D

V

AB

AB

DD

W

||

||

256

)

(

×













+

×

=

(17)

W

A

B

R1

R2

Figure 68. Decreasing Step Size by Lowering the Nominal Resistance

Figure 67 and Figure 68 show applications in which the digital

potentiometers change steps linearly. On the other hand, log

taper adjustment is usually preferred in applications such as

volume control. Figure 69 shows another method of resistance

scaling which produces a pseudolog taper output. In this circuit,

output approaches log type behavior.

A

B

R1

R2

Figure 69. Resistor Scaling with Log Adjustment Characteristics

RESISTANCE TOLERANCE, DRIFT, AND

TEMPERATURE COEFFICIENT MISMATCH

CONSIDERATIONS

In rheostat mode operation, such as the gain control circuit of

Figure 70, the tolerance mismatch between the digital potent-

iometer and the discrete resistor can cause repeatability issues

among various systems. Because of the inherent matching of the

silicon process, it is practical to apply the multichannel device

in this type of application. As such, R1 should be replaced by

one of the channels of the digital potentiometer. R1 should be

programmed to a specific value while R2 can be used for the

adjustable gain. Although it adds cost, this approach minimizes

the tolerance and temperature coefficient mismatch between R1

and R2. In addition, this approach also tracks the resistance drift

over time. As a result, these nonideal parameters become less

sensitive to system variations.

U1

C1

R2

+

–

AD8601

W

B

A

Figure 70. Linear Gain Control with Tracking Resistance Tolerance and Drift

Notice that the circuit in Figure 71 can also be used to track the

tolerance, temperature coefficient, and drift in this particular

application. However, the characteristics of the transfer function

change from a linear to a pseudologarithmic gain function.

U1

V+

+

–

AD8601

B

W

A

R

C1

Figure 71. Nonlinear Gain Control with Tracking Resistance Tolerance

and Drift