Datasheet

AD5263
Rev. 0 | Page 24 of 28
RESISTANCE SCALING RESISTANCE TOLERANCE, DRIFT, AND
TEMPERATURE COEFFICIENT MISMATCH
CONSIDERATIONS
The AD5263 offers 20kΩ, 50kΩ, and 200kΩ nominal
resistances. Users who need a lower resistance and the same
number of step adjustments can place multiple devices in
parallel. For example, Figure 66 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.
In the rheostat mode operation, such as the gain control circuit
of Figure 69, the tolerance mismatch between the digital
potentiometer 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 non-ideal
parameters become less sensitive to system variations.
03142-0-066
W2
A1
B1
A2
B2
LED
V
DD
W1
03142-0-070
AD8601
V
I
V
O
AB
C1
W
U1
R1
R2
*REPLACED WITH ANOTHER CHANNEL OF RDAC
Figure 66. 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 67, a
proportionately lower voltage appears at Terminal A. This
translates into a finer degree of precision because the step size at
Terminal W will be smaller. The voltage can be found as
()
(
R1R
R1RR2
VD
DV
AB
AB
DD
W
||
||256
)( ×








+
×=
)
(18)
Figure 69. Linear Gain Control with Tracking Resistance Tolerance and Drift
03142-0-067
W
A
B
R1
R2
R1 << R
AB
V
DD
Notice that the circuit in Figure 70 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 pseudo-logarithmic gain function.
03142-0-071
AD8601
V
I
V
O
A
R
B
C1
W
U1
Figure 67. Decreasing Step Size by Lowering the Nominal Resistance
Figure 66 and Figure 67 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 68 shows another method of resistance
scaling which produces a pseudo-log taper output. In this
circuit, the smaller the value of R2 with respect to R
AB
, the more
the output approaches log type behavior.
Figure 70. Nonlinear Gain Control with Tracking Resistance Tolerance and
Drift
03142-0-068
V
I
V
O
A
B
R1
R2
Figure 68. Resistor Scaling with Log Adjustment Characteristics