Improving the Resolution of Digital Potentiometer (POT) Applications

AND8420/D
Former Catalyst Document Number DN4
Improving the Resolution of
Digital Potentiometer (POT)
Applications
http://onsemi.com
Resolution is the ability to discern one of many. In
application circuits using digital POTs, resolution is both a
function of the potentiometer and the circuit design. The
objective of this application note is to illustrate a few basic
device and circuit ideas on resolution. This application note
focuses on resolving voltage.
There are millions of applications of the potentiometer,
however, the most common one is the programmable
voltage divider shown in Figure 1. For a potentiometer with
n taps (remember two of the taps are used for the bottom and
top of the resistive array), the resolution of the potentiometer
is given as:
V OUT (smallest) +
APPLICATION NOTE
If the range of the programmed voltage can be restricted
in the application, the pot can be inserted in a voltage divider
as shown in Figure 2. Resistors R1, R2, and the end-to-end
resistance of the potentiometer, RPOT, define the voltage
range and the number of pot taps defines the voltage
resolution within this range.
V OUT (smallest) +
V REF
For example, if the voltage range is restricted from 0.601 V
to 0.701 V, the resolution is for the tap pot 1.01 mV over this
range.
0 V v V OUT v V REF
The output of the circuit is the wiper voltage which may
be buffered (VOUT‘) or not (VOUT). The amount of loading
of the wiper will determine whether to use the buffered or
unbuffered version of the digital POT. For example, the
resolution is 50.5 mV for a 100 tap pot and a 5 V reference.
The number of taps of ON Semiconductor’s digital POTs
vary from 32 to 256.
VREF
R2
(432kW)
V 2 (0.701V)
VREF (5V)
RPOT
(10kW)
V1 (0.600V)
VOUT (50.5mV)
(CAT5113)
+
−
R1
(60.4kW)
 Semiconductor Components Industries, LLC, 2013
VOUT(1.01mV)
+
−
VOUT’(1.01mV)
VOUT’ (50.5mV)
(CAT5111)
Figure 2. Differential Voltage Divider
Figure 1. Programmable Voltage Divider
July, 2013 − Rev. 1
n * 1 R POT ) R 1 ) R 2
V 1 v V OUT v V 2
n*1
RPOT
R POT
V REF
1
Publication Order Number:
AND8420/D
AND8420/D
An amplifier and a few discrete resistors can extend the
resolution to satisfy any application requirement. The circuit
in Figure 3 consists of two buffered digitally controlled,
100 tap, potentiometers and a summing amplifier circuit
with weighted-value, input resistors. For this circuit,
V OUT +
99 V 1
100
)
VREF (+5V)
Digital POT 1
+
−
V2
V1
100
R
V 1,2
1
V OUT (smallest) +
n * 1 100
+
−
VREF (+5V)
The output of this circuit example varies from 0 to 5 V in
half-millivolt steps and is the equivalent to a 13+ binary bit
DAC (with memory!!). The use of summing amplifier
(inverting and noninverting) circuits, weighted-value input
resistors, and digital POTs can extend the resolution to any
required level.
Digital POT 2
99R
+
−
VOUT
(0.505mV)
V2
Figure 3. High Resolution Voltage Divider
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AND8420/D