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4346
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Maxim > Design support >
App notes
> Digital
Potentiometers > APP 4346

Dec 11, 2008

Keywords: digital pots, potentiometer, voltage
divider, sensor conditioning, ratiometric, precision resistor,
resistance, ratio, single power supply, temperature
coefficient, low drift, linear, log, logarithmic, arbitrary
waveform curve, string DAC, audio
Related
parts

Download,
PDF format (37kB)

[?]

APPLICATION NOTE 4346

Pots, Pans, Logs, and Linear Digital Pots Create
Arbitrary Voltage Curves

By: Bill
Laumeister

Abstract: Linear
digital potentiometers can be used to make arbitrary voltage
curves and waveforms. An alternative name for a digital pot is
a "string" digital-to-analog converter (DAC). Logging pots is
the art of making a linear pot produce a logarithmic response
for applications such as audio.

Attend
this brief webcast by Maxim on
TechOnline

Logging,
pots, and pans are about as diverse as subjects get. Well,
that is what most people might suspect because the common
definition of logging is harvesting trees. When logs are
mentioned, people often visualize cabins and former President
Lincoln seated at his desk. Pots and pans are typically
ignored as mundane. Nonetheless, an average cook needs great
pots to help prevent overcooking and burning food. Engineers,
however, have different ideas about logs and
pots.

Figure 1 compares pots and pans with logs
and log cabins. It also shows the linear and logarithmic plots
of a digital pot (potentiometer).

Figure
1. Pots and pans, and logs vs. potentiometers; the plots of
log and linear responses are resistance vs. pot
position.

The most popular electronic
potentiometers (pots) are linear (Figure 1, dotted line). Due
to their large sales volume, they tend to be among the less
expensive pots. Nevertheless, other applications like audio
require a logarithmic response (Figure 1, solid line) to match
the human ear's response. The dichotomy of these responses
inevitably leads to a designer's dilemma. How can we convert a
linear response to logarithmic? Maxim application note 3996,
"Logging the Linear
Digital Potentiometer," provides the math to do the
conversion.

How, then, can we visualize the simple
conversion? We will graphically illustrate a simple solution.
As application note 3996 suggests, the minimum number of taps
on the digital pot should be 128 or more. This number ensures
that enough steps are available to keep the errors under
reasonable control. With some thought we realize that half of
the taps always represents 6dB. Consequently, the other half
of the taps is the attenuations that are more than
6dB.

We can use a 256-step digital pot as an example.
The Maxim website has more than 30 different 256-step digital
pots available. The pots have a variety of serial
interfaces including I²C and SPI™, as well as up/down. The
parts have both volatile and nonvolatile
memories. An important parameter for audio use is the
glitchless switching between taps found in the MAX5403/MAX5404/MAX5405
family of parts.

To convert a linear pot to logarithmic
we just skip certain steps. A 256-tap linear pot can provide
38 log steps with about ±1% accuracy. The steps in dB are 48,
42, 38.5, 36, 34, 32.5, and in 1dB there are steps from 31 to
0. A spreadsheet with the exact numbers is available for download.
The spreadsheet also contains 0.5dB increments below the 32dB
point.

Using the same step-skipping method we can
generate other arbitrary voltage
curves. Thus we can compensate for other nonlinear responses
such as sensors. For example, if a given sensor changes with
temperature,
we could sense temperature and compensate.

An
alternative name for a digital pot is a "string
digital-to-analog converter" (DAC),
which means that digital pots are specified with parameters
like integral
nonlinearity and dynamic nonlinearity.

Now we
return to the logs, pots, and pans, and can use our
imagination to do creative designs. Usually designers do not
need to sit at Lincoln's desk for inspiration, and logs do not
always conjure up images of rustic cabins. However, we all can
go into our own kitchen, grab a pot, create good food, and be
inspired.

SPI
is a trademark of Motorola, Inc.

Related parts

MAX5403

Dual, 256-Tap,
Low-Drift, Digital Potentiometers in 10-µMAX

Free
samples

MAX5404

Dual, 256-Tap,
Low-Drift, Digital Potentiometers in 10-µMAX

Free
samples

MAX5405

Dual, 256-Tap,
Low-Drift, Digital Potentiometers in 10-µMAX

Free
samples

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Download,
PDF format (37kB)
APP 4346: Dec 11, 2008

AN4346, AN 4346, APP4346,
Appnote4346, Appnote 4346

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