Login
Register

ENGLISH简体中文
(Chinese)日本語
(Japanese)РУССКИЙ
(Russian)

Products

Solutions

Design

Buy

Support

About Us

My Maxim

Maxim > Design
support > App notes
> Audio
Circuits > APP 4292
Maxim > Design support >
App notes
> Circuit
Protection > APP 4292
Maxim > Design support >
App notes
> General
Engineering Topics > APP 4292

Sep 25, 2008

Keywords: Initial accuracy, digital pots,
potentiometer, resistance, voltage dividers, ratiometric,
ratio steps, LSB, taps error, reference decoupling, Earth
Ground, high push-button interface, common space shuttle,
local potential
Related
parts

Download,
PDF format (94kB)

[?]

APPLICATION NOTE 4292

Where Is Ground?

By: Bill
Laumeister

Abstract: For
safety reasons we keep our circuits referenced to our
surroundings. In most cases this is earth ground. This
application note explains how a proper understanding of local
circuit ground as a reference point frees our thinking to
consider other simple circuits.

Attend
this brief webcast by Maxim on
TechOnline

Where is ground in space?
Usually engineering is
strictly constrained by the laws of physics. Ground is an
exception, as engineers can get an ego boost
because we can finally control and define something. It is so,
just because we say so. It reminds us of the ego power
employed by Humpty Dumpty in Lewis Carroll's Through the
Looking-Glass.

"'When I use a word,' Humpty Dumpty said in a
rather scornful tone, 'it means just what I choose it to
mean—neither more nor less.'
'The question is,' said
Alice, 'whether you CAN
make words mean so many different things.'
'The question
is,' said Humpty Dumpty, 'which is to be master—that's
all.'"¹

Defining ground in earth orbit
emphasizes a possible confusion between "common" and
engineering language use. This dichotomy applies equally to
airplanes and automobiles driving on rubber tires. When we
engineers say "ground," we really mean "reference point." This
concept should be the beginning of our thought process. When
we realize that we can make the term relative, we gain a
certain freedom.

For safety reasons we keep our
circuits referenced to our surroundings. In most cases this is
earth ground. Common electrical building codes mandate that
the AC power line be bonded to the earth as it enters the
premises.

In power tools, insurance safety regulations
allow a choice between grounding the metal case with a third
wire safety ground or using double insulation. Similarly, we
can maintain safety and define the local ground or reference
point inside the equipment as we choose.

By separating
the reference point from safety ground we can look at an IC
data sheet in a new perspective.

A typical data sheet
shows a device's Absolute Maximum Ratings. Figure 1
says that a pin named VCC needs to be within a
certain voltage
range compared to a pin named GND.

Figure
1. Typical listings for absolute values taken from a data
sheet.

These values raise a question: could
VCC and GND meet their relative voltage
requirements and still be offset together by thousands of
volts compared to something else? Yes, of course!

Practical Examples

Figure
2. A diagram shows making ground a relative
+25V.

Assume for the moment that an application
needs a circuit to operate near a 30V supply, perhaps a sensor
or a LCD
contrast control. From Figure 2, the sensor can be
buffered with U1 and AC-coupled to other circuits. DP1 can
adjust bias gain or contrast. But how do you adjust
DP1?

Figure
3. A digital pot floating near +30V with self-debouncing
switches.

The MAX5174 up/down
switches are momentary in the up-arrow direction. They are
debounced because the switches mechanically chatter between
one end at a time. The switches do not bounce between the two
voltages. In simple terms, as the switch starts opening on the
+25V side, the contact will bounce between open circuit and
being connected to +25V. As the moving contact approaches the
+30V side, the bounce similarly will be between open and +30V.
The mechanics of the switch preclude a bounce between +25V and
+30V.

Figure
4. A digital pot floating between +25V and +30V with
internally debounced switches.

The DS1669, DS1809, and DS1869 digital
potentiometers have internal 100kΩ pullup resistors. They have
internal circuitry to debounce the switches and autoincrement
the wiper position. The data sheet explains the repetitive
pulse and continuous pulse ("push-and-hold") functions as well
as an unique single-switch control configuration.

Figure
5. A switch-driven up/down/increment interface.

The
up/down/increment interface of the MAX5450–MAX5455, MAX5471–MAX5475,
and DS4301
devices debounces the switches. If the switch is held down it
will autoincrement. See application note 1118, "Manually Operated
Digital Potentiometer Doesn't Need a Microprocessor," for
details.

Maxim's complete list of digital
pots is available on the website.

¹Carroll, Lewis,
Through the Looking-Glass, Chapter 6 (1872).

Related parts

DS1669

Dallastat Electronic
Digital Rheostat

DS1804

NV Trimmer
Potentiometer

Free
samples

DS1809

Dallastat

Free
samples

DS1869

3V Dallastat Electronic
Digital Rheostat

Free
samples

DS4301

Nonvolatile,
32-Position Digital Potentiometer

Free
samples

MAX5128

128-Tap, Nonvolatile,
Linear-Taper Digital Potentiometer in 2mm x 2mm
µDFN Package

Free
samples

MAX5160

Low-Power Digital
Potentiometers

Free
samples

MAX5450

Dual, 256-Tap, Up/Down
Interface, Digital Potentiometers

Free
samples

MAX5451

Dual, 256-Tap, Up/Down
Interface, Digital Potentiometers

Free
samples

MAX5452

Dual, 256-Tap, Up/Down
Interface, Digital Potentiometers

Free
samples

MAX5453

Dual, 256-Tap, Up/Down
Interface, Digital Potentiometers

Free
samples

MAX5454

Dual, 256-Tap, Up/Down
Interface, Digital Potentiometers

Free
samples

MAX5455

Dual, 256-Tap, Up/Down
Interface, Digital Potentiometers

Free
samples

MAX5471

32-Tap, Nonvolatile,
Linear-Taper Digital Potentiometers in SOT23

Free
samples

MAX5472

32-Tap, Nonvolatile,
Linear-Taper Digital Potentiometers in SOT23

MAX5474

32-Tap, Nonvolatile,
Linear-Taper Digital Potentiometers in SOT23

MAX5475

32-Tap, Nonvolatile,
Linear-Taper Digital Potentiometers in SOT23

Free
samples

MAX6302

+5V, Low-Power, µP
Supervisory Circuits with Adjustable
Reset/Watchdog

Free
samples

MAX6817

±15kV ESD-Protected,
Single/Dual/Octal, CMOS Switch Debouncers

Free
samples

Automatic updates
Would you like to be
automatically notified when new application notes are
published in your areas of interest? Sign
up for EE-Mail™.

Download,
PDF format (94kB)
APP 4292: Sep 25, 2008

AN4292, AN 4292, APP4292,
Appnote4292, Appnote 4292

|

|

|     Privacy Policy
|     Legal Notices

Copyright © 2011 by Maxim Integrated
Products