Analog Devices ee-59 Application Notes

Engineer To Engineer Note EE-59

Notes on using Analog Devices’ DSP, audio, & video components from the Computer Products Division

Phone: (800) ANALOG-D, FAX: [phone], EMAIL: [e-mail], FTP: ftp.analog.com

quartz crystals are used. Therefore,

CODEC SOUNDBYTES

AD1819 / AD1819A / AD1819B /
AD1881 Crystal Requirements

Last Modified: 10/14/98
Contributed by: Fred L.

Overview

This Edition of Engineer’s Notes will cover
crystal requirements for clocking Analog
Devices’ AC97 codecs .

1. Frequency Range.

All Analog Devices’ AC97 codecs are
designed to work from a crystal with a
frequency of 24.576MHz. This crystal
rate is required by AC97 1.03. Crystals
with a frequency of less than 24.576MHz
may be used if the codec is not being
used in a AC97 system. If a lower
frequency is used the sample rate of
the codec is affected and the engineer
must keep the following formula in
mind.

SAMPLE RATE = (SRG SETTING)*(XTAL RATE)

(512)*(48,000)

ADI recommends QUARTZ crystals only.

The use of other crystals such as

ceramic and/or LC resonators are not
recommended. These types of crystals

have frequency tolerances of greater
than ±5000 PPM. Using resonators with

such a wide tolerance my result in
audio performance less than AC97 or
PC98/98. If such high performance is
not a system requirement and low cost
is the design goal ceramic crystals may
be used.

3. Operating temperature.

All ADI AC97 codecs are design to
operate over the temperature range of

0°C to 70°C. Most standard crystal have
a temperature range of -10°C to 70°C.

However, a crystal with a limited
temperature range may be used if the
following considerations are made.

High temperature may affect start-up.
If the crystal and oscillator system
impedance are increased due to high
temp the conditions for oscillation,

namely Gain > 0 and Phase = n*360°
(n=0,1,2…), may not be met and
oscillation will not start. Therefore,
if the oscillator starts at the highest
ambient temperature required by the
design there will be no need for a
special crystal.

Where :

SRG SETTING = The current sample rate

register setting.

XTAL RATE = The crystal frequency in

Hz

For more information on using variable
samples rates with ADI’s AC97 codecs
see the engineers note titled HOW TO
USE VARIABLE SAMPLE RATES ON ANALOG
DEVICES AC97 CODECS.

2. Frequency Tolerance and Stability.

All Analog Devices’ AC97 codecs require
crystals with Frequency tolerance and

stability of less ±100 PPM. This
requirement can be met if standard if

Both high and low temperature may
affect the operating frequency of the
crystal, but this variation is usually
small and therefore not a problem.

4. Series vs. Parallel Crystals.

There is no such thing as a series cut
or parallel cut crystal. Series vs.
parallel has more to due with the
oscillator used in conjunction with the
crystal. If the oscillator is
inverting then the crystal/oscillator
system will oscillate in parallel
resonance. If the oscillator is non­inverting the crystal/oscillator system
will operate in series resonance.

a

Most frequency specs for crystals are

Fundamental Third-Overtone Fifth-

stated as “series”. This means that
the frequency specified for the crystal
is the series resonant frequency Fs.
All ADI’s AC97 codecs have inverting
oscillator circuits, therefore, the
codec/crystal system will oscillate in
parallel resonance Fp. Fp is usually
higher than Fs by about 0.02%. The
diagram on page 3 helps to describe the
difference between parallel and series
resonance.

5. Load Capacitance.

To operate at parallel resonance
requires capacitance external to the
crystal. The total external
capacitance required for parallel
resonance is referred to as the load
capacitance CL. Please refer to the
diagram on page 4 to see the
relationship between CL and the
external caps used in the oscillator
circuit.

6. Equivalent Series Resistance (ESR).

A crystal calibrated for parallel
resonance with a load capacitance CL
will have an equivalent series given by

ESR = Rs*(1+Co/CL)

2

For a description of Rs,Co and CL refer
to figures 1 and 2.

7. Drive Level.

The operating drive level is the power
dissipated internally in the crystal
blank. If the drive level is too low
(less than 100 micro-watts), starting
of oscillation may not occur. If the
drive level is to high (greater than
1mW) frequency shifts, poor long term
frequency aging and frequency
perturbations over temperature may
occur.

Aging is a general term used to
describe the gradual deterioration of
the operating characteristics of a
crystal unit over time. Many factors
contribute to this deterioration, such
as internal contamination, excessive
drive level, wire fatigue, frictional
wear, and surface erosion of the
crystal blank. Cleanliness of the
manufacturing process and of the quartz
blank can greatly reduce aging by
contamination. The most rapid aging
occurs within the first year. If aging
rates of a crystal must be low, the
crystal can be pre-aged by temperature­cycling or by high-temperature burn-in
for an extended period of time.

9. Spurious Modes.

All quartz crystals have multiple
vibrational modes. Spurious modes refer
to those that are unwanted and can be a
problem if the response is as strong as
the main mode. When the oscillator runs
on the spur instead of the main mode,
the frequency output is changed.
Spurious modes should be specified as
either a resistance ratio to the main
mode or dB suppression. A resistance
ratio of 1.5 or 2.0 to 1 is sufficient
to avoid mode hopping. A -3dB to -6dB
is an approximate equivalent for a
specification in terms of dB.

Analog Devices recommends using only
Fundamental mode crystals. The use of
third-overtone crystal may result a
decrease in audio performance due to
frequency inaccuracy and drift.

Frequency Spectrum

All ADI AC97 codecs work well with any
crystal that has a drive level of 1 mW
maximum.

8 Aging.

Notes on using Analog Devices’ DSP, audio, & video components from the Computer Products Division

Phone: (800) ANALOG-D or [phone], FAX: [phone], EMAIL: [e-mail]