Analog Devices ee-25 Application Notes

Engineer to Engineer EE-25

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]

AD1849K Jitter Requirements and
Input Signal Coupling

Last Modified: 6/25/97

Overview

This edition of Engineer’s Notes will shed some light on
AD1849K jitter requirements listed in the data sheet when
using an external clock source to drive the AD1849. Also
discussed are AC vs. DC coupling of the input signal.
The information is presented in a Q & A format.

Q & As

1. Please provide some specs for jitter
requirements for the clock inputs (both SCLK
and CLKIN and the relationship between them)
of the AD1849k. The AD1849K data sheet
mentions a low jitter requirement for external
clocks used to drive the part. (I am compelled to
use external clocks to synchronize with the rest
of the system.) Could you please identify which
clock signals in particular are susceptible to clock
jitter? I understand that the sampling clock
(SCLK) should be a low jitter clock but is this
also a requirement for the over-sampling filter
clock (MCLK) and the frame sync? Secondly, it
would be of assistance if you could quantify the
jitter requirements and outline the effects of clock
jitter on the distortion and noise specifications for
the part. How will the jitter affect THD and
IMD. Will -80dB THD still be attainable with a
jittery clock signal(s)?

Regarding jitter from an external clock source. Regardless
of the clock source for a mixed signal device, the effect of
jitter is to raise the noise floor and increase distortion.
The effect of such jitter will depend on the sample rate,
signal frequency and bandwidth and the amount of noise +
THD already present. For most applications using the
AD1849, i.e. 80dB S/N 50kHz sample rate and a 10kHz
tone, the jitter added noise will be much less than the
already present system noise (about -80dB) unless the
jitter exceeds about 2ns. For details of jitter effects I
would recommend one of the reference texts on digital
audio or telecommunications.

2. In order to sync with an existing clock we
wish to derive our 1849 clocks via a jittery PLL.
The question is: how much jitter is acceptable on
each of the SCLK and MCLK inputs?

The clocks most affected by jitter will be the Xtal inputs
(if external clocks are used) or the serial bit clock (SCLK)
if used as an input. Since usually only one clock source
is used at a time, this will normally be the only one you
have to worry about. As mentioned in item one above,
jitter shouldn't be a problem if it is less than 2ns, which
is much higher than obtained from most Xtal oscillators.
A well designed PLL should also have a much better jitter
performance than this.

3. Crystal data for the CS4215 says that the line
inputs should be DC coupled for minimum
output offset. The AD data says there is no
advantage to DC coupling. Will DC coupling to
the AD1849k generate loud pops when switching
between the AC coupled mic input and the DC
coupled line input as the data sheet suggests? I
would have expected no noise as the inputs in
both cases would be at the same DC potential. I
intend biasing the driving op-amp with the
AD1849k CMOUT, so that the only difference in
DC levels will be that caused by the op-amp's
output offset.

Regarding AC vs. DC input signal coupling. Yes, if
you switch between different DC bias voltage levels, you
will generate switching noise. The input to the CODEC
internal Op-Amps, is biased to the CMOUT voltage, +/­the input offset of the Op-Amps. This input offset is
typically less than 5mV and will not cause serious
problems, i.e. 'pops & clicks' unless gained-up in
subsequent signal processing. If you think your system
may generate total input offsets in excess of 10 to 20 mV
you may want to determine, by experiment, the maximum
tolerable level of switched noise you can tolerate. Wider
bandwidth applications where high S/N ratios are expected
will be subjectively worse, especially if augmented by
ringing in any filters.

EE-25 Page 2

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]