Analog Devices ee-28 Application Notes

Engineer To Engineer Note EE-28

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]

Daisy Chaining Multiple AD1847s
to a DSP - Ground Plane
Recommendations

Last Modified: 2/4/97

Overview

This Engineer’s Note will give some board layout tips
when designing a system that uses multiple AD1847
codecs.

AD1847 Daisy Chain Layout Issues

In my design I am using multiple codecs which
are daisy chained together, and I was wondering
about connecting the analog and digital planes.
The data sheet has recommendations but I still
need clarification on how to treat the power and
ground planes in the case of using multiple
codecs?

In contrast to the data sheet recommendations, subsequent
experience shows using a ferrite bead yields the best
results for achieving the highest S/N ratio.

EMI/RFI concerns on the analog side

What would happen if you have lots of RF noise (
switching noise generated by the CMOS circuitry? Where
do you bypass it to the analog ground plane?

For RF noise greater than 500 kHz, a
100 pF capacitor is best when bypassed
from the analog ground plane to the
chassis or cabinet ground.

We recommend a separate Digital Ground and Power Plane
for all codecs.

All codecs should share the same planes.

The best method of determining where the planes get
connected is as follows:

• Leave 3 places together on opposite sides of the

planes.

• Use a direct link (small narrow trace or wire) or a

ferrite bead to connect the two planes

• Of the 3 places to be linked, see which one gives you

the best signal-to-noise ratio. Be prepared to do some
measurements. To test for optimum performance,
you can set up a test by generating a tone to the input
of each codec, run an FFT on the DSP and view the
results with a scope on the codec output. There is no
solid proven method to yield the best result. The rule
of thumb is to do an educated guess on which link
between both planes gives you the best performance.

• Make sure that you only use one connection. Or else

you are in danger of creating ground loops!