CSP1027 Datasheet, PDF (37/64 Pages) Agere Systems – CSP1027 Voice Band Codec for Cellular Handset and Modem Applications

Data Sheet
December 1999
CSP1027 Voice Band Codec for
Cellular Handset and Modem Applications
7 Application Information (continued)
noise must be less than 36 µVrms referred to the
preamplifier input. As a worst-case analysis, assume
that all the digital noise is in the baseband (noise
source of 2.5 Vrms for a 5.0 V digital signal). The atten-
uation (isolation) between the digital signal and the
preamplifier for this case must be
isolation
≈
--²---^.--⁵-----^V----
36 µV
=
70,000:1
(97
dB)
Usually, only a small portion of a particular digital signal
ac-couples into the signal path, but this is tempered by
having many digital signals. It is the sum of these noise
sources that must be held to less than 36 µVrms. In
audio applications, the needed isolation is actually
greater than calculated above because the human ear
can detect tones that are 10 dB below the noise floor.
7.3.2 Typical Ways Digital Noise Couples into
Analog Circuits
1. Digital signals have overshoot or undershoot
because of circuit board impedance mismatches.
These reflections can turn on internal I/O protection
diodes. The diodes then inject the noise current into
the substrate as well as the chip power rails, and the
noise is distributed throughout the chip.
2. Fine-line CMOS (like that used to fabricate the
CSP1027) generates hot electron currents when the
logic gates change state. This current is injected into
the substrate and adds to the supply current. The
substrate current can couple directly into the analog
circuits, and the supply current transients can couple
through common supply impedance and by inductive
coupling.
3. Coupling off-chip, such as the package bond wires
and package pins, and coupling into the analog sig-
nals on the circuit board.
4. The classic coupling method: common power and
ground impedance.
7.3.3 The Problem with an Asynchronous Codec
Clock
When the I/O and sample clocks are not derived from
the same time base, their edges will drift with time.
Even if the I/O and sample time bases use master
oscillators that are stated to be the same frequency (or
one being a multiple of the other), they will differ by
some amount from their intended values. This differ-
ence in frequency will cause the digital circuit clock
edges to slide past the analog sample clock edges in a
periodic way, causing the sampled digital noise to also
vary in the same periodic way (noise tones in the base-
band).
7.3.4 The Advantage of Fully Synchronous
Operation
When all the clocks that are used in or connected to the
codec are generated from the same master time base,
the sampling switches sample in the quiet time before
the digital circuits change state, or at least sample the
same portion of the ground bounce transient. The por-
tion of the noise that does not change from one sample
to the next will alias to the signal path as a dc offset.
The portion that is signal dependent (like a data line
coupling into the analog signal path) can show up as a
tone even though its transitions occur synchronously
with the sample clock unless care is taken to ensure
that the analog sampling switches only open during a
quiet time (usually before the digital circuits change
state).
Lucent Technologies Inc.
37