CSP1027 datasheet(37/64 Pages) AGERE

37 / 64 page

Lucent Technologies Inc.

Data Sheet

CSP1027 Voice Band Codec for

December 1999

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

≈

= 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).

2.5 V

µV

----------------

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