The MC145557 has been discontinued. Suggested replacement: none.
The MC145554, MC145557, MC145564, and MC145567 are all per channel
PCM Codec–Filters. These devices perform the voice digitization and
reconstruction as well as the band limiting and smoothing required for PCM
systems. They are designed to operate in both synchronous and asynchronous
applications and contain an on–chip precision voltage reference. The
MC145554 (Mu–Law) and MC145557 (A–Law) are general purpose devices
that are offered in 16–pin packages. The MC145564 (Mu–Law) and MC145567
(A–Law), offered in 20–pin packages, add the capability of analog loopback and
push–pull power amplifiers with adjustable gain.
These devices have an input operational amplifier whose output is the input
to the encoder section. The encoder section immediately low–pass filters the
analog signal with an active R–C filter to eliminate very–high–frequency noise
from being modulated down to the pass band by the switched capacitor filter.
From the active R–C filter, the analog signal is converted to a differential signal.
From this point, all analog signal processing is done differentially. This allows
processing of an analog signal that is twice the amplitude allowed by a
single–ended design, which reduces the significance of noise to both the
inverted and non–inverted signal paths. Another advantage of this differential
design is that noise injected via the power supplies is a common–mode signal
that is cancelled when the inverted and non–inverted signals are recombined.
This dramatically improves the power supply rejection ratio.
After the differential converter, a differential switched capacitor filter band
passes the analog signal from 200 Hz to 3400 Hz before the signal is digitized
by the differential compressing A/D converter.
The decoder accepts PCM data and expands it using a differential D/A
converter. The output of the D/A is low–pass filtered at 3400 Hz and sinX/X
compensated by a differential switched capacitor filter. The signal is then filtered
by an active R–C filter to eliminate the out–of–band energy of the switched
These PCM Codec–Filters accept both long–frame and short–frame industry
standard clock formats. The MC145554/57/64/67 family of PCM Codec–Filters utilizes CMOS due to
its reliable low–power performance and proven capability for complex
analog/digital VLSI functions.