The MC145481 is a general purpose per channel PCM Codec-Filter with pin
selectable Mu-Law or A-Law companding, and is offered in 20-pin SOG and
SSOP packages. This device performs the voice digitization and reconstruction
as well as the band limiting and smoothing required for PCM systems. This
device is designed to operate in both synchronous and asynchronous
applications and contains an on-chip precision reference voltage.
This device has 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 passband 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 capacitor filter.
The MC145481 PCM Codec-Filter has a high impedance VAG reference pin
which allows for decoupling of the internal circuitry that generates the
mid-supply VAG reference voltage to the VSS power supply ground. This
reduces clock noise on the analog circuitry when external analog signals are referenced to the power supply ground.
The MC145481 PCM Codec-Filter accepts a variety of clock formats,
including Short Frame Sync, Long Frame Sync, IDL, and GCI timing
environments. This device also maintains compatibility with Our telecommunication products.
The MC145481 PCM Codec-Filter utilizes CMOS due to its reliable
low-power performance and proven capability for complex analog/digital VLSI functions.