MC145481

The MC145481 is a general purpose per channel PCM CodecFilter with pin selectable MuLaw or ALaw companding, and is offered in 20pin 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 onchip precision reference voltage.

This device has an input operational amplifier whose output is the input to the encoder section. The encoder section immediately lowpass filters the analog signal with an active RC filter to eliminate very high frequency noise from being modulated down to the passband by the switched capacitor filter. From the active RC 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 singleended design, which reduces the significance of noise to both the inverted and noninverted signal paths. Another advantage of this differential design is that noise injected via the power supplies is a commonmode signal that is cancelled when the inverted and noninverted 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 lowpass filtered at 3400 Hz and sinX/X compensated by a differential switched capacitor filter. The signal is then filtered by an active RC filter to eliminate the outofband energy of the switched capacitor filter.

The MC145481 PCM CodecFilter has a high impedance VAG reference pin which allows for decoupling of the internal circuitry that generates the midsupply 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 CodecFilter accepts a variety of clock formats, including Short Frame Sync, Long Frame Sync, IDL, and GCI timing environments. This device also maintains compatibility with Motorola's family of Telecommunication products, including the MC14LC5472 and MC145572 UInterface Transceivers, MC145474/75 and MC145574 S/TInterface Transceivers, MC145532 ADPCM Transcoder, MC145422/26 UDLT1, MC145421/25 UDLT2, and MC3419/MC33120 SLICs.

The MC145481 PCM CodecFilter utilizes CMOS due to its reliable lowpower performance and proven capability for complex analog/digital VLSI functions.