Описание
The MC145483 is a 13-bit linear PCM Codec-Filter with 2s complement data format, and is offered in 20-pin SOIC, SSOP, and TSSOP packages. This device performs the voice digitization and reconstruction as well as the band limiting and smoothing required for the voice coding in digital communication 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 13-bit linear A/D converter. The digital output is 2s complement format. The decoder digital input accepts 2s complement data and reconstructs it using a differential 13-bit linear 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 MC145483 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 MC145483 13-bit linear PCM Codec-Filter accepts both Short Frame Sync and Long Frame Sync clock formats, and utilizes CMOS due to its reliable low-power performance and proven capability for complex analog/digital VLSI functions.
