Sharp UX-B800A Service Manual page 77

UX-B800A
2.3. IC301 (CX20452) Modem block
2.3.1 Summary
The CONEXANT CX20452 Codec is an Integrated Analog (IA) circuit providing modem digital-to-analog and analog-to-digital conversion. It is pack-
aged in a 24-pin QFN. Ordering information is listed in Table 1-1.
The CX20452 Codec can serve as a line interface device between a CONEXANT® Modem Data Pump (MDP) and the PSTN. It can also serve as a
voice interface between the MDP and a microphone and speaker.
The MDP communicates with the CX20452 Codec via a serial interface. The CX20452 Codec has an 8-bit register that is used to configure the
device. This register may be written to or read from via the serial interface.
The CX20452 Codec receive path consists of line and microphone gain stages, line-input and mic-input anti-aliasing filters, a second order delta-
sigma ADC, and a third-order sinc decimation filter. The transmit path consists of a third-order interpolation filter, a second-order delta-sigma DAC, a
first-order lowpass switched capacitor filter, a second order lowpass continuous-time filter, a line-output driver, and a speaker-output driver. It has its
own control registers, timing logic, serial interface, references, and microphone bias circuit.
The device is designed to run at a clock rate of 1.9584 MHz. Adjustable oversampling ratios (OSRs) are provided to allow for other master clock fre-
quencies. Local and remote loopbacks are available for functionality and ease of testing.
The CX20452 Codec operates with digital power supply (VDD) and analog power supply (AVDD) = 3.3 V.
Order No./Part No. Package
CX20452-A 24-Pin QFN / Pb free
Table 1-1. Ordering Information
2.3.2 Functional description
Fig. 3
1) Microphone input
This input provides programmable gain for a microphone signal and low pass filtering to avoid aliasing before the signal is converted into a digital for-
mat. The microphone can be connected to this device as single-ended. A programmable gain amplifier provides four selectable gain settings from 6
to 26 dB, in steps of 5 dB. The input impedance is 150 kΩ minimum.
2) Line input
This input provides low pass filtering to avoid aliasing before the signal is converted into a digital format. The line input is single-ended and must be
AC coupled into the device. The filter structure realizes a two-pole LPF filter. The gain on the line input is selectable at 6dB or 16dB. The input imped-
ance is 150 kΩ minimum. Either Line input or Microphone input can be selected.
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