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TDA1549T

Stereo 4fs data input up-sampling filter with bitstream continuous calibration DAC (BCC-DAC1)

Philips Electronics 

Philips Semiconductors

Stereo 4fs data input up-sampling filter with

bitstream continuous calibration DAC (BCC-DAC1)

Objective specification

TDA1549T

Continuous calibration DAC

The stereo 5-bit DAC uses the dynamic continuous

calibration technique. The DAC currents (16 sources and

16 sinks) of each channel are repeatedly generated from

one single reference current. This duplication is based on

an internal charge storage principle and has an inherently

high accuracy which is insensitive to ageing, temperature

and process variations.

Figure 4 shows one such current calibration source.

During calibration the cell is connected to the reference

current sink Iref via switch S2. The calibration transistor M1

is connected as an MOS diode via the switch S1 forcing its

gate potential to assume a value so that the total current of

the calibration cell is equal to the reference current. After

calibration the gate of M1 is allowed to float. The gate

capacitance Cgs retains its potential and the current

through the cell remains exactly equal to the reference

current. This current is now connected to the output. Each

digital-to-analog current source and each current sink is

calibrated precisely in this way.

Operational amplifiers

High precision, low-noise amplifiers together with the

internal conversion resistors RCONV1 and RCONV2 convert

the DAC output current to a voltage capable of driving a

line output. This voltage is available at VOL and VOR

(1.5 V RMS typical).

Connecting external capacitors CEXT1 and CEXT2 between

FILTCL and VOL and FILTCR and VOR respectively,

provides the required 1st-order post-filtering for the left

and right channels (see Fig.1). The combinations of

RCONV1 with CEXT1 and RCONV2 with CEXT2 determine the

1st order fall-off frequencies.

Internal reference circuitry

Internal reference circuitry ensures that the output voltage

signal is proportional to the supply voltage, thereby

maintaining maximum dynamic range for supply voltages

from 3.8 to 5.5 V and making the circuit also suitable for

battery-powered applications.

August 1994

Fig.3 Filter and noise shaper characteristics.

7

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