Philips scored a first in 1987 by developing the bitstream data conversion technique which converts binary samples of an analog audio signal into a 1-bit code using oversampling, noise shaping and pulse density modulation (PDM). Compared with multi-bit DACs, our bitstream conversion DACs have improved small-signal linearity, no “glitches” and no crossover distortion effects. We have a wide range of DACs of this type plus digital filters for, noise shaping, digital filtering and post filtering requirements of all classes of digital audio equipment.
Essential functions in digital audio equipment are the analog-todigital conversion (ADC), and the digital-to-analog conversion (DAC). These signal processing stages play a major and critical role by converting audio signals into 16-bit sound data words (ADC), and by converting 16-bit sound data words which have been numerically decoded and filtered, into analog waveforms which can be made audible (DAC)
Philips has developed an extensive range of stereo data converters using several different conversion techniques to meet the diverse requirements of the digital audio market:
Standard 16-bit DACs
- Standard 16-bit stereo DACs
- Bitstream conversion stereo ADCs and DACs for top-class audio performance
- Continuous calibration stereo DACs which are the world’s smallest converters with the lowest power consumption
- Stereo filter-DACs that use bitstream conversion for small signals, and the continuous calibration technique for large signals
- A quadruple sign-magnitude filter-DAC for 4-channel (front and rear stereo) digital audio applications in cars
- Bitstream conversion ADCs
- Combined stereo bitstream ADCs with digital filter and bitstream/continuous calibration DACs
- Universal Serial Bus (USB) DAC and CODEC for USBcompliant audio and multimedia applications.
With these DACs, the ten lowest bits are derived from a reference current by means of a passive 10-bit current divider based on emitter scaling. The six highest bits are constructed by dynamic element matching (DEM), a technique patented by Philips.
As shown in the figure below, DEM uses matched resistors to divide a reference input current into four nearly equal parts. Under control of an internal shift register, the four currents are interchanged with a frequency of 250 kHz from an internal oscillator. After time-averaging, the resulting four output currents are very closely matched. Two of the four output currents generated in the first DEM stage are added to form the largest (MSB = bit 16) bit current. The third output current forms the second largest (MSB – 1 = bit 15) bit current. The fourth output current is fed into the next DEM stage to construct bits 13 and 14. Similarly, the fourth output current from the second DEM stage is fed into the final DEM stage to construct bits 11 and 12. The fourth output current from the final DEM stage is fed to the passive 10-bit current divider to construct the 10 LSBs (bits 1 to 10). In this manner, all sixteen bit currents are derived from the same current referenc.
The bitstream conversion technique
The bitstream digital-to-analog conversion technique, developed by Philips, converts binary samples (e.g. 16-bit words) into a 1-bit code representing two levels (0 or 1) using oversampling, noise shaping and pulse density modulation (PDM). This code is then converted into an analog form that closely resembles the original signal by a switched-capacitor conversion technique. Because bitstream converters process the signal in the digital domain, they have the following advantages over conventional multi-bit converters:
Continuous calibration DACs (CC-DACs)
- Considerable improvement of small-signal linearity
- Absence of glitches
- Complete elimination of cross-over effects.
Continuous calibration is an innovative data conversion technique patented by Philips and implemented in our new range of CMOS CC-DACs for cost-effective D to A conversion in portable equipment.
The technique of continuous calibration is based on the principle of storing charges on the gate-source capacitance of internal CMOS transistors. This charge storage principle allows the largest bit currents to be generated repeatedly from a single reference current. Because only one internal reference source is required, these coarse bit currents are extremely closely matched and immune to ageing, temperature and process variations or matching.
The basic operation of a CC-DAC (see figure below) consists of continuous cycles of calibrating the drain current of CMOS transistors to a reference current, and then extracting an exact duplicate of the reference current from the drain. During calibration, the MOS transistor is connected as a diode in parallelwith its gate-source capacitance by linking its drain and gate.
The drain of this diode-connected transistor is connected to a constant reference current source so that the intrinsic gatesource capacitance charges to a voltage determined by the characteristics of the transistor. The drain-gate link and reference current source are then disconnected and, since the charge on the intrinsic gate-source capacitance of the transistor is preserved, an exact duplicate of the reference current is available as an output at the drain.
CC-DACs use symmetrical offset decoding in which the bit switching is arranged so that the zero-crossing transition is performed by switching only the smallest currents. The intrinsic highly accurate coarse current, combined with the symmetrical offset principle, precludes any distortion at the zero-crossing or at any other small-signal transitions. CC-DACs are therefore capable of high-quality reproduction of low-level audio input signals.
Philips’ CC-DACs are fabricated in a 1.0 µm CMOS process and feature extremely low power dissipation, small packages and simple application.
Philips TDA1541A / R1 / S1 / S2
High-performance 16-bit DAC
- Dynamic Element Matching (DEM)
- 4fs or 8fs oversampling
- I2S input data format, up to 8fs
- TTL compatible inputs
- Marked with single crown (TDA1541A/S1 only)
- Marked with double crown (TDA1541A/S2 only
These monolithic integrated dual 16-bit digital-to-analogue converters are specifically designed for use in hi-fi digital audio equipment such as Compact Disc players, digital tape or cassette recorders. With this range of DACs, the ten lowest bits are derived from a reference current while the six highest bits are generated by time-averaging in the Dynamic Element Matching (DEM) parts. For a full description the DEM technique.
|Typ. THD + N at full-scale at 0 dB
|Typ. THD + N at –60 dB
|Typ. signal-to-noise ratio
|Full-scale output current
Top-performance bitstream DAC:
- Pulse density modulation
- Inherently monotonic
- No zero-crossing distortion
- High signal-to-noise ratio
- High over-sampling rate up to 192 fs
This top-performance bitstream DAC is a BiMOS circuit without noise shaping, digital filtering or post filtering. It is intended for use in combination with our advanced bitstream digital filter TDA1307. This results in optimum performance by reducing crosstalk between the analog and digital sections of the circuitry, extending the dynamic range, and increasing the signal-to-noise ratio. Furthermore, the BiMOS process used for the DAC allows MOS transistors to be used for the digital logic and drivers, and bipolar transistors to be used to achieve low noise analog circuitry. This optimizes speed and reduces digital noise. Other precautions taken in the DAC design to ensure maximum immunity to crosstalk are fully separated L and R channels and separate supply lines for the analog and digital sections. It is ideally suited to high-quality audio systems such as Compact Disc and DAT players, or in digital amplifiers and digital signal processing system.