DAC chip PCM1738 with dual functions of PCM and DSD

    Abstract: PCM1738 is an advanced digital-to-analog conversion digital audio chip produced by BB Company. It can be used to simultaneously perform data conversion and decoding on the DSD data stream of SACD audio format and the PCM encoding used in DVD-Audio. This article introduces the basic principles, main performance indicators and application information of PCM1738, and also describes the new technologies of its advanced segment DAC and current segment DAC.

1 Overview

PCM1718 is another dual-function decoding chip recently developed by BB Company and used in high-end audio equipment, following the 24bit/192kHz sinusoidal mode DAC decoding chip PCM1704 and SACD's DSD decoding chip PCM1700. This chip can not only decode DVD-Audio's 24bit/192kHz PCM encoded digital audio signal, but also decode SACD's 64fs/1bit DSD encoded digital audio, and has good performance. Therefore, it is currently an ideal device for developing high-end audio players or performing audio digital-to-analog conversion.

2 Features and performance of PCM1738

Figure 1 shows the structural and functional block diagram of PCM1738, and the function description of each pin is listed in Table 1. The main features of PCM1738 are as follows:

●It has 24bit resolution;

●Get the desired frequency from 10 to 192kHz;

●With multiple interface format functions, 16/20/24bit is the back-end format, 16/20/24bit I2S, and 24bit is the front-end format;

●The system clock has multiple choices of 128/192/256/384/512/768fs, where the range of fs is 32kHz~192kHz;

●Analog output differential current is ±2.5mA (Iout terminal output);

●Contains 8 times over-sampling digital filter, with two choices of fast and slow ramp-down, and no out-of-area noise rising phenomenon;

●With digital volume attenuation, soft squelch, zero detection latch and de-emphasis functions;

●With DSD mode and four filtering modes to choose from;

●With DF bypass mode, it can isolate the digital filter inside PCM1738, so that the input signal can be connected to a higher-end external digital filter;

●Adopt analog 5V and digital 3.3V power supplies;

●Adopts 28-pin SSOP package.

Table 1 PCM1738 pin function table

3 The working principle of the advanced segment of PCM1738

The Advance Segment DAC and current segment DAC of PCM1738 are novel digital-to-analog conversion methods specially developed by BB Company for PCM1738. Since the 24-bit resolution accuracy is as high as 1/1677216=0.00000006, the component accuracy and circuit structure both restrict the final actual accuracy. Under the current technological level of integrated circuit manufacturing, developing new circuits is a shortcut that can quickly improve the level.

The overall principle of the PCM1738 high-level segment DAC is shown in Figure 2. The DAC mode of the advanced stage is mainly composed of the inverted compensated offset bipolar ICOB decoding part, 5-level third-order Δ-Σ modulation, advanced stage data weighted average DWA and 67-level differential current sections. When the format-changed digital output data enters the digital filter, it is first transformed into 24bit/8fs data through eight times supersampling, and then divided into upper and lower segments. The upper segment is the 6-bit data excluding the highest MSB, while the lower segment is a total of 18 bits of data consisting of the MSB and the last 17 bits. During operation, the upper 6 bits are input to the ICOB decoding part, and the lower 18 bit data are input to the 5-level third-order Δ-Σ modulator. In this way, the upper 6 bits can be used to modulate a coarse waveform of 64 steps (m=2 to the 6th power = 64), and the lower 18 bits are the residual amount not included in the 6 bits of the data.

The decoder ICOB (Inverted Complementary Offset Binary) is called inverted compensation offset binary. It converts 6-bit data without MSB into 64-level data, and then divides it into 63-level working code output suitable for current segment operation.

The equivalent circuit of the 5-level third-order Δ-Σ modulator is shown in Figure 3. When modulating a signal with a large input amplitude, the movement of each sampling point in the amplitude direction is set between 1 level. Inside. This makes the error of the amplitude to the time axis (caused by the main clock) relatively small.

The level of the 63-level ICOB digital output and the 5-level third-order Δ-Σ modulator output is both 1. In the summation circuit, they can be directly added to become a 67-level ICOB code, and then added with Δ- The Σ modulated signal is transmitted and finally the digital conversion operation is performed.

The operation of digital conversion is divided into two steps, namely advanced DWA and current segment DAC. Advanced DWA (Advance Data Weighted Averaging) can be used to generate the most appropriate working clock required in the current segment after digital-to-analog conversion. In order to minimize analog errors, the PCM1738 uses a combination of independent timing control and first-order noise shaping operations to achieve high-precision low jitter.

The current segment (Segment) DAC consists of a balanced differential current part that weights the same analog quantity and a current switch that controls the on/off of this current segment. It is the heart of the DAC that converts digital signals into analog signals. Figure 4 is a simplified equivalent circuit of the current segment. It has a total of 75 pairs of differential current sources, which can be controlled by 67 levels from the advanced DWA according to the input quantity and control clock to complete the conversion work. Since these 75 pairs of current sources have enough headroom to handle 67 levels, each current source can work at its best without mismatch errors in silent tones or extremely large amplitudes.

In addition to digital-to-analog conversion of DVD-Audio's 24bit/192kHz PCM encoding, PCM1738 also has the ability to perform digital-to-analog conversion of SACD's DSD direct data stream.

The current source structure in Figure 4 can also be used for digital conversion of 64fs/1bit DSD signals. If it is used as an analog FIR filter, the basic function is equivalent to the FIR filter shown in Figure 5. The values ​​of the delay unit D and various current segments are the weights of each section. At this time, each current source works in a differential manner in order to obtain high-quality DSD/analog conversion.

4 Application circuit of PCM1738

Figure 6 is the application circuit of PCM1738. The audio data access, working clock, serial data interface for control and other parts have been omitted in the figure, and the analog output part is the actual circuit of the company's evaluation board. The analog output of the D/A converter in PCM1738 is a balanced differential current, which is ±2.5mA at full amplitude (0dB) and 5mA peak-to-peak. Therefore, an external current/voltage (I/V) conversion circuit must be added when used, and double-ended/single-ended conversion is also required. When the I0 terminal is at full amplitude, its output Iout is ±2.5mA. The feedback resistors R11, R12, R21, and R22 in the figure are all 620Ω. In this way, the voltage V0 converted by I/V will be ±(2.5Rf) mA. . In addition, since the gain of the double-ended/single-ended conversion output with filtering in the latter part is 1, the actual output voltage becomes the difference between the two input signals. Right now:

Vout=Vo-(-Vo)=2Vo,

This yields that the output peak-to-peak value Vp-p should be 2.192Vrms.

The frequency response is determined by the capacitor in parallel with the feedback resistor. According to the DVD-Audio and SACD standards, the signal band requirement should reach 100kHz. But in fact, it can still be adjusted within a certain range as needed. The final bandwidth determined by the capacitor and resistance value in this circuit is 70kHz.

    What needs to be mentioned in particular is that in order to meet the requirements of THD+N less than 0.001% and S/N higher than 120dB, a suitable op amp should be selected. Basic indicators that need to be considered when selecting an op amp include input converted noise voltage, input converted noise block current, open loop gain bandwidth, rise rate, settling time and nonlinear distortion.

According to the output level (2Vrms) and signal-to-noise ratio (-120dB) requirements of a general player, the noise level converted is only 2μv. This means that the noise of components such as op amps and resistors and capacitors determines the noise level of the final device. are the main factors, so we must pay attention to the quality indicators of each link in order to obtain the best sound quality.