"PCM2902 audio digital decoding circuit making headphone amplifier, PCM2902 Application
Keywords: PCM2902, Digital Decoding Circuit
Author: Wu Hongki Wu Hong Jing
Although the headphone amplifier is not large, it is not easy to do well under honesty, in order to achieve the ideal level, many headphone amplifiers use the discrete component, because the output power is not large, using the A class amplifier is not in a small number, this is in amateur Under conditions, it is not easy to do well. The headphone amplifier described below uses a special headphone amplifier integrated circuit that is from TI, which is suitable for homemade under amateur conditions. I. The main feature of the main feature headphone amplifier is: full-integrated circuit overall solutions, including all circuitry required for headphone amplifiers such as power, sound source, drive, and power amplifiers. Adopt common digital sound source input: S / PDIF and USB, adapt to home and computer multimedia audience. You can use the USB bus to power up without additional power. Low distortion (O.005%), large dynamic (93dB), high signal-to-noise ratio (93dB); key electronic volume control and mute control functions. Current feedback differential input power amplifier stages, with more than 120 dB dynamic range and signal-to-noise ratio and 0.00014% ultra-low distortion. Second, the principle and IC
This circuit includes all the integrated circuits of Ti (Texas Instruments, Texas Instruments), including input and DAC circuits, unbalanced balance line amplifiers, balanced input mode power output grades, and power supply circuits. Some, S / PDIF-like digital signal reception is completed by the integrated receiving component TROX173 produced by Toshiba Company, and the digital signal reception of S / PDIF coaxial mode is completed by SN75179. The remaining parts of the reception and codec function are all from TI. The PCM2902 of the produced, including USB input, analog input, S / PDIF digital signal, ADC (modulus converter), DAC (digital-to-analog converter), and analog filter. The entire system uses nine seven 7 integrated circuits, and the total circuit schematic is shown in the figure below.
1. Input and DAC circuit input and DAC circuit consist of PCM2902, TORX173, SN75179, which describe their characteristics and functionality: TORX173 is a common fiber signal integrated receiving component, the peripheral circuit is simple, output is a TTL level mode, 5V The power supply is powered, the working current is 40mA, and the data receiving rate is up to 6MB / s. SN75179 is the low-power difference transceiver of Ti, meets the standard requirements of TIA / EIA-422-B, TIA / EIA-485-A, and ITU V.11, and the receiving sensitivity is ± 200mV, and the working power supply voltage is 5V. Using DIP8 packages, SP490E is used instead. PCM2902 is a single-chip USB interface stereo band decoder, automatic identification of converting USB, S / PDIF, analog input, S / PDIF output interface, full-speed USB1.1 transmission interface; integrated ADC and DAC, using TI SPACTTM The structural system (a technical structure of the Audio clock signal from the USB data stream), the on-chip simulation of the locking rings (PLL), separate recording, and reproducing sampling rate. This circuit only uses the USB input of PCM2902, S / PDIF input mode, and analog input idle is unused. The main features and technical performance of the PCM2902 are as follows: ● Dual Channel 16-bit Δ -σ Digital Mode Converter (DAC) and Analog Converter (ADC). Among them, DAC supports common 32, 44.1, 48kHz sampling rates, ADC supports 8, 11.025, 16, 24.05, 32, 44.1, 48 kHz adoption rate; ● Single power supply mode, typical application use USB Bus 5V power supply; ● HID interface to achieve volume, mute and standby control; ● On-chip DAC main technical indicators: ● Analog audio performance: THD + N = 0.005%; SNR (signal to noise ratio) = 93db; DNR (dynamic range) = 93dB; ● On-chip super sample digital filter: inner dispensing = ± o. 1db; turning frequency attenuation capability = -43dB; ● On-chip simulation low-pass filter; ● single-end voltage output mode. The figure below shows the internal function block diagram of the PCM2902. In order to achieve better performance, ultra-low noise LDO (low pressure difference regulator) TPS79901 is powered by PCM 2902, and its output voltage is 3.6V in Fig. 1, and the value of adjustment R9 and R8 can be adjusted.
2. The unbalanced balance line amplifier DRV134PCM2902 is an unbalanced output method, while the post-level headphone amplifier TPA6120A2 uses a balance input mode. In order to implement the front-rear stage matching, the circuit uses TI's Burr-Brown's balanced audio line amplifier DRV134. DRV134 is an unbalanced input, balancing output method line amplifier, integrating the resistors required to set the gain without worrying about whether the peripheral component selection is not properly caused, the gain is fixed to 6dB of the device, 8-pin package, the internal equivalent The circuit is shown below. It can be seen from the figure below that the DRV134 contains circuit functions we usually require 3 operational amplifiers: input buffer amplification and balanced mode line amplifiers. The main features and technical performance of DRV134 are as follows: low distortion: THD + N = 0.0005%; high conversion rate: 15V / μs; wide power supply voltage: ± 4.5V ~ ± 18V; low power, quiescent current is ± 5.2 MA; CMRR (common mode signal suppression ratio): 68dB; PSRR (power supply voltage suppression ratio): 110dB; output offset voltage (differential output): ± 1mV typical value, maximum no more than ± 10mV.
If you want to use an operational amplifier, you can use a dual-handed composition to form a dual channel input; amplifying, a four-handed setup dual channel balanced line amplifier, such as Burr-Brown's low noise series audio operation OPA2134 (Double op amp) ) And OPA4134 (four operations). 3. Balancing input mode power output grade TPA6120A4 TPA6120A4 is a current feedback high-fidelity headphone amplifier, its main features and technical performance are as follows: ● Wide supply voltage range: ± 5V ~ ± 15V, independent power supply; ● Work current (VCC = ± 15V) Each channel, maximum value): 15MNA; ● ± 12V power drive 600Ω load and THD + N typical value of 0.00014%; ● IMD (transient intermodulation distortion) 0.00014%, typical Value; ● DNR (dynamic range) and SNR (signal to noise ratio): 120dB; ● The output noise is typically 5μV under 2V / V gain conditions; ● SR (conversion rate): 1300V / μs; ● CMRR: 100DB; ● PSRR (Power supply voltage suppression ratio): 75dB; ● Enter an offset voltage (differential input): 2mV, maximum no more than 5mV. ● Strous suppression capability (VIN = 1VRMS, RL = 32Ω ~ 64Ω, RF = 1kΩ, FIN = 1KHz): - 90dB. The current feedback amplifier has the following advantages: up to 1300V / s conversion rates can effectively prevent odd harmonic distortion; the speed and linearity of transient current output characteristics are not limited to output distortion, and the big dynamic output will not cause this Bottom noise increase or deterioration of signal-to-noise ratio; frequency response and gain is not related to the enlargement of the amplifier has a very wide stable gain range in the full-band, and the loop gain will not deteriorate within the 20DB / OCT range. This makes the TPA6120A2 based on the current feedback frame include low noise, high-opening gain, low distortion characteristics within the wide band range. 4, the power supply circuit PTN4050A and PTN4050C DRV134 and TPA6021A2 require a positive and negative dual power supply to work well, in order to be able to use the USB bus + 5V power supply, the +5 converted to a DC-DC power supply circuit for the ± 12.5V power supply. The DC-DC power converter generally adopts the PWM (pulse width modulation) switching power supply circuit, and the small power application can also be used with a charge pump (capacitor energy storage circuit) circuit. The switching power supply has high efficiency in a wide input supply voltage range, but will bring electromagnetic interference, which is also one of the main reasons for the audio amplifier. Optimized mature design can reduce electromagnetic interference to an ideal extent, which is not easy to do in amateur conditions, in order to simplify the circuit, the circuit uses the modular DC-DC power converter PTN4050A and PTN 4050C, Peripheral Application Circuit is simpler than the circuit of the three-end adjustable regulators we are frequent, only one resistor is required to set the output voltage. The PTN4050A is a positive source converter that output voltage adjustable. The output voltage is determined by the resistance between 1 pin and 4 pin. The main features and technical performance are as follows: The output power can reach 6W, the output circuit can reach 500mA; wide input voltage Range: 2.9 ~ 7V; adjustable wide output voltage range: port 3 to port 15V; high efficiency, can reach 84%; switching frequency (typ): 260kHz; output ripple (typ): 3%; enable Control terminals and overheating shutdown; output current limiting and input voltage underplete shutdown function. The PTN4050C is an output voltage adjustable boost power converter, and the output voltage is determined by the resistance between 1 foot and 3 pin. The main features and technical performance are as follows: The output power can reach 12W, the output current can reach 1oooma: wide input voltage Range: 2.95 ~ 5.5V; adjustable wide output voltage range: 5 ~ 15V; efficiency can reach 90%; switching frequency (typ): 525kHz; output ripple (typ): 1.5%; PTN4050A and PTN4050c belong to unification The series of products, shapes, and operating temperatures are the same, and the pin mounting is pinned in the inner outer dimensions of 22 × 12.6 × 12.5 mm, and provides a wireless welding ball connection installation, which is highly reduced to 10 mm.
Third, the assembly and adjustment of the main ICs in this circuit are flat patch packages, and the assembly is not very convenient when using the breadboard under amateur conditions, so that these ICs are not a lot, the spacing is not too small, as long as there is more Skilled welding technology is still possible. If the conditions are allowed to make a piece of print, it will not be a problem. The following is a simply mentioning the principles of the peripheral components of each unit circuit and the possible debugging issues. 1, PCM2902 and other input circuit PCM2902 include digital circuitry and analog circuitry, should pay attention to the decoupling of each power supply terminal, and the decoupling capacitance should be as close as possible to the power terminal, in general, not in order to seek digitally, simulate strictly separate, no need to force the last Little grounding, if the printed board is still recommended to improve the signal-to-noise ratio. The decoupling capacitance is used to use the X5R type capacitance and resistance when using a patch element. There is a voltage adjustment circuit in the PCM2902, and if the requirements are not high, the external LDO is TPS. 79901 can also be available. According to the component value in Figure 1, the voltage of the TPS79901 output (5 pin) is 3.6V, if it is below 3.6V, the value of R9 should be adjusted, but the voltage should not be higher than 3.8V. PCM 2902 conversion of multiple input modes is based on priority, from high to low, S / PDIF, Analog (analog signal), USB, if the connection of the S / PDIF port is effective, USB, Analog will not be effective If the S / PDLF connection is invalid, the Analog connection is valid, USB is invalid, otherwise USB is valid; the three will only be effective. If the S / DIF input mode is not required, in Figure 1. IC2 (SN75179) and IC9 (TORX173) and its peripheral circuits can be omitted. If you want to get a better performance and does not increase the complexity of the circuit, it is also possible to use PCM2702 that does not support the S / DIF input output function. Its SNR and DNR can reach 100dB; if the complexity of the circuit can be used, independent The input and output circuitry and high performance DAC, such as PCM1792 of Ti, which can reach 120 dB above 120 dB, and the full equilibrium output can be better in combination with the end-level ear. 2, DRV134 and peripheral circuits are extremely simple, in order to ensure circuit performance, you need to mind two DRV134 power terminals should first retract the retreat capacitor and then connect to the positive or negative power supply rather than simple connections. Retiles. 3, TPA6120A2 and peripheral circuits First: the accuracy of the TPA6120A2 peripheral element resistance, it is best to use a low noise resistance of 1% accuracy. If there is a digital multimeter, the resistance of the corresponding position of the two channels is as close as possible, ie R33 And R37, R32, R34, R30 and R36, R31 and R35, R40, and R41 may not be used to do this. Second: The retractable capacitance of the power supply should be connected to the capacitor and then connected to the power supply separately. Third: All peripheral components strive to connect to the TPA6120A2 lead connections, R33 and R37 priority to the input terminal (5 feet and 16 feet). Fourth: TPA6120A2 uses heat dissipation reinforced 20-pin flat packages, and the bottom has a bare heat dissipation pad, which is also the ground terminal of TPA6120A2, should be properly grounded and guaranteed to have a certain copper foil area to ensure heat dissipation; if used, if you use the bread, you can use The fine copper wire of 8 ~ 10 cm is symmetrically welded to the exposed heat dissipation pad to achieve heat dissipation purposes, while allowing the heat dissipation pad to facilitate heat dissipation. Fifth: About TPA6120A2 Empty Feet. The TPA6120A2 has 5 total 10 empty feet each, and these empty feet are not connected to the internal circuit. When the printing board is installed, it can be grounded to improve heat dissipation. If the use of the bread board can be vacant or only two of the outermost side Connectivity. The TPA6120A4 contains two independently powered amplifiers, and if only the amplifier needs to shut down the second channel to save electric energy and reduce heat generation, this approach improves the string suppression capability. Close the second channelThe amplifier only needs to close the positive power of the second channel, the negative power supply is not allowed to disconnect, and the second channel cannot work separately. If only one channel is used for some reason, it is not convenient to turn off the second channel, and the output of the second channel must access a load resistance of at least 25 Ω, that is, the lower left or the lower right figure, and the IC is close to IC. Output terminals to ensure the high-frequency oscillation of the amplifier, and RL and RO can be combined at this time. Finally: About TPA6120A2 Howling Problem Problem. The stability of the current feedback high-speed broadband amplifier is not as good as our common voltage feedback amplifier, and the bandwidth of TPA6120A2 is above 10MHz, and the peripheral component (resistance) is not very easy to cause high frequency oscillation. The instantaneous voltage is also easy to cause amplifiers. Random oscillation, although the frequency is as high as tens of trillion, the impact on audio performance is still not ignored. If there is no test condition, you can strictly adjust the selection principle of the peripheral component (resistance), which is in general, in general, the existence of oscillation amplifiers sound the treble will hair, even the halve, no matter how it sounds comfortable, the amplifier that sounds comfortable is good Amplifier. Usually we are accustomed to unbalanced single-end applications, as shown in the left, Ri and RF determine the gain of the amplifier, and the smaller resistance value can reduce the noise of the system, but it is more likely to cause the amplifier to generate oscillation, larger Although the RF of the resistance will bring more noise, but the amplifier is more stable, and it can improve the adaptability of the impedance load because the RF resistance can drive the amplifier to drive a higher impedance load. Typical RF resistance of TPA6021A2 is 1kΩ, in order to increase the gain of the amplifier to adapt to the high impedance load can be adjusted between 1 to 2 kΩ, it is not recommended to use an RFO of more than 10 kΩ, and the appropriate RO can also suppress the oscillation of the amplifier. The resistance of RO is generally not less than 10 Ω. This value can be reduced to the extent to ignore, and this value can be increased to 100 Ω or higher if necessary.
Unlike the common voltage feedback amplifier, the feedback circuit cannot be added such as a frequency compensation effect, otherwise it will cause the amplifier oscillation, the capacitance in the load loop, including the distribution capacitance of the output loop, and the equivalent capacitance of the headset, etc. These inherent capacitors are not easy to handle under normal conditions, and appropriate adjustments RF and RO can be well resolved. In order to prevent the aggregate circuit (signal source) and the effect of the line capacitor on the positive phase input, it also avoids the influence of the input input capacitance of this stage circuit on the pre-stage circuit, sometimes it is necessary to join the RS, 50Ω is enough to be What impact is caused. The lower right picture is a differential input application schematic diagram of a channel of TPA6021A2, which can be highly moderate signal suppression capabilities, and our common external noise and its interference, a large part is common mode signal. In case of condition, the high-precision resistance should be selected to ensure that the input resistance of the positive, the inverting circuit and the feedback resistor is close to the equal, and of course, the resistance of low noise is also necessary.
Regardless of single-end input or differential applications, peripheral resistors should be as close as possible to the IC lead terminals, for feedback resistors, should be preferentially close to the input terminal, if there is conditional as possible to use uncommon patch resistors. If there is an oscilloscope, it can be tested according to the following method. The oscillation of the amplifier can be observed by the oscilloscope, and the graph of the output characteristics and the frequency can be further drawn. It can be well measuring the stability of the amplifier: Is it oscillation or a critical point? Is it more stable? The image and the left figure displays two curve instances, and more graphs can be found in the electronic data documentation of TPA6021A2. The right picture is the case where the amplifier is relatively stable, the curve is smooth and there is no big peak, and the valley appears. At this time, the RF can be appropriately reduced to increase the amplifier performance; the left picture is an oscillation of the amplifier. At this time, the amplifier needs to be appropriate. Adjustment, including adjustment input resistance (RS), feedback resistance (RF), and output resistance (RO) and improve power supply decoupling and filtering, usually, these oscillations can be completely eliminated by these measures. If the curve appears 2dB of peaks, Valley, the amplifier has oscillate; if the capacitance curve connected to a 10PF in the output, there is a peak, Valley, and the magnifier is in a critical state.
4, the power supply circuit is made by a single power supply. There are many ways to generate a dual power supply. If you don't force the bus mode, you can use the external dual power supply to supply power, so that the cost will be low. Because the overall power consumption of the circuit is not large, the general three-terminal regulator can be powered, which can use ± 15V power supply instead of ± 12.5V in this circuit, because this is the design of power consumption. The LJSB bus can only provide the current of the maximum 500rrla for each terminal, which is a little smaller, because the circuit requires approximately 350mA total input current, so the circuit itself also designs an external power interface; use this external interface It is good to use the voltage stability + 5V power, because the TPS79901 and PTN4050C in this circuit are only 5.5V; if your computer or other digital source USB interface is insufficient, it is only used in an external power supply; even Using the LJSB bus is also needed to exclude a USB interface without sharing with keyboard, mouse, etc., otherwise it is easy to cause insufficient power supply and cause the circuit performance to decrease. The power circuit portion should be assembled separately. After installing, you should call the debugging and do not directly supply the real load. Connect the positive and negative power supply to check whether the output voltage is normal, whether the load adaptability is normal, that is, fast access And short-open load, whether the output voltage has a hopping phenomenon, whether or not to be swing from the pointer of the pointer type, if there is an electronic load, it is certainly better, but for modular circuits, simple testing is to meet the needs. . The power supply circuit should be able to provide an output current of not less than ± 50mA, so the load resistance can be tested loads using a resistance of 220Ω, 3 to 5W.
5. Total debugging all components After installation, take a closer check whether there is a installation connection error, check the connection to the load and the load after the check is correct, and the power supply part is normal, and the power supply section includes TPS79901 and PTN4050A, PTN4050C. After the power supply is normal, you can check it all. In the total power input part, enter current gauge or use the current gear of the multimeter, and instantaneously observe whether the current size is abnormal, especially the case; if the abnormality is rapidly shorted Whether the circuit has a short circuit, if it is not possible to discharge the electric load exclusion method one by one. After the current is normal, measure whether the output of TPA6120A2 (the place of the headset) is obviously DC voltage on the ground, and if not, it can be attached to the headset and the sound source. First connect the USB connection with the computer, to make sure the S / DIF is not connected and your operating system is a Windows98 operating system, this time the computer should automatically identify the USB device, in general, this time your computer The audio playback device will be automatically set to USB audio, or you can open the Sound and Audio Device option in the Control Panel to confirm that "Sound Option" in the Audio tab should be displayed as "USBSPEAKERS" or similar content. Otherwise, most of you have a problem with your circuit, of course, you can also see if there is a drop menu, at least there should be information about your original sound card. If you confirm that your circuit connection includes a USB connection and a computer's USB interface, you can change the PCM2902 to try. If everything is normal, you can play a well-known music to enjoy your labor results. After appreciating, if your computer sound card output has a S / DIF interface, it is best, connect the appropriate S / PDIF connection to the appropriate interface, this time the USB audio device should automatically disappear and convert it to you. The original sound card in the computer, because this is the computer sound card to provide S / PDIF digital audio signal for your ear. You can compare the sound difference between the USB connection method. If your computer sound card does not have S / PDIF output, you have to use your DVD or other devices with S / PDIF output feature to test your ear, you need to pay attention to the power supply, for convenience is Use an external power supply. If you can't find the S / PDIF output signal, it is generally, for this part of PCM2902, as long as the USB function is normal, other functions are generally normal, you can slowly wait, wait for the S / PDIF signal source; If you do not test the input conversion function, you can also use analog signal input to disconnect the connection of the 12, 13 feet of the PCM2902 and the connection of the ground wire. Connect a 1μF-free capacitance, if the electrolytic capacitor should pay attention to the capacitance The access polarity is then connected to the analog source to observe whether the input function conversion is normal. "
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