The current information in the circuit can provide useful information about the condition of the circuit. Current monitoring circuit is widely used in various instrument fields to realize protection, compensation and control. Common applications of current monitoring include battery monitoring system, motor control, overcurrent protection and 4 mA to 20 mA system, etc. In addition, current monitoring is also useful in commercial applications such as audio. One such application is to monitor the current output from the audio amplifier to the speaker in order to provide sound quality compensation and protection.
The audio amplifier must reproduce the input audio signal with high efficiency and low distortion. It shall have good frequency response performance in the audio frequency range of 20 Hz to 20 kHz, so as to faithfully reproduce sound and music. Audio amplifiers may need to provide output power ranging from a few milliwatts (for personal music players and headphones) to hundreds of watts (for home and commercial sound systems, such as theatres, auditoriums, outdoor sound systems, etc.). This paper focuses on the loudspeaker output current monitoring circuit working in the high voltage range. The main devices used in the circuit are class D amplifier, differential amplifier ad8479 and ada4805-1.
Basic class D amplifier signal flow
Audio amplifiers are divided into several categories: Class A, class AB, class B and class D. Compared with other types of amplifiers, class D amplifier has high efficiency and can provide high output power drive. Some commercial class D amplifiers provide power capabilities ranging from 1500 W per channel to 6000 W per channel.
Class D amplifiers can be simply described as switching amplifiers or pulse width modulation (PWM) amplifiers. The following figure shows the signal flow of a basic class D amplifier.
The working process of a typical class D amplifier starts with the comparator. A standard analog audio signal with a frequency usually between 20 Hz and 20 kHz is compared with a high-frequency triangular waveform to generate a PWM signal. Then, the PWM signal drives the output transistor to produce a series of pulses with potentially high voltage. After that, a low-pass filter recovers the sinusoidal audio signal. When not switching, the current through the output transistor is 0; Low on resistance reduces I2R loss, which significantly reduces the total power loss of the output stage. In this way, high efficiency can be achieved.
Even though class D amplifiers have the advantages of high efficiency and high power operation, some technologies can still improve audio quality, such as using feedback and predistortion mechanisms. The following figure shows a basic class D amplifier using feedback mechanism. In the feedback mechanism, the output signal (usually from the filter) is sent to the error correction module at the input. The error correction module can be fully analog, or use digital processing to deliberately predistorte the audio signal, so as to correct the output defects and improve the audio output quality. Except? In addition to the inherent nonlinearity of speakers, the tendency of speaker impedance to change due to temperature and aging may also cause this defect.
The current monitoring circuit can obtain the data to be fed back for error correction. The challenge of selecting a device suitable for this purpose is that the device must be robust enough to receive high-voltage pulses at the output of the audio amplifier. The ad8479 meets this requirement because it works even if there is a high input common mode voltage. Ada4805-1 is also added to the circuit as an analog-to-digital converter (ADC) driver with low offset and low noise.
Ad8479 is a precision differential amplifier that can measure differential signals even in the presence of common mode voltages up to ± 600 v. The relationship between input common mode voltage and output voltage shown in Figure 3 shows this capability. It has the following characteristics: low offset voltage, low offset voltage drift, low gain error drift, excellent common mode rejection ratio (CMRR) and wide frequency range. In this application note, ad8479 is configured as a high-end current detection amplifier to monitor the current of class D audio amplifier. The ad8479 also has a bandwidth of 130 kHz, which can meet the bandwidth requirements of audio applications.
Current monitoring circuit using ad8479 and ada4805-1
Ada4805-1 is a rail to rail amplifier with low input offset voltage and low input offset voltage drift. The gain of ada4805-1 is set to 10, and the output voltage generated is usually within the input voltage range of. In this case, successive approximation (SAR) ADC is usually used to process audio signals. The class D amplifier used is a class D power amplifier with expandable output power from 25 W to 500 W. The amplifier is configured with a ± 50 V supply voltage and provides a 1 kHz sinusoidal output. The ad8479 output is fed to the input of the ada4805-1, which is used as an ADC driver with a gain of 10. The resistance tolerance should be low to avoid large offset drift of the circuit.
For the class D amplifier used in this circuit, the current flowing through the detection resistance (RSENSE) is 4.74 a, producing a full-scale voltage of 475.71 MV peak. The common mode voltage is 37.9 V peak.
Analysis of main error sources of current monitoring
CMRR indicates the ability of the device to suppress common mode interference signals at each input. Mathematically, it refers to the ratio of common mode gain change to differential gain. If there is a high common mode voltage, especially when measuring small differential signals, this parameter is often one of the large error contributors. CMRR generates a corresponding output offset voltage error, which is part of the total system error. The rated CMRR of ad8479 is 96 dB. Another error source is offset voltage. The smaller the full-scale signal, the greater the error of offset voltage contribution.
The input offset voltage of ad8479 is 1 MV, which contributes 2102 ppm of full scale (FS) when converted to ppm. Ada4805-1 introduction 125 μ V offset voltage multiplied by gain 10, so the total error caused by offset voltage is 3352 ppm of full scale (FS). In addition, the data book shows that the ad8479 has a gain error of 0.02% FS, so the error brought by ad8479 to the circuit is 200 ppm FS.
Table 1 and table 2 summarize the main error sources of ad8479 and ada4805-1 respectively. The error of ad8479 offset voltage contribution is large, which is 2102 ppm FS at 37.9 V input common mode voltage. The error of common mode voltage contribution is 1262 ppm FS. Here, for the 37.9 V common mode voltage and 0.1 detection resistance (see Figure 1), the error contributed by the offset voltage is large, followed by the input common mode voltage. However, if the common mode voltage is larger, it will become a large error source. For example, at 250 V
At mode voltage, the common mode error contribution is 8329 ppm FS. This common mode voltage is common for high voltage class D amplifiers. In addition, the greater the detection resistance, the greater the voltage drop caused by it, resulting in the increase of full-scale voltage, which will eventually reduce all error contributions.
The following figure shows the response test results of the current detection circuit. It also includes the input voltage of ad8479, the output voltage of ad8479 and the amplified signal at the output of ada4805-1. A current of approximately 4.74 a flows into the detection resistance and load. The inverted input signal is about ± 38 V, and about ± 500 MV appears at the output of ad8479, which shows the ability of ad8479 to measure the differential signal in the presence of high common mode voltage.
Measured current and voltage
Real time monitoring requires not only high-speed devices, but also rapid response to deal with the sudden change of target current. The change speed of the output signal must keep up with the change speed of the input signal, which requires a correct interpretation of the electrical state of the speaker in a very short time, even short circuit events.
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