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AD1990ACPZ bảng dữ liệu(PDF) 11 Page - Analog Devices |
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AD1990ACPZ bảng dữ liệu(HTML) 11 Page - Analog Devices |
11 / 16 page AD1990 Rev. 0 | Page 11 of 16 THEORY OF OPERATION OVERVIEW The AD1990 is a 2-channel, high performance, switching, audio power amplifier. Each of the two Σ-Δ modulators converts a single-ended analog input into a 2-level pulse stream that controls the differential, full H-bridge, power output stage. The combination of an Σ-Δ modulator and a switching power stage provides an inherently linear and efficient means of amplifying the entire range of audio frequencies. The AD1990 also offers warning and protection circuits for overcurrent and over- temperature conditions, as well as silent turn-on and turn-off transitions. Σ-Δ MODULATOR The AD1990 is a switching type, also known as a Class-D, audio power amplifier. This class of amplifiers maximizes efficiency by only using its power output devices in full-on or full-off states. While most Class-D amplifiers use some variation of pulse-width modulation (PWM), the AD1990 uses Σ-Δ modulation to determine the switching pattern of the output devices. This provides a number of important benefits. Σ-Δ modulators do not produce a sharp peak with many harmonics in the AM frequency band as pulse-width modulators (PWM) often do. In addition, the 1-bit quantizer produces excellent linearity across the full amplitude range. Σ-Δ modulators require feedback to generate an error signal with respect to the input. The feedback voltages for the AD1990 modulators come from the outputs of the power devices and before the passive low-pass filters (see Figure 23). This compensates for nonlinear behavior in the power stage, such as nonoverlap time, mismatched rise and fall times, and propagation delays. It also reduces sensitivity to both dc and transient changes of the power supply voltage. Σ-Δ modulators operate in discrete time. As with all time- quantized systems, the Nyquist frequency is equal to half of the sampling frequency and input signals above that point aliases back into the base band. The AD1990 sampling frequency (master clock) is equal to half the frequency of the input clock, approximately 6 MHz, so images only alias for input frequencies above approximately 3 MHz. This is far enough above the audio band that bandwidth and aliasing are not a problem in real applications. The modulator has a noise shaping effect, and SNR is increased in the audio band by shifting the quantization noise upward in frequency. For a nominal input clock frequency of 12.288 MHz, the noise floor rises sharply above 20 kHz. The actual clock frequency used in an application circuit can deviate from this rate by as much as ±10%, and the corner frequency of the noise scales proportionately. The frequency at which the quantization noise dominates the output determines the amplifier’s practical bandwidth. MUTE AND RESET When power is applied and the RESET pin remains asserted, the AD1990 is in its lowest power consumption mode. The analog modulator is not running, and the power stage is tri- stated. On deasserting the RESET pin, the modulator begins a start-up sequence that includes initialization of the modulator, the protection circuits, and other functions. Once the start-up sequence is complete, the amplifier is in a state in which the modulator is running, but the output stage is not driven. When MUTE is deasserted, the output is started using a soft-start sequence that avoids any audible pop or click noise in the output signal. The output power transistors do not switch while MUTE remains asserted. Unlike the analog mute circuits found on some amplifiers that can be limited in their attenuation by the control logic or crosstalk, the mute attenuation on the AD1990 is greater than its dynamic range. The noise floor of the output signal also drops while in MUTE because the output transistors are not switching. Power-Up Sequencing Careful power-up is necessary when using the AD1990 to ensure correct operation and to avoid possible latch-up issues. The AD1990 should be powered up with RESET and MUTE held low until all the power supplies have stabilized. Once the supplies have stabilized, bring the AD1990 out of RESET by bringing RESET high. Begin the soft unmute sequence by bringing MUTE high at least 1 sec after the RESET rising edge. The amplifier produces audio using a shorter start-up sequence (as shown in Table 7), but the amplifier can produce an audible pop or click noise as the output starts switching. This is because the ac coupling capacitors at the analog input have a long time constant. If MUTE is deasserted substantially less than 1 sec after deasserting RESET, then these capacitors may not have charged to a steady state. They need ample time to settle at a bias voltage of VREF, the reference voltage for the single-ended inputs, or the amplifier starts with a slight dc offset. GAIN STRUCTURE Analog Input Levels The AD1990 has single-ended inputs for the left and right channels. The analog input section uses an internal amplifier to bias the input signal to the reference level, VREF, which is nominally equal to AVDD/2. A dc-blocking capacitor, as shown in Figure 22, prevents this bias voltage from affecting the signal source. In combination with the nominal 20 kΩ input impedance, the value of this capacitor should be large enough to produce a flat frequency response at the lowest input frequency of interest. |
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