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MAX9814ETD+T Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX9814ETD+T Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 14 page Attack Time The attack time is the time it takes for the AGC to reduce the gain after the input signal has exceeded the threshold level. The gain attenuation during the attack time is exponential, and defined as one-time constant. The time constant of the attack is given by 2400 x CCT seconds (where CCT is the external timing capacitor): • Use a short attack time for the AGC to react quickly to transient signals, such as snare drum beats (music) or gun shots (DVD). • Use a longer attack time to allow the AGC to ignore short-duration peaks and only reduce the gain when a noticeable increase in loudness occurs. Short- duration peaks are not reduced, but louder pas- sages are. This allows the louder passages to be reduced in volume, thereby maximizing output dynamic range. Hold Time Hold time is the delay after the signal falls below the threshold level before the release phase is initiated. Hold time is internally set to 30ms and nonadjustable. The hold time is cancelled by any signal exceeding the set threshold level, and the attack time is reinitiated. Release Time The release time is how long it takes for the gain to return to its normal level after the output signal has fall- en below the threshold level and 30ms hold time has expired. Release time is defined as release from a 20dB gain compression to 10% of the nominal gain set- ting after the input signal has fallen below the TH threshold and the 30ms hold time has expired. Release time is adjustable and has a minimum of 25s. The release time is set by picking an attack time using CCT and setting the attack-to-release time ratio by configur- ing A/R as shown in Table 1: • Use a small ratio to maximize the speed of the AGC. • Use a large ratio to maximize the sound quality and prevent repeated excursions above the threshold from being independently adjusted by the AGC. AGC Output Threshold The output threshold that activates AGC is adjustable through the use of an external resistive divider. Once the divider is set, AGC reduces the gain to match the output voltage to the voltage set at the TH input. Microphone Bias The MAX9814 features an internal low-noise micro- phone bias voltage capable of driving most electret condenser microphones. The microphone bias is regu- lated at 2V to provide that the input signal to the low- noise preamplifier does not clip to ground. Applications Information Programming Attack and Release Times Attack and release times are set by selecting the capacitance value between CT and GND, and by set- ting the logic state of A/R (Table 1). A/R is a tri-level logic input that sets the attack-to-release time ratio. The attack and release times can be selected by utiliz- ing the corresponding capacitances listed in Table 2. Microphone Amplifier with AGC and Low-Noise Microphone Bias 8 _______________________________________________________________________________________ 10ms/div VCT 500mV/div VMICOUT (AC-COUPLED) 1V/div ATTACK RELEASE HOLD 0V 0V A/R ATTACK/RELEASE RATIO GND 1:500 VDD 1:2000 Unconnected 1:4000 Figure 2. Input Burst Exceeding AGC Limit Table 1. Attack-and-Release Ratios tRELEASE (ms) CCT tATTACK (ms) A/R = GND A/R = VDD A/R = UNCONNECTED 22nF 0.05 25 100 200 47nF 0.11 55 220 440 68nF 0.16 80 320 640 100nF 0.24 120 480 960 220nF 0.53 265 1060 2120 470nF 1.1 550 2200 4400 680nF 1.63 815 3260 6520 1µF 2.4 1200 4800 9600 Table 2. Attack-and-Release Time |
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