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MAX9814ETD Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX9814ETD Datasheet(HTML) 9 Page - Maxim Integrated Products |
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9 / 14 page  Setting the AGC Threshold To set the output-voltage threshold at which the micro- phone output is clamped, an external resistor-divider must be connected from MICBIAS to ground with the out- put of the resistor-divider applied to TH. The voltage VTH determines the peak output-voltage threshold at which the output becomes clamped. The maximum signal swing at the output is then limited to two times VTH and remains at that level until the amplitude of the input signal is reduced. To disable AGC, connect TH to MICBIAS. Microphone Bias Resistor MICBIAS is capable of sourcing 20mA. Select a value for RMICBIAS that provides the desired bias current for the electret microphone. A value of 2.2k Ω is usually sufficient for a microphone of typical sensitivity. Consult the micro- phone data sheet for the recommended bias resistor. Bias Capacitor The BIAS output of the MAX9814 is internally buffered and provides a low-noise bias. Bypass BIAS with a 470nF capacitor to ground. Input Capacitor The input AC-coupling capacitor (CIN) and the input resistance (RIN) to the microphone amplifier form a highpass filter that removes any DC bias from an input signal (see the Typical Application Circuit/Functional Diagram). CIN prevents any DC components from the input-signal source from appearing at the amplifier out- puts. The -3dB point of the highpass filter, assuming zero source impedance due to the input signal source, is given by: Choose CIN such that f-3dB_IN is well below the lowest frequency of interest. Setting f-3dB_IN too high affects the amplifier’s low-frequency response. Use capacitors with low-voltage coefficient dielectrics. Aluminum electrolytic, tantalum, or film dielectric capacitors are good choices for AC-coupling capacitors. Capacitors with high-voltage coefficients, such as ceramics (non-C0G dielectrics), can result in increased distortion at low frequencies. Output Capacitor The output of the MAX9814 is biased at 1.23V. To elimi- nate the DC offset, an AC-coupling capacitor (COUT) must be used. Depending on the input resistance (RL) of the following stage, COUT and RL effectively form a highpass filter. The -3dB point of the highpass filter, assuming zero output impedance, is given by: Shutdown The MAX9814 features a low-power shutdown mode. When SHDN goes low, the supply current drops to 0.01µA, the output enters a high-impedance state, and the bias current to the microphone is switched off. Driving SHDN high enables the amplifier. Do not leave SHDN unconnected. Power-Supply Bypassing and PCB Layout Bypass the power supply with a 0.1µF capacitor to ground. Reduce stray capacitance by minimizing trace lengths and place external components as close to the device as possible. Surface-mount components are recommended. In systems where analog and digital grounds are available, connect the MAX9814 to analog ground. f 1 2R C 3dB_OUT L OUT − = ×× π f 1 2R C 3dB_IN IN IN − = ×× π Microphone Amplifier with AGC and Low-Noise Microphone Bias MAX9814 Maxim Integrated 9 |
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