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OP283GS Folha de dados(PDF) 10 Page - Analog Devices |
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OP283GS Folha de dados(HTML) 10 Page - Analog Devices |
10 / 12 page REV. B OP183/OP283 Low Noise Microphone Amplifier for Multimedia The OP183 family is ideally suited as a low noise microphone preamp for low voltage audio applications. Figure 40 shows a gain of 100 stereo preamp for the AD1849 16-bit SoundPort Stereo Codec chip. The common-mode output buffer serves as a “phan- tom power” driver for the microphones. bandwidth and is not sensitive to false-ground perturbations. The simple false-ground circuit shown achieves good rejection of low frequency interference using standard off-the-shelf components. Amplifier A3 biases A1 and A2 to the middle of their input common-mode range. When operating on a +3 V supply, the center of the OP283’s common-mode range is 0.75 V. This notch filter effectively squelches 60 Hz pickup at a filter Q of 0.75. To reject 50 Hz interference, simply change the resistors in the twin-T section (R1 through R5) from 2.67 k Ω to 3.16 kΩ. The filter section uses an OP283 dual op amp in a twin-T configu- ration whose frequency selectivity is very sensitive to the relative matching of the capacitors and resistors in the twin-T section. Mylar is the material of choice for the capacitors, and the relative matching of the capacitors and resistors determines the filter’s pass band symmetry. Using 1% resistors and 5% capacitors produces satisfactory results. A Low Voltage Frequency Synthesizer for Wireless Transceiver The OP183’s low noise and the low voltage operation capability serves well for the loop filter of a frequency synthesizer. Figure 42 shows a typical application in a radio transceiver. The phase noise performance of the synthesizer depends on low noise contribution from each component in the loop as the noise is amplified by the frequency division factor of the prescaler. CRYSTAL +3V VARACTER DIODE 900MHz REFERENCE OSCILLATOR PHASE DETECTOR ÷ PRESCALER RF OUT V CONTROL VCO OP183 Figure 42. A Low Voltage Frequency Synthesizer for a Wireless Transceiver The resistors used in the low-pass filter should be of low to moderate values to reduce noise contribution due to the input bias current as well as the resistors themselves. The filter cutoff frequency should be chosen to optimize the loop constant. Ω10kΩ Ω50Ω Ω20Ω Ω100Ω Ω20Ω Ω50Ω Ω10kΩ Ω10kΩ Ω100Ω Ω10kΩ +5V 1/2 OP283 10µF +5V 1/2 OP213 10µF 1/2 OP283 15 17 MINL MINR 19 CMOUT AD1849 LEFT ELECTRET CONDENSER MIC INPUT RIGHT ELECTRET CONDENSER MIC INPUT Figure 40. Low Noise Stereo Microphone Amplifier for Multimedia Sound CODEC A +3 Volt 50 Hz/60 Hz Active Notch Filter with False Ground To process ac signals, it may be easier to use a false-ground bias rather than the negative supply as a reference ground. This would reject the power-line frequency interference which oftentimes can obscure low frequency physiological signals, such as heart rates, blood pressures, EEGs, ECGs, et cetera. ΩR10 25k Ω ΩR8 1k Ω 8 1 4 3 2 A1 +3V C1 1µF ΩR1 2.67k Ω ΩR3 2.67k Ω C2 1µF ΩR4 2.67k Ω 1/2 OP283 ΩR6 10k Ω V IN Ω R5 1.33k Ω (2.67k Ω ÷ 2) ×C3 2µF (1µF × 2) 7 6 5 A2 V O 1/2 OP283 ΩR7 1k Ω 1 3 2 A3 OP183 C5 0.015µF ΩR11 10k Ω ΩR12 70 Ω 0.75V C6 1µF +3V C4 1µF A1, A2, AND A3 = 1/2 OP283 Ω Q = 0.75 NOTE: FOR 50Hz APPLICATIONS CHANGE R1–R4 TO 3.1k Ω AND R5 TO 1.58k Ω (3.16kΩ ÷ 2). Ω R2 2.67k Ω ΩR9 75k Ω Figure 41. +3 Volt Supply 50 Hz/60 Hz Notch Filter with Pseudo Ground Figure 41 shows a 50 Hz/60 Hz active notch filter for eliminating line noise in patient monitoring equipment. It has several kilohertz –10– |
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