A microphone preamp circuit is used to amplify a microphone’s output signal to match the input level of the devices following it in the signal chain. Matching the peaks of the microphone’s signal level to the full-scale input voltage of an ADC makes maximum use of the ADC’s dynamic range and reduces the noise that subsequent processing may add to the signal.
A single op amp can be easily used in a circuit as a preamp for a MEMS microphone output. The MEMS microphone is a single-ended output device, so a single op amp stage can be used to add gain to the microphone signal or just to buffer the output.
This application note covers some of the key op amp specifications to consider in a preamp design, shows a few basic circuits, and provides a table of Analog Devices, Inc., op amps that may be appropriate for a preamp design. The ADMP504 MEMS microphone is used as an example in this application note to describe different design choices. This is an analog microphone with 65 dB SNR. Designs using different microphones may require adjustment from what is described in this application note, depending on the microphone noise, sensitivity, maximum acoustic input and other specifications.