This paper examines the overall system noise performance of a microphone preamplifier system and the impact of providing some of the gain in both the analog and digital domains. Whereas some applications require up to 70 dB of system gain to bring microphone levels up to drive an A/D converter, the designer can elect to provide part of the gain in an analog stage and the remaining necessary gain in the DSP following the converter. As newer high-performance A/D converters realize dynamic range and THD specs greater than 100 dB, the use of digital gain post conversion becomes feasible. The noise impact of this approach is investigated and compared to others. Models are presented for the various noise sources in the system including source resistance, amplifier equivalent input noise, converter noise, and how they contribute to the total output noise of the system.
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