A 50μW biasing feedback loop with 6ms settling time for a MEMS microphone with digital output

Abstract

A MEMS microphone is a variable capacitor where the distance d between the compliant membrane and the rigid backplate is proportional to the air (acoustic) pressure. If the charge Q at the plates remains constant, there is a linear relationship between the voltage across the plates and the distance given by: ΔV=Q×Δd/εA. A practical charge source implementation (Fig. 11.3.1) is a DC-DC converter (V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</inf> ) with a very high series resistance in the form of bipolar diodes (D <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1, 2</inf> ) [1]. A capacitor (C <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</inf> ) provides the signal ground for the microphone (C <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mems</inf> ). An ADC performs continuous-time voltage readout of the microphone signal pin. Preferably its DC bias is equal to the ADC common-mode voltage. This paper deals with a biasing feedback loop (BFL) that realizes this function.