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

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 (Vd) with a very high series resistance in the form of bipolar diodes (D1, 2) [1]. A capacitor (Cg) provides the signal ground for the microphone (Cmems). 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.