Method and apparatus of an acoustic energy detection circuit for a pulse density modulation microphone

A PDM (pulse density modulation) signal energy detection circuit, comprising:

• a digital-to-analog converter circuit for receiving a PDM digital input signal and producing an analog output signal based on the PDM digital input signal;
• a comparator circuit for receiving the analog output signal from the digital-to-analog converter circuit and producing a pulsed signal when a magnitude of the analog output signal exceeds a pre-set threshold;
• and
• a counter circuit for receiving the pulsed signal from the comparator circuit and producing an energy detection signal when a number of consecutive pulsed signals exceed a pre-set count;
• wherein the digital-to-analog converter circuit comprises a single MOSFET gain stage, a MOSFET current source carrying a current Ibias and a switched capacitor network, the single MOSFET gain stage being biased at a current Ibias and having an average voltage of Vnbias at a drain and gate connection, wherein Vnbias is a bias voltage derived from an external MOSFET gate voltage having drain and gate tied together and biased at a current Ibias.

A circuit for receiving a PDM digital input signal and producing an analog output signal based on the PDM digital input signal, comprising:

• an MOSFET for receiving a bias current Ibias from a current source and having an average voltage of Vnbias at a drain and gate connection;
• and
• a plurality of capacitors coupled to the MOSFET;
• wherein the circuit is configured to sample an input voltage Vnbias through switches controlled by the PDM digital input signal, wherein Vnbias is a bias voltage derived from a gate-to-source voltage of an external MOSFET having drain and gate tied together and biased at a current Ibias;
• and
• wherein the circuit is configured to produce an analog output voltage with a magnitude that is based on a density of the PDM digital input signal, the analog output voltage having an expression:
• Vnbias
• ±
• (
• 2
• ×
• Dp
• -
• 50
• 100
• )
• ×
• Vnbias
• ,
• where Dp is a density of pulses of the PDM digital input signal, expressed in a percentage.