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An AC-Biased Microphone

I’ll round out the last of the things that caught my attention at this year’s ISSCC with a proposal and implementation of an AC-biased microphone. This is done based on projections that the biasing resistor for a traditional DC approach will head into ridiculously high territory – teraohms and higher.

The team, from NXP and Delft University, lists a number of problems that this causes.

  • The connection between the MEMS die and the ASIC can easily pick up stray electrical noise due to its high impedance, meaning expensive packaging is required to shield this node.
  • Creating a poly resistor of this size would be enormous; instead, active devices biased below their turn-on voltages are used. But leakage currents from neighboring ESD structures can find their way through these, with the ultimate result being increased noise.
  • They say that chopping can’t be used to reduce flicker noise because of the extra input current the switching would cause; increased transistor sizes are needed instead.
  • The on-chip bias generator will typically be a charge pump, and ripple noise could push the shut-off active devices used as resistors to turn on slightly; therefore large filtering caps are needed.

Their approach is differential, and they modulate the signal while cancelling out the carrier using cross-coupling caps; there are, in fact, three caps that have to be tuned to match the microphone sensing cap, and they have an 11-bit register for each of them.

Critically, feedback resistors are used to set the common-mode bias level; that and the fact that their contribution to in-band noise is now low due to the modulation mean that resistor values can be brought back down well below a gigaohm.

While you might expect the increased complexity to make the ASIC larger, in fact quite the reverse is true (presumably due to smaller components): the ASIC is 1/12 the size of the current state of the art. Expensive shielding is also no longer required to reject external noise.

They weren’t overwhelmed by the SNR they achieved, in the 58/60-dB range, but they commented that, with some focus, they could easily get to 64/65-dB levels.

For those of you with the proceedings, you can get much more detail in session 22.2.

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