1M in parallel with 46nF isn't high impedance at timescales of 10us...
However I've just realised there will be cross coupling at lower frequencies because the piezo elements and the S/H cap form a switched capacitor filter!
1M in parallel with 46nF isn't high impedance at timescales of 10us...
However I've just realised there will be cross coupling at lower frequencies because the piezo elements and the S/H cap form a switched capacitor filter!