Help with Piezo conditioning circuit schematic
Vbias is just a mid-rail voltage level that may be regarded as 0V or it may be slightly different to 0V. I've never seen the symbol before but I'm fairly sure it will connect directly to 0V or maybe a 1 or 2V dc reference voltage that is itself referenced to the 0V/earth symbol in the right hand box.
I'd say the top triangular symbol is a positive power rail even though I've seen that symbol used for ground on other drawings. Knowing that the circuit is a comparator and that it needs a "threshold/trigger-point" voltage assures me of that.
Impossible to say but, it is an output!!
The piezo earth connection should ideally be strongly AC coupled to the Vbias voltage - that is critical then if you need to make an arduino 0V/ground/earth connection, do so at the comparator end of the circuit to avoid digital currents upsetting the front-end sensitive amplifier/rectifier.
It means, as I read it, that the op-amps should share the same bias point.
The threshold voltage is that directly onto the "-" input pin on the comparator - this can be a common connection to all comparators - i.e. no need to repeat the two resistors for each comparator circuit.