// the 24 bit adc value from the chip is actually the fraction of the full
// dynamic range,the actual full dynamic range voltage is fixed by hardware
// design,and it‘s defined as VOLTAGE_SCALE. so the formular will be:
//
// vrms(in milliVolt) = adc * VOLTAGE_SCALE * 1000 / 2^24
//
// if we multiply everything first,we might overrun the 32 bit value
// but if we do all the bit shift first,we will lost to much accuracy
// so we do the this step by step.
//
// the last 5 bits are insignificant to milliV,and good enough for us to do
// a mulitplication without overrun.
?
v=adc/?2^N? (N表示几位)? //公式
?例子
0.75? ?011? ?==3/(2^2) =3/4=0.75
