在这个例程中，进入锁相环之前，为什么要把采样的交流电压除以2送给锁相环函数？如下面第二行代码
Vac_in=(long)((long)Vac_FB<<9)-Offset_Volt; // shift to convert to Q21
spll1.AC_input=Vac_in>>1;
SPLL_1ph_MACRO(spll1);
此外，后面计算中对电流、电压又有乘以2
inv_meas_cur_lleg1_inst=(((int32) Ileg1_fb) <<12)-_IQ24(0.5);
inv_meas_cur_lleg2_inst=(((int32) Ileg2_fb) <<12)-_IQ24(0.5);
inv_meas_cur_diff_inst = (inv_meas_cur_lleg1_inst - inv_meas_cur_lleg2_inst)<<1;
inv_meas_vol_inst =((long)((long)Vac_FB<<12)-_IQ24(0.5))<<1; // shift to convert to Q24