求助:流水线ADC中MDAC的问题

frankchan23

当MDAC从采样间段转到放大间段时,Cs和Cf的底板要先与运放的输入共模电平断开,以实现底板采样,
我的问题是,既然已经与输入共模电压断开,在放大间段,运放的输入共模电压是怎么建立产生的?
请诸位高人帮忙解答一下,在下多谢谢了!!!

prof3

1. During sampling period, assume VCMI1(nT-T/2), VCMI2(nT-T/2), VCMO(nT-T/2) are input common mode voltage, summing node common voltage and output common voltage respecitively.  
2. During amplifying period, assume VCMI1(nT), VCMI2(nT), VCMO(nT) are input common mode voltage, summing node common voltage and output common voltage respecitively.  
3. As long as
   VCMI1(nT) = VCMI1(nT-T/2)
   VCMO(nT) = VCMO(nT-T/2)
are meet, then VCMI2(nT) = VCMI2(nT-T/2) will be satisfied(charge conversation). That is to say, the common mode voltage will be the same during the whole cycle(sampling and amplifying).

Hopefully it helps.

frankchan23

我的一种理解是偏置电压建立到了寄生电容上去了,

apprenticewyc

如果不考虑电荷注入和时钟馈通,共模电压是保存在寄生电容上的,因为没有额外的电荷注入,所以共模电压是不变的.
如果计入电荷注入和时钟馈通,则共模电压会抖动.
输出共模电压,反馈电容和寄生电容共同决定了输入共模电压的变化量.
而输出共模电压是由共模反馈电路决定的.