[SOLVED] 9-input NAND gate problem

[ylhsheep]

Many 9-input NAND gate are present in a mixed signal IC chip under analysis. In my opinion, the nmos in series of the NAND will not turn on, for the VDD is 5V,and the threshold voltage of the low-threshold nmos is about 0.65V. So how to apprehend the function of the 9-input NAND gate.

 

[FvM]

I don't see immediately how the NMOS series connection should cause other problems than making the device slow.

 

[RobG]

Assuming the nand gate is not sinking dc current, the final voltage drop across the drain-source of the NMOS will be zero, so each gate-source voltage will be the full supply. Thus the NMOS will all be on. However, like FvM says, it will be slow. This is because you have nine NMOS devices in series which have to be very big to keep the impedance low. The big devices will add extra capacitance.

rg