8 IN AN OPEN BITLINE, THE SENSE AMPLIFER COMPARES THE VOLTAGE ON A BITLINE FROM THE ACTIVE SUBARRAY TO THE VOLTAGE ON A BITLINE FROM A QUIESCENT SUBARRAY

11.8 In an open bitline, the sense amplifer compares the voltage on a bitline from the

active subarray to the voltage on a bitline from a quiescent subarray. Power supply

noise between subarrays makes sensing a small swing impossible. In a closed bit-

line, the two sense amplifier inputs come from bitlines in the same subarray, but

only one of the two is activated. This design requires somewhat more layout area

but eliminates most supply noise problems. It is still sensitive to coupling that

affects one sense amplifier input more than the other. Twisted bitlines route the

folded bitlines in such a way that each one sees exactly the same coupling capaci-

tances, hence making coupling noise common mode as well. This is necessary in

modern DRAM designs and costs slightly more area to perform the twists.