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.
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