1. S. Reda EN160 SP’07 Design and Implementation of VLSI Systems (EN0160) Lecture 21: Differential Circuits and Sense Amplifiers Prof. Sherief Reda Division of Engineering, Brown University Spring 2007 [sources: Weste/Addison Wesley – Rabaey/Pearson]

2. S. Reda EN160 SP’07 Remember Cascode Voltage Switch Logic Cascode voltage Switch logic Adv: no static power dissipation compared to pseudo-NMOS radioed circuits differential signaling Disadv: potentially large diffusion capacitance (slowing things down) How can we cut down the diffusion capacitance?

3. S. Reda EN160 SP’07 Differential Split-Level Circuits Adv: The reference voltage (V DD /2+V t ) limits the swing on the internal diffusion nodes (X) to 0-V DD /2  reduces parasitic delay Disadv: Introduces series resistance does not help the delay much Brings static power back into the picture

4. S. Reda EN160 SP’07 Sense Amplifiers Function:  Senses small differential input signals and magnify them into larger out signals  reduces delay  Commonly used in memory in which differential bit lines have huge capacitances

5. S. Reda EN160 SP’07 Sense Amplifiers

6. S. Reda EN160 SP’07 Sample Set Differential Logic (SSDL) During sample, when Ф is low: Both the precharge and evaluation transistors are ON  static current consumed One of the internal nodes (X or Xbar) is precharged high; contention on the other During set, when Ф is high: Precharge and evaluation transistors are OFF Sense amplifier turns on pulling the lower of the two nodes to the ground

7. S. Reda EN160 SP’07 Enable/Disable CMOS Differential Logic When Ф is high both outputs are pulled low pull-up stack is turned off (no static power) When Ф is low output gate is enabled one output will be held high, the other pulled down sense-amp uses positive feedback to produce a faster swing The sense amplified uses cross-coupled inverters

8. S. Reda EN160 SP’07 Summary Done with chapter 6 (Combinational Circuit Design). Ignore Section 6.7 (Silicon-on- Insulator) Next time: material review for the exam (or better on Monday?)