Download lecture21 - Brown University

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]
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?
S. Reda EN160 SP’07
Differential Split-Level Circuits
Adv: The reference voltage (VDD/2+Vt) limits the swing on the
internal diffusion nodes (X) to 0-VDD/2  reduces parasitic delay
Disadv:
• Introduces series resistance does not help the delay much
• Brings static power back into the picture
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
S. Reda EN160 SP’07
Sense Amplifiers
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
S. Reda EN160 SP’07
Enable/Disable CMOS Differential Logic
The sense amplified uses cross-coupled inverters
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
S. Reda EN160 SP’07
Summary
• Done with chapter 6 (Combinational Circuit
Design). Ignore Section 6.7 (Silicon-onInsulator)
• Next time: material review for the exam (or
better on Monday?)
S. Reda EN160 SP’07