Normal order of Failures watch your DC bias

During our group meeting on Friday we discussed a recent Journal of Solid State circuits paper.

Yunzhi Dong and Kenneth Martin "A High-Speed Fully-Integrated POF Receiver with Large-area Photo Detectors in 65nm CMOS" IEEE JSSC Sept 2012 pp2080.

This is a very well written paper starting from the dynamic behavior of photo diodes to the implementation of the receiver.  The receiver looks similar to those I have worked on in the past and the line-up clever with a fully-differential transimpedance amplifier with a replica diode at the input.  A single point gain control after the TIA followed by filtering and a buffer amplifier to get the signal out.  Dong and Martin chose a continuous time high-frequency boost approach these have advantages in power and jitter tolerance but are more difficult to design.  Thats why its Dong's Ph.D.    I really liked the impedance analysis at the TIA input.  The TIA itself was very clever and amazingly works at 65nm voltage levels.  The eye diagrams look great that tells you something.

Dong did a good job in his design and his layout.  If you built a circuit with the exact same schematics as Dong and Martin and did a different layout, it may not work at all.  At GHz rates the layout is so critical that a missing shield or imbalanced line could ruin, or worse yet, degrade performance.  When debugging a receivers like this in the past I have seen the same issues over and over (at the chip level):
#1  Bad circuit architecture (wrong circuit, incorrect design, incorrect specs. )
#2  Bad Layout (imbalanced, mirror symmetric, IR drops, coupling)
#3  DC Bias (improper DC bias point)

#1 is obvious.  #2, I have met analog designers who claim that the layout is not important.  Maybe they don't respect the job of a good layout designer.  Depending on the SNR and bandwidth requirements your layout sensitivity can go from non-existent to extreme.  #3.  Its amazing how many 15yr+ analog designers mess up DC bias.  Famous quote "That voltage was close to what I needed so I simply used it."  If you hear that quote, get ready for the lab.