Extended case study

Signoff review: supply ripple, spur coupling, analog performance degradation regressed after a change touching Analog Supply Noise.

Background

Team had prior closure; latest integration run shows instability concentrated in one operating condition.

Symptoms observed

• supply ripple, spur coupling, analog performance degradation degradation

• Cross-team disagreement on root cause

• Directed tests pass but system scenario fails

Investigation timeline

1. Hour 0: freeze data tags, firmware build, and environmental conditions

2. Hour 1: diff boundary assumptions and handoff revisions

3. Hour 2: isolate first failing mode and trigger sequence

4. Hour 3: correlate waveform/log/report evidence

5. Hour 4: classify root cause and ownership

6. Hour 5: apply bounded fix

7. Hour 6: execute full regression and issue signoff memo

Root cause

Root cause tied to Analog Supply Noise: Switching current bursts from digital domains couple through shared impedance and package parasitics, modulating analog supply quality and phase noise.

Fix and validation

• Bounded RTL/config/layout correction

• Re-run supply noise simulation, ripple spectrum, decap strategy report

• Full cross-domain regression matrix

Lessons learned

• Tag everything

• Mechanism before commands

• No signoff without owner agreement

CASE STUDY — Analog Supply Noise
baseline metric / regressed metric / post-fix metric

Sequence under stress

AMS BOUNDARY SEQUENCE — Analog Supply Noise

analog macro -> wrapper / boundary cell -> synchronizer or sampler -> digital consumer
      |                 |                         |                     |
  analog assumptions    legal voltage/state       clock/reset contract   protocol/data validity

metric under watch: supply ripple, spur coupling, analog performance degradation

AMS deep dive

AMS signoff is a combined noise, IR, timing, and package decision.

Concept diagram

BOUNDARY SIGNOFF

noise + IR + package + timing -> release decision

Metric graph

RISK DASHBOARD

open blockers █████
waivers       ███

Reports and artifacts

• supply ripple spectrum

• IR around analog islands

• package PI summary

• release dashboard

Mini case study

IR hotspot at analog island caused jitter spikes only under burst traffic.

Debug branches

• Correlate droop and jitter

• Check package return path

• Validate mode-specific activity profile

Senior review question

Ask: what boundary condition proves this topic is actually closed?

Key takeaways

• State boundary, mode, and evidence tag with every claim.

• Always align analog, digital, and physical owners before signoff decisions.

Common pitfalls

• Fixing averages while tails still fail.

• Skipping package/supply evidence in jitter or SerDes issues.

• Shipping with waivers that lack owner and expiration criteria.

Principal AMS review addendum

Switching current bursts from digital domains couple through shared impedance and package parasitics, modulating analog supply quality and phase noise.

Metric: supply ripple, spur coupling, analog performance degradation