Analog blocks do not fail loudly; they degrade. A converter that met specification on a bench meets something else beside a switching regulator on the same die, and the symptom appears three levels away as a system specification that will not close for reasons nobody can locate.
| Mechanism | Effect | Countermeasure |
|---|---|---|
| Supply noise | Switching current on a shared rail modulates analog bias | Separate supplies and returns, on-die decoupling, careful pad assignment |
| Substrate coupling | Digital switching injects current into the substrate | Guard rings, deep wells, physical separation, triple-well where available |
| Capacitive coupling | Aggressor nets couple into high-impedance analog nodes | Shielding, routing discipline, keeping sensitive nodes short |
| Thermal gradient | A hot digital block shifts nearby analog bias and matching | Floorplan separation, symmetric placement of matched devices |
| Clock feedthrough | Periodic disturbance appears as a spur in the spectrum | Clock routing kept away from analog, sensitive timing aligned to quiet periods |
| Layout parasitics | Extracted resistance and capacitance shift performance from schematic | Parasitic-aware layout, post-layout simulation as the real signoff |
The instinct on discovering an integration problem is to modify the circuit. Usually the circuit was fine and the floorplan was not, and the same circuit placed differently would have met specification.
Which means the analog constraints have to enter the floorplan at the point the floorplan is made, not be accommodated afterwards. Where the sensitive blocks sit, where the switching regulator sits, where the supplies and returns run, and how far apart the matched devices are — these are decided once, early, and are expensive to revisit.
Post-layout simulation across corners is then the actual signoff. Schematic-level results are a design aid; they are not a prediction of silicon.
Because the failure modes are coupled and continuous. Supply noise, substrate coupling, thermal gradients and layout parasitics all shift analog performance, and none of them appear in a standalone schematic simulation.
Mostly a floorplan and layout problem. The instinct is to modify the circuit; usually the circuit was adequate and the placement was not, which is why analog constraints must enter the floorplan when it is made rather than afterwards.
Post-layout simulation across the corners that matter for that block, with extracted parasitics. Schematic-level results are a design aid, not a prediction of silicon.
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