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ADR-0042

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ADR-0042: Ahuja over Miller compensation for the ramp-generator OTA

ID ADR-0042
Status accepted
Date 2026-05-20
Chips RC2.0
Blocks ramp-generator, OTA
Sources timeline/weekly/2026-W20.md

Context

The ramp-generator OTA needed frequency compensation. Two candidates: classic Miller compensation, and Ahuja (cascode) compensation.

Decision

Use Ahuja compensation. It gives better power-supply rejection and removes the feedforward zero that Miller introduces, at the cost of an extra bias branch.

Rejected alternatives and why

  • Miller compensation — simpler, but the feedforward zero degrades phase margin at the ramp speeds required, and PSRR is worse on the noisy 28nm supply.

Consequences

  • Extra current in the cascode branch within power budget.
  • PSRR improved enough to sit on the dirty supply without a dedicated LDO for this block.

What would make us revisit this

A move to a cleaner supply rail for the ramp generator, or a power budget cut that makes the extra branch unacceptable.

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