Designing Smart AC PDU

flux

I reviewed the uploaded Claude spec and worklist. I would not ingest it blindly because it conflicts with our current project spec in several important places:
  1. MCU conflict
    • Current spec: PSB3014, 87-BGA.
    • Claude spec: PSB3112, 68-DRQFN.
    • This is a major architecture/package change and needs your approval before schematic.
  2. Current transformer conflict
    • Current spec: Triad CST-1020.
    • Claude spec/worklist: Talema AC-1020.
    • Talema may be better thermally, but we need to verify UL file, ratio, footprint, accuracy, phase, and stock before replacing CST-1020.
  3. Power tree changed
    • Current spec has +12 V and +3.3 V.
    • Claude adds a dedicated +5 V LED buck and USB/debug/service additions. Reasonable, but not previously committed.
  4. Schematic capture order is good
    • I agree with Claude’s 12-sheet structure and with capturing Sheet 1: AC input & protection first after reconciliation.
Recommended next step: I should create a short spec reconciliation checklist and then update the canonical project spec to either adopt or reject Claude’s changes. The key decision needed from you first is:
Do you want to switch from PSB3014 to PSB3112, or keep PSB3014 as originally selected?
gabekassel

gabekassel

Keep PSB3014 and reject the PSB3112 change unless you find a strong reason to switch.

flux

Understood — I’ll keep PSB3014 as the canonical MCU and mark the PSB3112 change from the Claude spec as rejected unless future verification shows a compelling reason to switch.

flux

Confirmed and documented: PSB3014 remains the canonical MCU, and the Claude-spec PSB3112 migration is rejected unless future evidence gives a strong reason to switch.
Updated:
Next best step: rebuild the MCU pin assignment for PSB3014, then continue reconciling CT choice, ADC clock source, and LED/power architecture before Sheet 1 schematic capture.
Rebuild PSB3014 pins
Verify CT options
Finish ADC clock
Reconcile LED power
Prepare Sheet 1
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Current Sensing Component Selection
Decision
Use one isolated PCB current transformer per outlet feeding one 8-channel simultaneous-sampling delta-sigma ADC. Add one isolated AC voltage waveform channel in a separate front end so firmware can compute watts, VA, power factor, and energy.
Preferred sensing chain:
Diagram



  
    
  
  
    
  
















Outlet switched line conductor
Triad CST-1020 current transformer
Split burden resistor network
Input clamp / protection
1st-order differential anti-alias RC filter
TI ADS131M08 8-channel 24-bit ADC
Infineon PSB3014 SPI
Isolated AC voltage sensing: AMC3330 + ADS131M02
Preferred Current Transformer
Triad Magnetics CST-1020
  • Flux library part UID: 1607bdb6-7893-4465-86d2-504e96fb46a5
  • Type: 50/60 Hz PCB-mount current sense transformer
  • Primary rating: 20 A
  • Turns ratio: 1000:1
  • Recommended / datasheet terminating resistor example: 100 ohm
  • Datasheet accuracy: ±3% from 2 A to 20 A
  • Operating temperature: -10 °C to +65 °C
  • UL file: E205349, component instrument transformer
  • Size: approximately 23.5 mm × 24.8 mm × 12.0 mm body class
Fit to PDU
  • 20 A CT rating gives margin over the 15 A total PDU limit and 15 A-capable NEMA 5-15R outlets.
  • ±3% CT accuracy from 2–20 A supports the project’s ±5% per-outlet current target after calibration.
  • Operating temperature is only rated to +65 °C, so enclosure internal temperature must be checked carefully because project ambient target is ~50 °C. If the CT location can exceed 65 °C, select a higher-temperature CT before release.
Preferred ADC / Front-End IC
Texas Instruments ADS131M08IPBSR
  • Flux library part UID: c5120e59-1358-4711-80d1-e606b88e5286
  • Type: 8-channel simultaneous-sampling 24-bit delta-sigma ADC
  • Inputs: 8 differential channels, ideal channel count for 8 CTs
  • Programmable gain: 1, 2, 4, 8, 16, 32, 64, 128 per channel
  • Internal reference: 1.2 V low-drift reference
  • Data rate: up to 32 kSPS per channel; 8 kSPS is a good starting point for 50/60 Hz RMS
  • Supply: 2.7–3.6 V AVDD/DVDD, compatible with a 3.3 V logic domain
  • Interface: SPI, with DRDY and SYNC/RESET pins recommended
  • Temperature range: -40 °C to +125 °C
  • Package: 32-pin TQFP or WQFN; library part is TQFP-32 5 mm × 5 mm
  • Power: about 21.5 mW in high-resolution mode, about 11.3 mW low-power, about 6.2 mW very-low-power
Why ADS131M08 over simpler ADCs
  • Measures all 8 outlet channels simultaneously, avoiding mux timing/phase artifacts.
  • Differential inputs match split-burden CT circuits.
  • 24-bit delta-sigma architecture and PGA give large dynamic range for RMS measurement.
  • TI explicitly documents CT/energy-metering front-end circuits for this ADC family.
  • The ±5% target is modest for this ADC, so system accuracy will mainly be set by CT tolerance, burden resistor tolerance/tempco, phase/amplitude calibration, and firmware RMS processing.
Front-End Starting Point
For CST-1020 with 1000:1 ratio:
  • Secondary current at 15 A RMS primary: 15 mA RMS.
  • Recommended initial effective burden: about 39 ohm total, implemented as split burdens around ADC ground, e.g. 19.6 ohm + 19.6 ohm precision resistors.
  • Resulting ADC differential signal:
    • 15 A RMS: 0.585 V RMS, about 0.827 V peak.
    • 20 A RMS: 0.780 V RMS, about 1.10 V peak.
  • This stays below the ADS131M08 ±1.2 V full-scale range at gain = 1 with margin for 20 A overload measurement.
Recommended per-channel ADC input network:
  • Split burden: 19.6 ohm + 19.6 ohm, 0.1% or 0.5%, low-tempco preferred.
  • Differential anti-alias filter per TI guidance: 1 kΩ series in each ADC input leg and 10 nF C0G differential capacitor across AINxP/AINxN.
  • Add input clamp/protection so an open burden, surge, or CT transient cannot overstress ADC pins.
  • Place burden and protection close to CT/ADC interface; never allow CT secondary to run open-circuit in normal operation.
Alternate ADC Options Considered
AD7329
  • 8 channels, 12-bit plus sign, 1 MSPS SAR ADC.
  • Accepts bipolar inputs directly.
  • Requires ±12 V analog supplies for full specified performance, adding unnecessary rails.
  • Rejected as primary choice because power architecture is more complex and resolution/dynamic range are worse for this use case.
ADC1283 / ADC128D818 class
  • 8 channels, 12-bit, 3.3 V-compatible.
  • Simpler and lower cost, but multiplexed and lower dynamic range.
  • Usable for coarse current monitoring, but less robust for calibrated RMS across eight outlets.
ADE7953
  • High-accuracy single-phase metering IC with two current channels and one voltage channel.
  • Excellent metering performance, but one IC cannot measure 8 independent outlets.
  • Would require four ICs for eight current channels, so rejected for this architecture.
Voltage / Energy Sensing Update
Add one isolated AC line-voltage sensing channel for the shared PDU input voltage. Preferred architecture is a high-value mains-rated L-N divider feeding a TI AMC3330 reinforced isolated voltage-sensing amplifier, then a TI ADS131M02 2-channel simultaneous delta-sigma ADC. The ADS131M02 should be clock/sync aligned with the ADS131M08 as much as practical so firmware can calculate per-outlet real power from v(t) × i(t), plus VA, power factor, and Wh.
Do not consume an ADS131M08 CT channel for voltage; all 8 channels are reserved for the eight outlets. Detailed voltage-sensing notes are captured in AC Voltage and Energy Sensing Selection.
Open Items Before Schematic Capture
  1. Confirm calibrated accuracy range for ±5% target. Recommended target: 0.5–15 A per outlet, with lower currents reported as approximate.
  2. Confirm whether 65 °C CT operating limit is acceptable based on enclosure thermal estimates. If not, find a higher-temperature CT.
  3. Select exact burden resistor power rating and protection clamp after final ADC input range and surge strategy are set.
  4. Verify ADS131M08 and ADS131M02 synchronization strategy for calibrated real-power calculations.

  • Decision

  • Preferred Current Transformer

  • Fit to PDU

  • Preferred ADC / Front-End IC

  • Why ADS131M08 over simpler ADCs

  • Front-End Starting Point

  • Alternate ADC Options Considered

  • AD7329

  • ADC1283 / ADC128D818 class

  • ADE7953

  • Voltage / Energy Sensing Update

  • Open Items Before Schematic Capture