RP2350A + ADS1220 Molecule

rp2350a-ads1220 v1.0.0 · 24-bit thrust-acquisition molecule for the drone motor carrier

26.4 × 36.0 mm4-layer, 1.6 mm RP2350A QFN-60ADS1220 24-bit100 parts

Project overview

From the project's own overview document (author-written).

RP2350A + ADS1220 Molecule — Function

24-bit thrust acquisition. RP2350A (QFN-60) + ADS1220 + W25Q16JV + AP2112K-3.3, two differential load-cell channels. 26.40 × 36.00 mm, 4-layer, 70 placeable parts.

Signal chain

bridge -> [RF 2.49k 1%] -+- [CDIF 470n] -+- ADS1220 AINx
                         +- [CCM 47n 5%] -- GND (shared via per pair)

R sits at the contact end, C at the ADC end. A shared GND via per pair removes CM-to-DM conversion error from CCM mismatch; CDIF >= 10 x CCM swamps the remainder.

Filter constants

VIN(MAX)  = +/-[(AVDD - AVSS) - 0.4] / Gain  =  +/-36 mV at gain 128
                                                PGA output stays 200 mV off each rail
CM window = 1.25 - 4.16 V                       Eq 12/13/14, Eq 14 dominates
f_cm / f_diff = 2(CDIF/CCM) + 1 = 21            f_diff 64.8 Hz, f_cm 1360 Hz

Noise 0.41 µVpp at gain 128 / 20 SPS. −3 dB bandwidth 13.1 Hz at 20 SPS, 39.6 Hz at 90 SPS (Table 8-4). Reference input current ±10 nA, buffered.

Ratiometric topology

REFP0 (pin 7), AVDD (pin 10), MC16 and MC23 all sit on the same +5V node, downstream of F1.

Fuse drop and the AP2112's ~70 mA are common to reference and excitation, so they cancel
Resistance from that node out to the cell is outside the loop and appears as gain error — ~0.029 % for 0.1 Ω, calibratable

Power

+5V_IN → F1 polyfuse → +5V → D1 SMF6.0A → GND. The fuse is upstream of the TVS, so a reverse polarity fault drives TVS current through the fuse.

AP2112K-3.3 with 10 µF in (C5) and out (C6). C1 100 n sits 1.67 mm from AVDD.

Indicator

WS2812B-2020-V6 (C52917434). VIH is 0.55 × VDD and it runs from 3.3 V. Specifies 100 nF. Not stocked at Mouser or DigiKey in the PLCC4 2020 package — LCSC only.

Board state

0 DRC violations, 0 unconnected. All 30 contacts on the 2 mm lattice, MP datum 16.000 × 32.000 mm.

Three silk DRC checks are ignored. Five BOM/schematic LCSC mismatches exist in jlcpcb/project.db — the BOM generates from that database, not the schematic fields.

What it does

A self-contained 24-bit acquisition molecule that nests on the drone motor carrier over its machine-pin datum. It takes +5 V in, makes its own 3.3 V on-board, and digitises two ratiometric load-cell channels with a TI ADS1220, run by an RP2350A. The load-cell excitation and the differential pairs cross the machine contacts to the carrier, so the same nest that mechanically seats the molecule also carries its analogue signals.

The board, section by section

Power

The 5 V input is protected then regulated: +5V_IN → F1 polyfuse → +5V → D1 SMF6.0A TVS → GND, and an AP2112K-3.3 (U1) makes the 3.3 V rail for the MCU and ADC. The fuse sits upstream of the TVS, so a reverse-polarity fault drives clamp current through the fuse. Both the 5 V and 3.3 V rails carry a "present" indicator.

MCU — RP2350A

RP2350A QFN-60 (U3) with a 12 MHz X322512MSB4SI crystal (Y1). Its 4.7 µF decoupling sits on pins 23/25, 49, 50 and 46; R7 and R8 are placed close to the RP2350 as annotated on the schematic. The MCU runs acquisition and presents DFU over its own USB.

QSPI flash

W25Q16JV (U4) program/data flash on the RP2350A's QSPI bus.

ADS1220 front-end

ADS1220IRVAR (U2), a 24-bit delta-sigma ADC, with a ratiometric RC front-end: RF 2.49 kΩ · CDIF 470 nF · CCM 47 nF — R at the contact end, C at the ADC end. Filter corners are f_diff 64.8 Hz and f_cm 1360 Hz; differential rejection is 51 dB @ 24 kHz PWM and 72 dB @ 256 kHz, common-mode rejection 24.9 dB @ 24 kHz. REFP, AVDD and the excitation contacts share the +5V node, so the fuse and LDO drops are common to reference and excitation and cancel. Design notes on the schematic: PSW must remain 0b, and the usable input swing is 5 V − (2 × 200 mV) = 4.6 V.

Status LEDs

LED1 is a WS2812B-2020 addressable RGB used for STATUS / DFU. LED2 (blue, "5 V Present", ≈0.51 mA), LED3 (yellow) and LED4 (emerald green, ≈0.45 mA) are 0402 indicators — the LED currents are as annotated on the schematic. Illustrative: the Power toggle in Static mode lights these indicators.

DFU controls

Three SKRPACE010 tacts (SW1–SW3). The DFU function is: a local manual assert (drive low to request an update), a status LED, and a remote assert input.

Debug

J1 is a BM03B-SRSS-TB JST connector — the SWD debug landing.

Machine pins

MC1–MC26 are the machine-pin nesting interface. The molecule seats on the motor carrier over these contacts, and the load-cell excitation and differential pairs cross here — the contacts sit upstream of the front-end filter.

Layout

Interactive PCB — click any pad, trace or zone to highlight its whole net; hover for pad and live part data; toggle layers and mirror Top/Bottom.

Schematic

Interactive schematic — hover a symbol for its KiCad highlight plus live MPN / stock / price / LCSC.

How this page was built

Generated from the design files. The 3D model, the section membership, and the part list are derived mechanically from rp2350a-ads1220.kicad_pcb / the schematic and the STEP export. Coverage: 100 placed parts, all resolved to a solid, no stale geometry, no DNP.
Author-asserted. The section narrative and the design intent (filter corners, DFU behaviour, ratiometric topology) come from the designer's own notes embedded in the schematic, not from a prior page. Part-level values are as-drawn from the schematic, not re-verified against each part's own datasheet.
Illustrative. The Static-mode Power toggle is a visual aid, not captured data.
LEDs. The 0402 indicators use the Adom in-s42atr LED model, recoloured per the schematic value (LED2 blue, LED3 yellow, LED4 green); LED1 keeps its real WS2812 geometry.