STM32F7 USB-ADC Multiplexer Board

Purpose

This board provides ten isolated, differential analog acquisition channels, multiplexed into a single 12-bit ADC on an STM32F7 microcontroller. The host communicates with the board over USB. Channel selection is handled autonomously by the firmware using an I2C-controlled analog multiplexer, so the host receives a sequenced stream of converted values without managing the mux directly. Primary use case: slow multi-point temperature or voltage monitoring in industrial environments with significant common-mode noise on the sensor lines.

External Interfaces

Interface	Connector	Protocol	Direction
Host communication	USB-C	USB 2.0 FS	bidirectional
Power input	USB-C	5 V / 500 mA max	in
Analog inputs	10 × 2-pin terminal	differential, ±10 V max	in

Functional Blocks

1. USB Interface

USB-C receptacle
EMC filter: common-mode choke + ESD protection clamp (TVS)
Connected directly to STM32F7 USB FS peripheral

2. Power Supply

Input: 5 V from USB
Rail 1: 3.3 V LDO — powers digital logic, MCU VDD, I2C mux
Rail 2: 1.7 V LDO — powers MCU VDDA (ADC reference domain)
Sequencing: 3.3 V must be stable before 1.7 V is enabled (soft-start or PGOOD tie)

3. Analog Frontend (×10, identical instances)

Each channel consists of:

EMC filter — common-mode choke + differential RC filter at board entry
Low-pass filter — single-pole passive, sets anti-alias corner frequency
Diff-to-single converter — discrete or IC-based; converts differential signal to single-ended 0–1.7 V range for ADC input

4. Analog Multiplexer

I2C-controlled 10:1 analog mux
Driven by STM32F7 I2C master
Single output routed to STM32F7 ADC_IN pin

5. Microcontroller — STM32F7

USB FS/HS peripheral for host communication
I2C master for mux control
12-bit SAR ADC for signal conversion
Firmware sequences channels, buffers readings, reports over USB CDC or custom protocol

Power Architecture

USB 5V ──► 3V3 LDO ──► MCU VDD, I2C Mux, USB EMC filter
       └──► 1V7 LDO ──► MCU VDDA (ADC reference domain)

Estimated current budget:

STM32F7 active: ~100 mA (3V3) + ~5 mA (1V7)
I2C Mux: ~1 mA
10× frontend passives: negligible
Total: < 150 mA @ 3V3 → well within USB 500 mA

Signal Chain

Differential input
  → EMC filter (CM rejection at board entry)
  → Low-pass filter (anti-alias, sets BW)
  → Diff-to-single converter (level shift to 0–1.7 V)
  → I2C Analog Mux (channel selected via I2C from STM32)
  → STM32F7 ADC (12-bit, single-ended)
  → USB (host reads converted values)

Known Constraints

Form factor: must fit standard 3U rack enclosure, max PCB 160 × 100 mm
Cost target: < 35 € BOM at 50 units
EMC: must pass EN 55032 Class B without additional shielding enclosure
Operating temperature: −10 °C to +60 °C
Input protection: must survive ±30 V on any analog input without damage

Open Questions

Which diff-to-single IC? INA828, AD8221, discrete? — depends on CMRR requirement
Anti-alias filter corner frequency — need to define max signal BW first
I2C mux part selection — ADG728? MAX4617? check availability
Does the ADC need an external reference or is VDDA sufficient?
USB CDC vs. custom HID class — ask firmware team
Do we need a reverse-polarity protection FET on the 5V input?