Medical Electronics13 min read2026-01-15

Designing a Medical Signal Chain: From Photodiode to ADC for SpO2 and ECG

Medical device signal chains demand sub-μV noise floors, strict power budgets, and IEC 60601 compliance. This guide walks through the critical design decisions for precision photoplethysmography (PPG) and electrocardiography (ECG) front-ends using ADI components.

Medical Signal Chain Requirements


Designing a medical-grade analog front-end (AFE) is fundamentally different from industrial or consumer signal chain design. The three non-negotiable requirements:


1. **Noise**: SpO2 systems must resolve photodiode currents of 10nA to 1μA in the presence of 1μA to 100μA ambient light interference

2. **Power**: Portable medical devices must run from coin-cell or rechargeable Li-ion with weeks of battery life

3. **Compliance**: IEC 60601-1-2 EMC compliance requires careful PCB layout and filtering


SpO2 AFE Design with ADI OPAx392 + AD7175


Photodiode Current Amplification


A transimpedance amplifier (TIA) converts photodiode current to voltage. The OPA392 (single, dual, or quad) is ideal:

  • 10pA input bias current (negligible compared to 10nA photodiode current)
  • 4.5MHz GBP at 3.3V — supports >100kHz LED drive frequencies for ambient light rejection
  • Single 3.3V supply operation

    • Critical design: use a 2-stage TIA — first stage at 10MΩ gain, second stage as driver — to manage bandwidth while keeping noise low.


    ADC Requirements


    The AD7175 24-bit Σ-Δ ADC is the standard choice for medical AFE:

  • 19.2 kSPS maximum sample rate — filter to 100Hz for SpO2, 500Hz for ECG
  • 85nV RMS noise (20kHz BW) — achieves 22-bit effective resolution
  • Integrated PGA with gains from 1× to 64×