Impedance

The Impedance node monitors electrode contact quality in real time, helping you ensure reliable EEG signal acquisition before and during recording sessions. It reads raw data from the EEG device’s shared Metal buffer and computes per-channel impedance or quality metrics.

Two Measurement Modes

The node supports two fundamentally different approaches to impedance measurement:

Signal Quality mode estimates electrode contact quality from the noise characteristics of each channel — without injecting any excitation current. This is the default and recommended mode for most EEG applications.

How It Works

The fundamental principle: poorly connected electrodes have higher impedance, which amplifies both thermal noise and mains interference. The node measures three independent metrics per channel:

Signal Quality Index (SQI)

The three metrics are combined into a single score from 0 to 100:

GPU Acceleration

All six Goertzel frequency filters (5 high-band + 1 mains) run in parallel per channel via a Metal compute kernel. A single pass over the ring buffer computes all metrics simultaneously. GPU acceleration reduced computation time from 2.13 ms to 0.39 ms (5.5× speedup).

LOFF mode uses the ADS1299’s built-in AC lead-off detection circuit for true impedance measurement in kiloohms.

How It Works

The ADS1299 ADC injects a small AC excitation current at f_DR/4 (500 Hz at 2 kSPS) through each electrode. The Impedance node then:

Both measurement modes use the same color-coded quality classification system. Thresholds are configurable via the Good Threshold and Bad Threshold inputs.

Output: Impedances (JSON)

The Impedances output port provides per-electrode values as an OrderedJSON object with anatomical electrode ordering:

// LOFF mode (kΩ):
{"Fp1": 4.2, "Fp2": 3.8, "C3": 12.1, "C4": 8.7, "P3": 5.4, "P4": 6.2, "O1": 45.3, "O2": 7.1}

// Signal Quality mode (SQI 0–100):
{"Fp1": 92, "Fp2": 95, "C3": 67, "C4": 78, "P3": 88, "P4": 91, "O1": 12, "O2": 85}

Output: All Good (Bool)

The All Good output is true when every electrode is classified as “good”. Use this as a gate signal to start recordings only when all electrodes are properly connected.

Measurement

Display

Pre-Recording Electrode Check

Monitor electrode quality before starting an EEG recording session:

Quality-Gated Recording

Only allow recording when all electrodes show good contact:

Per-Electrode Impedance Routing

Extract individual electrode values for custom processing:

GPU Acceleration

Both LOFF and Signal Quality modes use Metal compute shaders for parallel processing across all channels. The GPU performs the numerically intensive Goertzel DFT filtering, while lightweight score computation runs on the CPU.