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Impedance Measurement

This document describes the impedance measurement implementation for EGI Net Amps amplifiers.

Background

Electrode impedance measurement is essential for ensuring good signal quality in EEG recordings. High impedance electrodes result in noisy signals and increased susceptibility to interference. EGI amplifiers (NA400/NA410) include built-in circuitry for measuring electrode impedances without requiring external equipment.

Measurement Principle

The impedance measurement uses a voltage divider technique:

  1. A known calibration signal is injected through all electrodes
  2. Each electrode to be measured is connected to a precision 10 kΩ reference resistor
  3. The resulting signal amplitude is measured
  4. Impedance is calculated from the voltage divider ratio
Calibration Signal → [Electrode Impedance Z] → [10K Reference] → Ground
                                             ↓
                                    Measured Amplitude

The impedance formula:

Z = (idealSignal - measuredAmplitude) / (measuredAmplitude / 10.0)

Where:

  • idealSignal = expected amplitude with 0Ω electrode impedance
  • measuredAmplitude = peak-to-peak amplitude of the measured signal
  • 10.0 = reference resistor value in kΩ

Amplifier States

The measurement process uses three amplifier states:

Excitation State (Initial/Driving)

All electrodes actively drive the calibration signal onto the scalp:

Setting Value Description
All 10K Resistors OFF Electrodes not connected to reference
All Drive Signals ON All electrodes driving calibration signal
Oscillator Gate ON Enable 20 Hz sine wave generator
Calibration Frequency 20 Hz Low frequency for skin penetration
Calibration Amplitude 4095 Maximum (12-bit DAC)
Wave Shape Sine (0) Clean sinusoidal signal
Subject Ground OFF Not grounded during measurement
Current Source OFF Voltage mode measurement

Measurement State (Per-Channel)

For the channel being measured:

Setting Value Description
Channel Drive Signal OFF Stop driving this electrode
Channel 10K Resistor ON Connect to reference resistor

All other channels remain in excitation state, creating the voltage divider.

Reset State

After measurement, return channel to excitation state:

Setting Value Description
Channel Drive Signal ON Resume driving
Channel 10K Resistor OFF Disconnect reference

Measurement Algorithm

1. Configure amplifier to Excitation State (all channels driving)

2. For each channel to measure:
   a. Set channel to Measurement State (drive OFF, 10K ON)
   b. Wait for settling time (~30ms command + ~1s filter time)
   c. Collect samples for peak-to-peak calculation (~51 samples)
   d. Calculate amplitude = max(samples) - min(samples)
   e. Calculate impedance from amplitude
   f. Reset channel to Excitation State (drive ON, 10K OFF)

3. Measure the Reference (Cz) electrode each cycle (COM is optional)

4. When done, reset amplifier to Default Acquisition State

Timing Parameters

Parameter Value Description
Command Time 30 ms Time for command to reach amplifier
Settle Time 0 ms Additional settling
Filter Time 1.0 s Duration for filter/measurement
Peak-to-Peak Samples 51 Samples for amplitude calculation

Total time per collection: ~1.03 seconds

Each collection measures a whole tiling set at once, so a full scan costs roughly one collection per tiling set (plus one for the reference) rather than one per channel. A 256-channel net has 6 tiling sets, so a full scan is ~6 collections (about a minute). Nets without a tiling layout fall back to one collection per channel (~4-5 minutes for 256 channels).

LSL Streaming

Impedance values are streamed via LSL at 1 Hz:

  • Stream Type: Impedance
  • Sample Rate: 1 Hz (one sample per second)
  • Data Format: float32 (kΩ)
  • Channel Labels: E1, E2, ..., E256, Cz

The last channel, Cz, is the reference electrode and is always appended after the per-electrode channels (so the impedance stream has one more channel than a standard-net EEG stream). Unlike the E-channels, its impedance is derived from the amplifier's dedicated reference monitor field (refMonitor) rather than a regular EEG channel — see measureReference().

The stream publishes the current known values for all channels every second, even during scanning. Channels not yet measured show the maximum/invalid sentinel value.

Stream Metadata

Each channel includes 3D electrode position in the stream description:

<channel>
  <label>E1</label>
  <type>Impedance</type>
  <unit>kohms</unit>
  <location>
    <X>0.123</X>
    <Y>0.456</Y>
    <Z>0.789</Z>
    <unit>normalized</unit>
  </location>
</channel>

Coordinates are on a unit sphere:

  • X: positive toward right ear
  • Y: positive toward nose
  • Z: positive toward vertex (Cz)

Implementation

Key Source Files

File Description
ImpedanceMeasurement.h Class declaration and timing structures
ImpedanceMeasurement.cpp Measurement algorithm implementation
LSLStreamer.cpp LSL outlet creation with electrode positions
ElectrodePositions.h Electrode coordinate definitions
ElectrodePositions.cpp Geodesic position generation
EGIAmpClient.cpp Integration with streaming client

Key Classes

ImpedanceMeasurement - Core measurement logic:

  • setupImpedanceState() - Configure amplifier for impedance mode
  • startContinuousScan() - Begin background scanning thread
  • measureChannel(int ch) - Measure single channel (blocking)
  • feedSample(PacketFormat2&) - Feed EEG samples for amplitude calculation
  • publishThread() - Push values to LSL at 1 Hz

LSLStreamer - LSL stream management:

  • createImpedanceOutlet() - Create impedance stream with metadata
  • pushSample() - Push impedance values

AmpServer Commands

Command Description
cmd_TurnAll10KOhms Set all 10K resistors (0=off, 1=on)
cmd_TurnAllDriveSignals Set all drive signals (0=off, 1=on)
cmd_TurnChannel10KOhms Set single channel 10K resistor
cmd_TurnChannelDriveSignals Set single channel drive signal
cmd_SetOscillatorGate Enable/disable calibration oscillator
cmd_SetCalibrationSignalFreq Set frequency (20 Hz for impedance)
cmd_SetCalibrationSignalAmplitude Set amplitude (0-4095)
cmd_SetWaveShape Set waveform (0=sine, 1=square)
cmd_DefaultAcquisitionState Reset to normal EEG acquisition

Usage

CLI

./EGIAmpServerCLI --address 10.10.10.51 --impedance

GUI

  1. Link to the amplifier
  2. Check "Measure Impedances" checkbox
  3. Impedance stream appears on LSL network
  4. Uncheck to stop measurement and return to normal EEG

Python Visualization

cd scripts
pip install -r requirements.txt
python impedance_viewer.py --threshold 50

Values of 1000 kΩ indicate:

  • Electrode not yet measured
  • No contact with scalp
  • Broken or disconnected electrode

References

  • EGI AmpServer SDK Documentation
  • Net Station Acquisition source code (NSAAmpSettings.m, ImpedancesController.m)
  • EGI Geodesic Sensor Net Technical Manual