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‎Impedance.md‎

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# Impedance Measurement
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This document describes the impedance measurement implementation for EGI Net Amps amplifiers.
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## Background
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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.
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## Measurement Principle
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The impedance measurement uses a **voltage divider** technique:
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1. A known calibration signal is injected through all electrodes
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2. Each electrode to be measured is connected to a precision 10 kΩ reference resistor
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3. The resulting signal amplitude is measured
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4. Impedance is calculated from the voltage divider ratio
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```
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Calibration Signal → [Electrode Impedance Z] → [10K Reference] → Ground
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↓
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Measured Amplitude
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```
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The impedance formula:
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```
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Z = (idealSignal - measuredAmplitude) / (measuredAmplitude / 10.0)
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```
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Where:
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- `idealSignal` = expected amplitude with 0Ω electrode impedance
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- `measuredAmplitude` = peak-to-peak amplitude of the measured signal
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- `10.0` = reference resistor value in kΩ
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## Amplifier States
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The measurement process uses three amplifier states:
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### Excitation State (Initial/Driving)
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All electrodes actively drive the calibration signal onto the scalp:
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| Setting | Value | Description |
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|---------|-------|-------------|
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| All 10K Resistors | OFF | Electrodes not connected to reference |
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| All Drive Signals | ON | All electrodes driving calibration signal |
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| Oscillator Gate | ON | Enable 20 Hz sine wave generator |
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| Calibration Frequency | 20 Hz | Low frequency for skin penetration |
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| Calibration Amplitude | 4095 | Maximum (12-bit DAC) |
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| Wave Shape | Sine (0) | Clean sinusoidal signal |
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| Subject Ground | OFF | Not grounded during measurement |
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| Current Source | OFF | Voltage mode measurement |
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### Measurement State (Per-Channel)
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For the channel being measured:
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| Setting | Value | Description |
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|---------|-------|-------------|
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| Channel Drive Signal | OFF | Stop driving this electrode |
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| Channel 10K Resistor | ON | Connect to reference resistor |
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All other channels remain in excitation state, creating the voltage divider.
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### Reset State
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After measurement, return channel to excitation state:
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| Setting | Value | Description |
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|---------|-------|-------------|
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| Channel Drive Signal | ON | Resume driving |
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| Channel 10K Resistor | OFF | Disconnect reference |
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## Measurement Algorithm
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```
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1. Configure amplifier to Excitation State (all channels driving)
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2. For each channel to measure:
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a. Set channel to Measurement State (drive OFF, 10K ON)
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b. Wait for settling time (~30ms command + ~1s filter time)
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c. Collect samples for peak-to-peak calculation (~51 samples)
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d. Calculate amplitude = max(samples) - min(samples)
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e. Calculate impedance from amplitude
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f. Reset channel to Excitation State (drive ON, 10K OFF)
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3. Optionally measure Reference (Cz) and COM electrodes
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4. When done, reset amplifier to Default Acquisition State
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```
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## Timing Parameters
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From EGI's Net Station implementation:
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| Parameter | Value | Description |
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|-----------|-------|-------------|
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| Command Time | 30 ms | Time for command to reach amplifier |
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| Settle Time | 0 ms | Additional settling (Net Station uses 0) |
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| Filter Time | 1.0 s | Duration for filter/measurement |
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| Peak-to-Peak Samples | 51 | Samples for amplitude calculation |
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Total time per channel: ~1.03 seconds
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For a 256-channel net, a full scan takes approximately 4-5 minutes.
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## LSL Streaming
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Impedance values are streamed via LSL at 1 Hz:
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- **Stream Type**: `Impedance`
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- **Sample Rate**: 1 Hz (one sample per second)
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- **Data Format**: float32 (kΩ)
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- **Channel Labels**: E1, E2, ..., E256, Cz
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The stream publishes the **current known values** for all channels every second, even during scanning. Channels not yet measured show 1000 kΩ (maximum/invalid value).
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### Stream Metadata
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Each channel includes 3D electrode position in the stream description:
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```xml
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<channel>
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<label>E1</label>
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<type>Impedance</type>
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<unit>kohms</unit>
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<location>
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<X>0.123</X>
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<Y>0.456</Y>
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<Z>0.789</Z>
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<unit>normalized</unit>
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</location>
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</channel>
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```
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Coordinates are on a unit sphere:
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- X: positive toward right ear
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- Y: positive toward nose
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- Z: positive toward vertex (Cz)
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## Implementation
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### Key Source Files
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| File | Description |
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|------|-------------|
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| [ImpedanceMeasurement.h](src/core/include/egiamp/ImpedanceMeasurement.h) | Class declaration and timing structures |
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| [ImpedanceMeasurement.cpp](src/core/src/ImpedanceMeasurement.cpp) | Measurement algorithm implementation |
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| [LSLStreamer.cpp](src/core/src/LSLStreamer.cpp) | LSL outlet creation with electrode positions |
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| [ElectrodePositions.h](src/core/include/egiamp/ElectrodePositions.h) | Electrode coordinate definitions |
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| [ElectrodePositions.cpp](src/core/src/ElectrodePositions.cpp) | Geodesic position generation |
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| [EGIAmpClient.cpp](src/core/src/EGIAmpClient.cpp) | Integration with streaming client |
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### Key Classes
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**`ImpedanceMeasurement`** - Core measurement logic:
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- `setupImpedanceState()` - Configure amplifier for impedance mode
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- `startContinuousScan()` - Begin background scanning thread
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- `measureChannel(int ch)` - Measure single channel (blocking)
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- `feedSample(PacketFormat2&)` - Feed EEG samples for amplitude calculation
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- `publishThread()` - Push values to LSL at 1 Hz
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**`LSLStreamer`** - LSL stream management:
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- `createImpedanceOutlet()` - Create impedance stream with metadata
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- `pushSample()` - Push impedance values
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### AmpServer Commands
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| Command | Description |
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|---------|-------------|
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| `cmd_TurnAll10KOhms` | Set all 10K resistors (0=off, 1=on) |
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| `cmd_TurnAllDriveSignals` | Set all drive signals (0=off, 1=on) |
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| `cmd_TurnChannel10KOhms` | Set single channel 10K resistor |
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| `cmd_TurnChannelDriveSignals` | Set single channel drive signal |
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| `cmd_SetOscillatorGate` | Enable/disable calibration oscillator |
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| `cmd_SetCalibrationSignalFreq` | Set frequency (20 Hz for impedance) |
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| `cmd_SetCalibrationSignalAmplitude` | Set amplitude (0-4095) |
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| `cmd_SetWaveShape` | Set waveform (0=sine, 1=square) |
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| `cmd_DefaultAcquisitionState` | Reset to normal EEG acquisition |
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## Usage
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### CLI
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```bash
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./EGIAmpServerCLI --address 10.10.10.51 --impedance
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```
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### GUI
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1. Link to the amplifier
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2. Check "Measure Impedances" checkbox
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3. Impedance stream appears on LSL network
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4. Uncheck to stop measurement and return to normal EEG
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### Python Visualization
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```bash
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cd scripts
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pip install -r requirements.txt
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python impedance_viewer.py --threshold 50
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```
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## Impedance Thresholds
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Recommended thresholds for EGI saline-based nets:
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| Quality | Impedance (kΩ) | Color |
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|---------|----------------|-------|
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| Excellent | < 20 | Green |
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| Good | 20 - 50 | Yellow-Green |
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| Acceptable | 50 - 100 | Yellow |
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| Poor | 100 - 200 | Orange |
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| Bad | > 200 | Red |
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Values of 1000 kΩ indicate:
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- Electrode not yet measured
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- No contact with scalp
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- Broken or disconnected electrode
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## References
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- EGI AmpServer SDK Documentation
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- EGI Geodesic Sensor Net Technical Manual

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