Project D: The phantom head
Saline gelatin with buried wire dipoles driven at known amplitudes. Now you can measure your amplifier's noise floor, crosstalk, and rejection instead of guessing. The difference between 'I made an EEG' and 'I characterized an EEG.'
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You cannot characterize an amplifier on a head, because you do not know what the head is doing. A phantomPhantom A physical stand-in for tissue, such as saline gelatin with embedded wires, used to test an instrument against known signals. Related: CharacterizationCharacterization Measuring what your instrument actually does, such as its noise floor, bandwidth, and rejection, instead of assuming the datasheet.. Glossary entry is a head that you control: a bowl of salty gelatin with the conductivity of tissue, and inside it two or three tiny current sources you drive from a signal generator at any frequency and amplitude you like. With it you can measure what your system records when the truth is known, which is the only kind of measurement that means anything.
You drive a buried dipole with 10 µA and a 5 mm tip separation. Three centimetres away, on the surface, roughly how large is the potential?
About 10 microvolts, in the range of real EEGElectroencephalography (EEG) Recording the brain's electrical activity from electrodes on the scalp, a few tens of microvolts of blurred, summed cortical activity. Related: Electrocorticography (ECoG)Electrocorticography (ECoG) Recording from electrodes placed on the surface of the brain under the skull, usually during epilepsy surgery evaluation; cleaner and sharper than EEG. , Magnetoencephalography (MEG)Magnetoencephalography (MEG) Recording the tiny magnetic fields produced by cortical currents with superconducting sensors in a shielded room; less blurred by the skull than EEG, and far more expensive. , Volume conductionVolume conduction The spreading of currents from a source through the conductive tissues of the head, which is why one source shows up on many electrodes and why scalp EEG is blurry. , Microvolt (µV)Microvolt (µV) One millionth of a volt. Scalp EEG is tens of microvolts; a AA battery is 1.5 million of them. . Glossary entry. For a current dipoleCurrent dipole The simplest model of a patch of active cortex: a tiny current source and sink close together, with a strength and a direction. Related: Volume conductionVolume conduction The spreading of currents from a source through the conductive tissues of the head, which is why one source shows up on many electrodes and why scalp EEG is blurry. , Forward problemForward problem Given a source in the head, compute the voltages on the scalp; it has a unique answer. . Glossary entry in a medium of conductivity σ, the potential at distance r along the axis is roughly I·d / (4π σ r²); with 10 µA, 5 mm, 0.33 S/m, and 3 cm that is about 13 µV, a bit more near a boundary. You can now inject an EEG-sized signal with a known amplitude, which is the whole point.
Parts
| Part | Where | Qty | Approx. |
|---|---|---|---|
| 3D printing access (campus makerspace) or a Bambu A1 mini Free on campus after the safety certification, which is the right answer for almost everyone. Buying your own is $299 and is worth it by Phase 2 only if you print weekly. | Marriott Library or Lassonde makerspace; Bambu Lab if buying | 1 | free |
| Handheld oscilloscope with a built-in signal generator (FNIRSI DSO-510) $44.99 for 10 MHz, 48 MSa/s, and a DDS generator in one box, which replaces two purchases. 10 MHz is plenty for biopotentials and enough to eyeball SPI, but not enough for careful digital timing; the engineering building has real bench scopes for that. A Hantek 6022BE USB scope is $65.90 and a Rigol DHO800 is the one to want. | Amazon | 1 | $45 |
| Signal generator (cheap DDS module, or use the scope's built-in) Drives the phantom head dipoles at known amplitudes. Skip this if you bought the DSO-510, whose generator does the same job. | Amazon | 1 | $19 |
| Unflavored gelatin or agar, table salt, plastic bowl $14.17 for a pound of gelatin, which is about what a head-sized phantom takes, plus salt and a bowl. Saline concentration around 0.9% by weight matches tissue conductivity to within a factor you can calibrate. | Grocery store, Amazon | 1 | $16 |
| Total (prices drift; treat as a ceiling) | $80 | ||
Building it
The medium. Brain tissue has a conductivity around 0.3 siemens per metre. A 0.2 percent salt solution by weight (2 grams per litre) is close; physiological saline at 0.9 percent is about five times too conductive, which is fine for an amplifier test but wrong if you want the potentials to match a head. Dissolve unflavored gelatin at about 8 percent by weight in the warm salt solution, pour into a bowl a bit smaller than a head, and refrigerate overnight. Agar works too and does not melt on a warm day.
The dipoles. Twisted pairs of insulated wire, with 5 mm of insulation stripped from each conductor at the tip and the two bare tips held 5 mm apart with a dab of epoxy. Make three. Push them into the gelatin before it sets, at different depths and orientations, with the wires leaving through the bottom or side. Note their positions.
The current source. A signal generator output in series with a 100 kΩ resistor. The resistor makes the current nearly independent of the phantomPhantom A physical stand-in for tissue, such as saline gelatin with embedded wires, used to test an instrument against known signals. Related: CharacterizationCharacterization Measuring what your instrument actually does, such as its noise floor, bandwidth, and rejection, instead of assuming the datasheet. . Glossary entry’s impedance: 1 volt peak gives 10 microamps peak, and you can dial the generator down to 10 mV for 100 nA, which produces potentials of a fraction of a microvolt for noise-floor work.
The electrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry. Your own Ag/AgClSilver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. Related: ElectrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry wires from the electrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry project, or gold cups, pushed a millimetre into the surface at marked positions. Eight in a ring around the top plus a reference at the rim and a bias electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry at the far side.
The measurements
Each one gets a row in the characterization report.
Noise floorNoise floor What your instrument records when the input is shorted or connected to a perfectly quiet source; everything smaller than this is invisible to you. Related: Thermal (Johnson) noiseThermal (Johnson) noise The random voltage every resistor produces just by being warm; about 13 nanovolts per root hertz for 10 kilohms, which matters at microvolt levels. , CharacterizationCharacterization Measuring what your instrument actually does, such as its noise floor, bandwidth, and rejection, instead of assuming the datasheet. . Glossary entry. All inputs shorted to the reference at the connectors, no phantomPhantom A physical stand-in for tissue, such as saline gelatin with embedded wires, used to test an instrument against known signals. Related: CharacterizationCharacterization Measuring what your instrument actually does, such as its noise floor, bandwidth, and rejection, instead of assuming the datasheet. . Glossary entry. Sixty seconds. RMS in 0.5 to 40 Hz, input-referred. Then the same with the electrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry in the phantomPhantom A physical stand-in for tissue, such as saline gelatin with embedded wires, used to test an instrument against known signals. Related: CharacterizationCharacterization Measuring what your instrument actually does, such as its noise floor, bandwidth, and rejection, instead of assuming the datasheet. . Glossary entry and the dipoles unpowered: this adds the electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry noise, and the difference tells you what the electrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry cost.
GainGain How many times an amplifier multiplies its input; 1000× turns 50 microvolts into 50 millivolts, enough for an ADC to see. Related: Instrumentation amplifierInstrumentation amplifier A precision differential amplifier with very high input impedance and high CMRR, built from three op-amps and sold as a single chip such as the INA128 or AD8232. . Glossary entry and bandwidth. Drive one dipole with a sine at 10 µA. Step the frequency from 0.5 Hz to 100 Hz. Plot recorded amplitude against frequency on the channel nearest the dipole. The flat part is your passband; the frequency where it falls to 70 percent is your bandwidth. Compare to the filterFilter An operation that passes some frequencies and attenuates others; every filter also delays or distorts the timing of what it passes. Related: High-pass filterHigh-pass filter Removes slow drifts and DC offsets, passing everything above a cutoff; set it too high and you erase slow brain activity. , Low-pass filterLow-pass filter Removes fast content above a cutoff; the anti-alias filter is one, and so is the smoothing you apply to a noisy trace. , Notch filterNotch filter A very narrow filter that removes one frequency, usually 60 Hz mains hum; it also rings and distorts nearby content, so fix the hum at the source first. , Phase distortionPhase distortion Different frequencies being delayed by different amounts, which smears and shifts waveforms such as an ERP peak. . Glossary entry settings you thought you had.
Common-mode rejection. Drive the bias electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry against the reference electrodeReference electrode The electrode every other channel is measured against; there is no neutral spot on the head, so the choice shapes every trace. Related: MontageMontage The particular arrangement of which electrodes are subtracted from which, such as referential, bipolar, or average reference. , Common average referenceCommon average reference Re-referencing every channel to the mean of all channels, which approximates a neutral reference when electrodes cover the head well. . Glossary entry with a 60 Hz sine of 100 mV through a 1 kΩ resistor, so the whole phantomPhantom A physical stand-in for tissue, such as saline gelatin with embedded wires, used to test an instrument against known signals. Related: CharacterizationCharacterization Measuring what your instrument actually does, such as its noise floor, bandwidth, and rejection, instead of assuming the datasheet. . Glossary entry’s potential swings, equally at every electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry. Record. The 60 Hz amplitude you see, divided by 100 mV, is your common-mode gainGain How many times an amplifier multiplies its input; 1000× turns 50 microvolts into 50 millivolts, enough for an ADC to see. Related: Instrumentation amplifierInstrumentation amplifier A precision differential amplifier with very high input impedance and high CMRR, built from three op-amps and sold as a single chip such as the INA128 or AD8232. . Glossary entry; its ratio to your differential gainGain How many times an amplifier multiplies its input; 1000× turns 50 microvolts into 50 millivolts, enough for an ADC to see. Related: Instrumentation amplifierInstrumentation amplifier A precision differential amplifier with very high input impedance and high CMRR, built from three op-amps and sold as a single chip such as the INA128 or AD8232. . Glossary entry, in dB, is the CMRRCommon-mode rejection ratio (CMRR) How much better an amplifier responds to a difference between its inputs than to a signal shared by both, in decibels; 100 dB means 100,000 times. Related: Common-mode signalCommon-mode signal A voltage that appears equally on both inputs of a differential amplifier, such as 60 Hz hum picked up by the whole body. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. . Glossary entry of the system, electrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry included. Repeat with the bias drive on and off.
Crosstalk. Drive one dipole at 20 Hz. Measure the 20 Hz amplitude on every channel. Channels far from the dipole should see only what volume conductionVolume conduction The spreading of currents from a source through the conductive tissues of the head, which is why one source shows up on many electrodes and why scalp EEG is blurry. Related: Current dipoleCurrent dipole The simplest model of a patch of active cortex: a tiny current source and sink close together, with a strength and a direction. , Forward problemForward problem Given a source in the head, compute the voltages on the scalp; it has a unique answer. , Surface LaplacianSurface Laplacian Subtracting the average of neighbouring electrodes from each electrode, which sharpens spatial detail and suppresses distant sources. . Glossary entry predicts; anything more is crosstalk in the amplifier or the cable.
Linearity. Drive at 1, 3, 10, 30, and 100 µA. The recorded amplitude should scale exactly. Where it stops, you have found the input range.
A spatial map. Drive one dipole and record on all eight electrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry. Plot amplitude against electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry position. This is a forward model measured rather than computed, and in Phase 4 you will compute it and compare.
- Mix and pour the phantomPhantom A physical stand-in for tissue, such as saline gelatin with embedded wires, used to test an instrument against known signals. Related: CharacterizationCharacterization Measuring what your instrument actually does, such as its noise floor, bandwidth, and rejection, instead of assuming the datasheet. . Glossary entry with three dipoles at recorded positions. Refrigerate overnight.
- Build the current source. Verify with the multimeter that 1 V into 100 kΩ into a 10 kΩ dummy load gives about 10 µA.
- Measure the shorted-input noise floorNoise floor What your instrument records when the input is shorted or connected to a perfectly quiet source; everything smaller than this is invisible to you. Related: Thermal (Johnson) noiseThermal (Johnson) noise The random voltage every resistor produces just by being warm; about 13 nanovolts per root hertz for 10 kilohms, which matters at microvolt levels. , CharacterizationCharacterization Measuring what your instrument actually does, such as its noise floor, bandwidth, and rejection, instead of assuming the datasheet. . Glossary entry and the in-phantomPhantom A physical stand-in for tissue, such as saline gelatin with embedded wires, used to test an instrument against known signals. Related: CharacterizationCharacterization Measuring what your instrument actually does, such as its noise floor, bandwidth, and rejection, instead of assuming the datasheet. . Glossary entry noise floorNoise floor What your instrument records when the input is shorted or connected to a perfectly quiet source; everything smaller than this is invisible to you. Related: Thermal (Johnson) noiseThermal (Johnson) noise The random voltage every resistor produces just by being warm; about 13 nanovolts per root hertz for 10 kilohms, which matters at microvolt levels. , CharacterizationCharacterization Measuring what your instrument actually does, such as its noise floor, bandwidth, and rejection, instead of assuming the datasheet. . Glossary entry. Record both.
- Sweep frequency for gainGain How many times an amplifier multiplies its input; 1000× turns 50 microvolts into 50 millivolts, enough for an ADC to see. Related: Instrumentation amplifierInstrumentation amplifier A precision differential amplifier with very high input impedance and high CMRR, built from three op-amps and sold as a single chip such as the INA128 or AD8232. . Glossary entry and bandwidth. Plot.
- Measure CMRRCommon-mode rejection ratio (CMRR) How much better an amplifier responds to a difference between its inputs than to a signal shared by both, in decibels; 100 dB means 100,000 times. Related: Common-mode signalCommon-mode signal A voltage that appears equally on both inputs of a differential amplifier, such as 60 Hz hum picked up by the whole body. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. . Glossary entry with and without bias drive. Compute the dB values.
- Measure crosstalk and linearity.
- Record the eight-channel spatial map for one dipole and save the CSV; you will need it in Phase 4.