Project B: One channel of you

This is the project that makes the field feel real. Three 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, one on the back of your head, one on an earlobe, one on your forehead. An amplifier with a 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 of about two thousand and a band-pass from half a hertz to forty. An Arduino to sample it and the Web Serial plotter to show it. Close your eyes and watch a peak grow at about 10 Hz in the spectrumSpectrum A plot of how much of a signal lives at each frequency. Related: Fast Fourier transform (FFT)Fast Fourier transform (FFT) The algorithm that breaks a stretch of signal into the sinusoids that would add up to it, giving amplitude and phase at each frequency. , Power spectral density (PSD)Power spectral density (PSD) The spectrum expressed as power per hertz, the standard way to quantify how strong alpha is. . Glossary entry. Open them and watch it disappear within a second. That peak is your alpha rhythmAlpha rhythm A roughly 8 to 12 Hz oscillation over the back of the head that appears when the eyes close and fades when they open. Related: Beta rhythmBeta rhythm Roughly 13 to 30 Hz activity, prominent over motor areas and associated with active concentration or holding still., Theta rhythmTheta rhythm Roughly 4 to 8 Hz activity, prominent in drowsiness, some memory tasks, and strongly in the rodent hippocampus., Thalamocortical loopThalamocortical loop The two-way circuit between the thalamus and the cortex whose rhythmic activity produces alpha, sleep spindles, and other EEG rhythms., Sensorimotor rhythm (mu)Sensorimotor rhythm (mu) An 8 to 13 Hz rhythm over motor cortex that weakens when you move, or imagine moving, the corresponding body part.. Glossary entry, and once you have seen it you will not stop seeing it.

Parts

The circuit

Compared to the SpikerBoxSpikerBox Backyard Brains' small battery amplifier that lets you hear action potentials from a cockroach leg on a speaker; easy to build yourself. Related: Action potentialAction potential The brief, all-or-nothing voltage spike a neuron fires, about a millisecond long and about 100 millivolts tall across its membrane. , 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, two things change: 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 moves down to 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 frequencies, and a third 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 with a feedback circuit is added to cancel hum.

Input protection. Each 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 lead goes through a 100 kΩ resistor before the amplifier input. With the INA128’s 10 GΩ input impedanceInput impedance How hard the amplifier's inputs are to push current into; it must be far higher than the electrode impedance or the signal is lost in the divider. Related: 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. , Input bias currentInput bias current The tiny current an amplifier input draws; through a high electrode impedance it produces an offset and noise. . Glossary entry this costs nothing, and it limits any fault current to microamps.

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. Related: Differential measurementDifferential measurement Measuring the difference between two electrodes rather than one electrode against ground, so anything common to both cancels. , Common-mode rejection ratio (CMRR)Common-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. , Input impedanceInput impedance How hard the amplifier's inputs are to push current into; it must be far higher than the electrode impedance or the signal is lost in the divider. . Glossary entry. INA128, 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 resistor 500 Ω, 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 about 100. The 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 resistor is split into two 250 Ω resistors in series, because the junction between them carries the average of the two inputs, which is the common-modeCommon-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. Related: Common-mode rejection ratio (CMRR)Common-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., Differential measurementDifferential measurement Measuring the difference between two electrodes rather than one electrode against ground, so anything common to both cancels., Driven-right-leg circuitDriven-right-leg circuit A third electrode fed with an inverted copy of the common-mode signal, so the body's hum is actively cancelled instead of merely rejected.. Glossary entry voltage. You need it for the next part.

Driven-right-leg circuitDriven-right-leg circuit A third electrode fed with an inverted copy of the common-mode signal, so the body's hum is actively cancelled instead of merely rejected. 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. , Ground electrodeGround electrode The electrode that ties the body to the amplifier's zero and, in a good design, is driven to cancel common-mode hum. . Glossary entry. A TL072 stage takes the common-mode voltage from that junction, inverts and amplifies it (1 MΩ feedback, 25 kΩ input, so about −40), and drives the third 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 through a 390 kΩ resistor. Now the body’s hum is actively cancelled instead of only rejected. The 390 kΩ limits the current the circuit can push into you to about 25 microamps even if everything fails, which is what IEC 60601IEC 60601 The safety standard for medical electrical equipment, including how much current may leak into a patient (microamps). Related: Leakage currentLeakage current Current that flows into the patient through unintended paths; limited to tens of microamps for anything touching the body., Electrical isolationElectrical isolation A barrier, such as a battery, an optocoupler, or an isolated USB link, with no conductive path from the mains to the patient.. Glossary entry requires.

High-pass at 0.5 Hz. A 1 µF capacitor into 330 kΩ. This removes 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 offset, which can be tens of millivolts and would saturate the next stage. It also removes sweat drift.

Second 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 stage. TL072 non-inverting, 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 20 (1 kΩ to ground, 19 kΩ feedback; 18 kΩ plus 1 kΩ works). Total 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 about 2000, so 50 µV of alpha becomes 100 mV.

Low-pass at about 35 Hz. 10 kΩ into 0.47 µF. This is the anti-alias filterAnti-alias filter A low-pass filter placed before the ADC to remove everything above Nyquist, so nothing can alias. Related: AliasingAliasing A frequency above Nyquist masquerading as a lower one after sampling; nothing downstream can undo it. , 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. . Glossary entry for a 250 Hz sample rate, and it knocks down a good part of the 60 Hz.

Level shift for the ADCAnalog-to-digital converter (ADC) The chip that measures a voltage at regular instants and turns each into a number. Related: Sampling rateSampling rate How many measurements per second the ADC takes; EEG is usually 250 to 1000 per second. , QuantizationQuantization Rounding each sample to one of a finite set of levels; a 10-bit ADC has 1024 of them, a 24-bit one about 16 million. , Delta-sigma ADCDelta-sigma ADC A converter that samples very fast with a crude 1-bit decision and averages, achieving 24-bit resolution at modest rates; what the ADS1299 uses. . Glossary entry. The output swings around ground. The Arduino reads 0 to 5 V. A 10 µF capacitor into two 1 MΩ resistors, one to 5 V and one to ground, centres the signal at 2.5 V without adding a meaningful high-pass (0.03 Hz).

What the numbers should look like

Input-referred noise with a shorted input: a few microvolts RMS if soldered, ten or twenty on a breadboard. Alpha, eyes closed, at O1: 20 to 60 µV peak in most people. A blink at O1: 50 to 150 µV. The same blink on the forehead: 300 µV or more. Sixty-hertz hum after the DRLDriven-right-leg circuit A third electrode fed with an inverted copy of the common-mode signal, so the body's hum is actively cancelled instead of merely rejected. 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. , Ground electrodeGround electrode The electrode that ties the body to the amplifier's zero and, in a good design, is driven to cancel common-mode hum. . Glossary entry and low-pass, with the laptop unplugged: under 5 µV, ideally under 1.

Going further

Replace the breadboard with a soldered board and watch the 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 drop by a factor of five. Compare your amplifier to the AD8232 breakout, which has a DRLDriven-right-leg circuit A third electrode fed with an inverted copy of the common-mode signal, so the body's hum is actively cancelled instead of merely rejected. 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. , Ground electrodeGround electrode The electrode that ties the body to the amplifier's zero and, in a good design, is driven to cancel common-mode hum. . Glossary entry and filtersFilter 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 built in, and see what a professional designer did differently. Move 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 to C3 (above the left ear, toward the top) and open and close your right hand: the mu rhythm is alpha’s cousin and it drops when you move. Add a second channel and a second Arduino pin.

What to read now

Why so small? is about the 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, the difference measurement, and the third 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 you just used. Why the hum? is the debugging list above, with the reasons. Alpha and the back of the head is about what you saw. And the artifact bestiary will name every strange thing on your trace.