Why so small?
Microvolts, gain, and why the amplifier measured a difference between two electrodes instead of one electrode against ground. And the reason for the third electrode.
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The 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 you saw was about fifty microvoltsMicrovolt (µV) One millionth of a volt. Scalp EEG is tens of microvolts; a AA battery is 1.5 million of them. Related: GainGain How many times an amplifier multiplies its input; 1000× turns 50 microvolts into 50 millivolts, enough for an ADC to see.. Glossary entry. The hum your body picks up from the room is about ten to a hundred millivolts, a thousand times bigger. And your amplifier still showed you alpha. It did that with three ideas: 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, which made everything bigger; a difference measurementDifferential measurement Measuring the difference between two electrodes rather than one electrode against ground, so anything common to both cancels. 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., 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., 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, which made everything shared between two 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 disappear; 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 that actively cancelled what was left. Let’s see each one work.
How small, exactly
| Signal | Typical size on the skin |
|---|---|
| Scalp 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 (alpha, eyes closed) | 20 to 60 µV |
| Scalp 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 (an ERPEvent-related potential (ERP) The small, stereotyped voltage waveform the brain produces in response to an event, invisible in a single trial and revealed by averaging many. Related: P300P300 A positive bump about 300 milliseconds after a rare or meaningful stimulus, the basis of the classic BCI speller. , N170N170 A negative deflection about 170 milliseconds after seeing a face, over the back of the head on the right. , Error-related negativity (ERN)Error-related negativity (ERN) A sharp negative wave about 50 to 100 milliseconds after you make a mistake, before you consciously notice. , Trial averagingTrial averaging Adding many time-locked epochs together so the consistent response grows and the random noise shrinks by the square root of the number of trials. . Glossary entry, before averaging) | 1 to 10 µV |
| EOGElectrooculography (EOG) Recording eye position from the standing voltage across the eyeball, which moves with the eyes; also the main source of blink artifacts in EEG. Related: Blink artifactBlink artifact The large, slow deflection an eye blink produces in frontal channels, often ten times the size of the brain signal. , Electroencephalography (EEG)Electroencephalography (EEG) Recording the brain's electrical activity from electrodes on the scalp, a few tens of microvolts of blurred, summed cortical activity. . Glossary entry (eye movement) | about 15 µV per degree; hundreds of µV for a big look |
| ECGElectrocardiography (ECG) Recording the heart's electrical activity, about a millivolt on the chest; it leaks into EEG as a small regular spike. Related: ArtifactArtifact Anything in the recording that did not come from the brain: blinks, muscle, heartbeat, movement, sweat, mains hum, electrode pops. . Glossary entry on the chest | about 1 mV |
| Surface EMGElectromyography (EMG) Recording the electrical activity of muscles, hundreds of microvolts to millivolts, much larger than EEG. Related: Myoelectric prosthesisMyoelectric prosthesis An artificial limb controlled by EMG from the remaining muscles. , Electroencephalography (EEG)Electroencephalography (EEG) Recording the brain's electrical activity from electrodes on the scalp, a few tens of microvolts of blurred, summed cortical activity. . Glossary entry, clenched | 0.1 to 5 mV |
| Extracellular spikeSpike Shorthand for an action potential as seen by an electrode, a brief blip about a millisecond wide. 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. , Spike sortingSpike sorting Deciding which spikes on an electrode came from which neuron, by clustering their waveform shapes. , Extracellular recordingExtracellular recording Recording with an electrode near neurons rather than inside one; you see small, fast blips from nearby cells rather than the full membrane swing. . Glossary entry, needle near a nerve | 50 to 500 µV |
| 60 Hz hum on a body in a normal room | 10 to 100 mV, sometimes volts |
| A AA battery | 1,500,000 µV |
Two things to notice. 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 is the smallest thing on the list. And the hum is bigger than everything you want to measure, sometimes by ten thousand times.
Gain
An analog-to-digital converterAnalog-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 like the Arduino’s measures 0 to 5 volts in 1024 steps, so each step is about 5 millivolts. Fifty microvolts is a hundredth of a step. Invisible. Multiply by 2000 and alpha becomes 100 millivolts, twenty steps. That is what 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 is for, and that is all 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 is for: matching the signal to the converter. It does nothing about the hum, which it also multiplies by 2000.
The difference
Here is the trick. The hum is picked up by your whole body, so it is very nearly the same at every point on your head. Alpha is generated under one spot and is different at different points. Measure the difference between two 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 and the hum, being the same on both, cancels, while alpha, being different, survives. A signal that appears equally on both inputs is called 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. A signal that appears as a difference is called differential. An 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 is a chip built to amplify the second and ignore the first, and how well it ignores the first is its common-mode rejection ratioCommon-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.
- Slide 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 from 80 dB down to 40. Watch the hum reappear in the bottom trace.
- Put 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 back to 120 dB. Now slide the 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. Related: Dry electrodeDry electrode An electrode used without gel, faster to apply but with higher, less stable impedance and more motion noise; the consumer-headset compromise. , Conductive gel or pasteConductive gel or paste Salty gel between electrode and skin that lowers impedance, stabilizes the chemistry, and is the reason research EEG takes half an hour to set up. , Electrochemical impedance spectroscopy (EIS)Electrochemical impedance spectroscopy (EIS) Measuring an electrode's impedance across many frequencies to characterize its interface, standard in electrode development. . Glossary entry mismatch to 100 percent. The hum comes back even though the amplifier is perfect. Read the readout.
- Turn on the 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 and watch what happens to the hum before it reaches the amplifier.
The readout in Figure 1 is worth staring at. A 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 100 dB means the amplifier treats a 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. 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 as a hundred thousand times smaller than a differential one. Ten millivolts of hum becomes a tenth of a microvolt. That would be the end of the story, except for the second experiment you just did.
Why perfect amplifiers still hum
The two 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 do not have the same 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. Related: Dry electrodeDry electrode An electrode used without gel, faster to apply but with higher, less stable impedance and more motion noise; the consumer-headset compromise., Conductive gel or pasteConductive gel or paste Salty gel between electrode and skin that lowers impedance, stabilizes the chemistry, and is the reason research EEG takes half an hour to set up., Electrochemical impedance spectroscopy (EIS)Electrochemical impedance spectroscopy (EIS) Measuring an electrode's impedance across many frequencies to characterize its interface, standard in electrode development.. Glossary entry. One has more paste, or sits on drier skin, or has a half-detached edge. Now the common-mode hum, pushing current through two unequal impedances into the amplifier’s (very large but finite) 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, produces two slightly different voltages at the two inputs. A difference. The amplifier, correctly, amplifies it. The hum has been converted from 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 to differential before the chip ever saw it, and no 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 can remove it.
This is the physics behind the debugging advice “re-paste 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.” It is also why the 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 of a biopotential amplifier is enormous (10 gigohms for the INA128): the bigger it is compared to the 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. Related: Dry electrodeDry electrode An electrode used without gel, faster to apply but with higher, less stable impedance and more motion noise; the consumer-headset compromise. , Conductive gel or pasteConductive gel or paste Salty gel between electrode and skin that lowers impedance, stabilizes the chemistry, and is the reason research EEG takes half an hour to set up. , Electrochemical impedance spectroscopy (EIS)Electrochemical impedance spectroscopy (EIS) Measuring an electrode's impedance across many frequencies to characterize its interface, standard in electrode development. . Glossary entry, the smaller the conversion.
The third electrode
You could try to reject the hum. Or you could make it smaller before it arrives. The driven-right-legDriven-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 circuit measures the common-mode voltage (that is what the split 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 gave you), inverts it, amplifies it, and drives it back into the body through 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. The body’s hum is now being actively pushed toward zero by a feedback loop. Typical 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: another 30 to 40 dB of rejection, which is the difference between a trace that is mostly hum and one where you have to look for it.The name is from electrocardiography, where 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 was traditionally on the right leg. In 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 it goes on the forehead or an earlobe; the physics does not care.
Without 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 you have no reference for the body at all, and the common-mode voltage can float wherever the room’s fields push it, including outside the amplifier’s supply rails, at which point it stops working entirely. So 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 does two jobs: it gives the body a reference, and it cancels hum. Every clinical 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, ECGElectrocardiography (ECG) Recording the heart's electrical activity, about a millivolt on the chest; it leaks into EEG as a small regular spike. Related: ArtifactArtifact Anything in the recording that did not come from the brain: blinks, muscle, heartbeat, movement, sweat, mains hum, electrode pops. . Glossary entry, and EMGElectromyography (EMG) Recording the electrical activity of muscles, hundreds of microvolts to millivolts, much larger than EEG. Related: Myoelectric prosthesisMyoelectric prosthesis An artificial limb controlled by EMG from the remaining muscles. , Electroencephalography (EEG)Electroencephalography (EEG) Recording the brain's electrical activity from electrodes on the scalp, a few tens of microvolts of blurred, summed cortical activity. . Glossary entry system has one.
Deep dive Decibels, since everyone uses them 3 min
A decibel is a ratio on a logarithmic scale. For voltages, 20 dB is a factor of 10, 40 dB is 100, 60 dB is 1000, 100 dB is 100,000, 120 dB is a million. Amplifier 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 is quoted in dB. 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 attenuation is quoted in dB. Signal-to-noise ratio is quoted in dB. The advantage is that gainsGain 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 attenuations along a chain simply add: 40 dB from the amplifier’s 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 plus 35 dB from 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 is 75 dB of hum rejection, a factor of about 5,600.
Deep dive What sets the amplifier's own CMRR 4 min
Inside an 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 are three op-amps and a set of precisely matched resistors. The common-mode rejection comes from the matching: if the resistors that set the difference are equal to one part in ten thousand, 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 is about 80 dB. Laser-trimmed chips like the INA128 reach 120 dB at DC. But the matching gets worse with frequency because of tiny stray capacitances, so 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 at 60 Hz is typically 20 dB lower than the DC number on the datasheetDatasheet The manufacturer's document for a chip: pinout, limits, typical performance, and the reference circuit you should start from. Glossary entry’s first page. That is exactly where you need it most, which is one reason 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 is not optional.
Deep dive Why the reference electrode is not the ground electrode 3 min
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, three jobs. The active electrodeActive electrode An electrode with a small amplifier or buffer built into it, so the high-impedance signal travels only millimetres before being made robust. Related: Dry electrodeDry electrode An electrode used without gel, faster to apply but with higher, less stable impedance and more motion noise; the consumer-headset compromise. , 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 sits over the thing you want (O1). 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 is the other input of the difference (the earlobe); everything you see is active minus reference, so a “quiet” reference matters and there is no truly quiet spot on the head. The groundGround electrode The electrode that ties the body to the amplifier's zero and, in a good design, is driven to cancel common-mode hum. Related: 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., 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.. Glossary entry or 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 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 gives the body a voltage reference and cancels hum; it is not part of the difference at all. Beginners connect the reference to ground and wonder why the hum is terrible. In Phase 2 you will meet montagesMontage The particular arrangement of which electrodes are subtracted from which, such as referential, bipolar, or average reference. Related: 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., 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., Surface LaplacianSurface Laplacian Subtracting the average of neighbouring electrodes from each electrode, which sharpens spatial detail and suppresses distant sources.. Glossary entry, which are just different choices of what to subtract from what.
Explain what this page was about to your roommate in three sentences. No jargon they would not know.
We amplify scalp EEG of 20 to 60 microvolts by a gain near 2000 so it spans tens of steps on a converter that moves about 5 millivolts per step. We measure the difference between two head electrodes so shared hum of 10 to 100 millivolts cancels while local brain activity survives, helped by rejection near 100 decibels. We add a third electrode that feeds the inverted shared voltage back into the body for another 30 to 40 decibels of quiet, plus a stable reference that keeps the amplifier inside its range.
Yours does not have to match. Check that the three ideas are there.