How to read a datasheet
The manufacturer's document for a chip is long, and you need about four pages of it. Here is which four, and what the numbers on them mean.
On this page10 min
Open the datasheetDatasheet The manufacturer's document for a chip: pinout, limits, typical performance, and the reference circuit you should start from. Glossary entry for the INA128, the 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 you will use in Phase 1. It is about thirty pages. You will read four of them: the first page, the absolute maximum ratings, the electrical characteristics table, and the typical application circuit. Everything else is for later, when you have a specific question.
Page one
The summary. What the chip is, its headline numbers, a block diagram, and a pinout. For 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 the headline numbers are the common-mode rejectionCommon-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 (120 dB), 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 (10 GΩ), the noise (8 nV/√Hz), and the supply range (±2.25 to ±18 V). You do not yet know what all of those mean. You will after Phase 2. For now, note them, and note that every one of them is a “typical” value, which is a marketing word for “on a good day at 25 °C.”
Absolute maximum ratings
The table of things that destroy the chip. Supply voltage, input voltage, temperature. Exceed any of them, even briefly, and you may have a dead part that looks fine. The one that bites biopotential work is input voltage: it must stay within the supply rails. If your 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 pick up a static discharge or your 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 loop saturates, the inputs can leave the rails. The reference circuits include resistors in series with the inputs for this reason.
Electrical characteristics
The long table. Three columns matter: min, typ, and max, at stated conditions. Design to min and max, not typ. When the table says 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 120 dB typical and 100 dB minimum, your design must work at 100 dB, and your characterizationCharacterization Measuring what your instrument actually does, such as its noise floor, bandwidth, and rejection, instead of assuming the datasheet. Related: 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., PhantomPhantom A physical stand-in for tissue, such as saline gelatin with embedded wires, used to test an instrument against known signals., 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.. Glossary entry will probably measure something between the two.
Read the conditions in the header. “At ±15 V, 25 °C, G = 100” is not the same as your circuit at ±9 V and G = 1000. Numbers change with 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 especially; the bandwidth of 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 drops as 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 rises, and the table has a row for each.
Typical application circuit
Near the end. This is the circuit the manufacturer’s engineers tested. Start from it, exactly, before you improve it. Nine tenths of “my amplifier oscillates” and “my amplifier is noisy” come from a bypass capacitor the reference circuit had and yours does not.
The plots
DatasheetsDatasheet The manufacturer's document for a chip: pinout, limits, typical performance, and the reference circuit you should start from. Glossary entry carry a set of typical-performance curves. Three to look at for a biopotential front end:
- Voltage noise density versus frequency. Flat at high frequencies (white), rising below some corner (1/f). 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 lives at 0.5 to 40 Hz, in the rising part. A chip that is quiet at 1 kHz may not be quiet at 1 Hz.
- 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 versus frequency. It falls as frequency rises. The number on page one is at DC. At 60 Hz it is usually 20 dB lower, and 60 Hz is exactly where you need it.
- 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 versus frequency. The bandwidth at your 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. At G = 1000 an INA128 is good to a few kilohertz, which is more than enough for 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 and fine for 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.
Reading the pinout without frying anything
Pin 1 is marked with a dot or a notchNotch 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. Related: FilterFilter An operation that passes some frequencies and attenuates others; every filter also delays or distorts the timing of what it passes. , 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. . Glossary entry, and the numbering goes counterclockwise viewed from above. Check twice. A chip inserted backwards on a split supply is dead in under a second.
Deep dive Vocabulary you will meet on every biopotential datasheet 4 min
Input bias currentInput bias current The tiny current an amplifier input draws; through a high electrode impedance it produces an offset and noise. Related: 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: the tiny current the inputs draw; through a high 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 it makes an offset and noise. Input offset voltage: the DC error the amplifier adds, which you will high-pass away. Slew rate: how fast the output can move; irrelevant for 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. 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 error and nonlinearity: how far from ideal; irrelevant at this stage. PSRR: power supply rejection, how much supply ripple leaks to the output; with batteries this is a non-issue, which is another argument for batteries. Settling time: irrelevant. Quiescent current: what it draws doing nothing; decides how long your battery lasts.