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.
You are skimming: the title, the first figure, and the short version. Switch to Read in the header for the full page, or Deep to open every deep dive.
Open the datasheet for the INA128, the instrumentation amplifier 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 amplifier 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. Glossary entry (120 dB), the input impedanceInput impedanceHow 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 (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 electrodes pick up a static discharge or your DRLDriven-right-leg circuitA 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 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 CMRR is 120 dB typical and 100 dB minimum, your design must work at 100 dB, and your characterizationCharacterizationMeasuring what your instrument actually does, such as its noise floor, bandwidth, and rejection, instead of assuming the datasheet. 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 gain especially; the bandwidth of an instrumentation amplifier drops as gain 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
Datasheets 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). EEG 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.
- CMRR 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.
- Gain versus frequency. The bandwidth at your gain. At G = 1000 an INA128 is good to a few kilohertz, which is more than enough for EEG and fine for EMG.
Reading the pinout without frying anything
Pin 1 is marked with a dot or a notch, 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 current: the tiny current the inputs draw; through a high electrode impedanceElectrode impedanceHow 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 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 EEG. Gain 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.