Instrumentation amplifier (in-amp IC)

01

Set gain and accuracy

TI's INA828 datasheet describes a precision instrumentation amplifier (second generation of the INA128) whose gain is set by one external resistor, G = 1 + 50 kΩ ÷ RG. It lists 50 µV maximum offset voltage, gain drift of 5 ppm/°C at G = 1 and 50 ppm/°C for G > 1 (excluding the external resistor), common-mode rejection of 110 dB minimum at G = 10, power-supply rejection of 100 dB minimum at G = 1 and voltage noise of 7 nV/√Hz. [1]

The gain equation differs between parts even though each uses one resistor. TI's INA333 uses G = 1 + 100 kΩ ÷ RG for gains from 1 to 1,000. [2] Analog Devices' AD620 uses G = 1 + 49.4 kΩ ÷ RG for gains from 1 to 10,000. [3] A resistor chosen for one part does not give the same gain on another.

Offset and drift also depend on the design. The zero-drift INA333 lists 25 µV maximum offset and 0.1 µV/°C drift at G ≥ 100, with 100 dB minimum CMRR at G ≥ 10 and 50 nV/√Hz noise. [2] The AD620 (B grade) lists 50 µV maximum offset, 0.6 µV/°C maximum offset drift, 100 dB minimum CMRR at G = 10, 9 nV/√Hz noise at 1 kHz and 40 ppm maximum nonlinearity. [3]

Editorial calculation: for a gain of 100, RG = 50 kΩ ÷ (100 − 1) ≈ 505 Ω (nearest standard value 499 Ω gives G ≈ 101.2). A 10 mV bridge signal then becomes about 1 V, and the 50 µV offset is 0.5% of that input signal before calibration. [1]

Editorial note: the resistor's tolerance and drift add to the amplifier's gain error and drift, so specify RG with matching accuracy. [1]

Give the supplier the sensor and signal range, required gain, offset and drift limits, CMRR at your frequency, noise and bandwidth, input common-mode range and protection, supply rails and current, temperature range, package and quantity.

02

Check bandwidth, input protection and supply

The INA828 lists bandwidth of 2 MHz at G = 1 and 260 kHz at G = 100, stability with 1 nF capacitive loads, input protection up to ±40 V, supply current of 650 µA maximum, single-supply operation from 4.5 to 36 V or dual supplies of ±2.25 to ±18 V, a specified temperature range of −40 to 125 °C and an 8-pin SOIC package. [1]

Supply range and current separate low-power parts from wide-supply parts. The INA333 runs from 1.8 to 5.5 V with 50 µA quiescent current and a rail-to-rail output, specified from −40 to +125 °C. [2] The AD620 runs from ±2.3 to ±18 V with 1.3 mA maximum supply current and lists 120 kHz bandwidth at G = 100 and 15 µs settling to 0.01%. [3] Compare these with the INA828's 4.5–36 V range and 650 µA maximum below.

Editorial calculation: bandwidth falls with gain — 260 kHz at G = 100 corresponds to a gain-bandwidth of about 26 MHz at that gain, well above a 1 kHz bridge signal. [1]

Editorial note: check that both inputs stay inside the amplifier's input common-mode range at the chosen gain and reference voltage, not only within the supply rails. [1]

03

Consider pin-selected gain instead of a gain resistor

Renesas' datasheet FN8364 (Rev.2.02, Jun 4, 2020) covers the ISL28533, ISL28534, ISL28535, ISL28633, ISL28634 and ISL28635, 5V zero-drift rail-to-rail input/output programmable gain instrumentation amplifiers. A 2-bit, 3-state logic interface on pins G0 and G1 selects one of 9 gain settings from G = 1 to 1,000. Its feature list gives 5µV maximum input offset, 50nV/°C maximum offset drift, 138dB CMRR at G = 100, gain error below 0.4% at all gains, 2.3MHz gain bandwidth, 0.4µVP-P noise from 0.1Hz to 10Hz, a 2.5V to 5.5V single supply and −40°C to +125°C operation. [4]

Renesas' electrical table gives CMRR rising with gain: 100 dB at G = 1, 114 dB at G = 10, 138 dB at G = 100 and 150 dB at G = 1000 (typ.; minimum 80, 100, 110 and 120 dB), gain drift of 10 ppm/°C, and a supply current of 2.9 mA (typ.), 3.4 mA maximum, for the single-ended parts with no load. [4] Against these, TI's INA828 sets gain with one external resistor, G = 1 + 50 kΩ ÷ RG, with gain drift of 50 ppm/°C for G > 1 excluding the resistor, and runs from 4.5 to 36 V at 650 µA maximum [1].

The choice depends on the measurement. A pin-selected part avoids an external gain resistor and allows gain to be changed by logic, but it is a 5 V part; the INA828 accepts up to 36 V and ±40 V input protection [1], and TI's INA333 targets micro-power battery systems [2]. Give the supplier your sensor's full-scale signal, common-mode voltage, supply rail and whether gain must change in operation.

Editorial calculation: 10 ppm/°C over a 50 °C temperature change is 500 ppm, or 0.05% of gain; 50 ppm/°C over the same change is 2,500 ppm, or 0.25%. [4] [1]

Values are from the Renesas FN8364 datasheet (text extracted from the PDF); Renesas' product pages show the ISL28533 family as active.

04

Wide-supply, micro-power and low-cost families side by side

The three datasheets reviewed show different families: a precision wide-supply part (INA828), a micro-power zero-drift part for battery-operated systems (INA333), and a low-cost low-power part available in DIP and SOIC (AD620). The applications listed include bridge and weigh-scale amplifiers, ECG and medical instrumentation, pressure, thermocouple and RTD sensor amplifiers, and data acquisition. [1] [2] [3]

Ask each supplier for the datasheet with gain equation, offset, drift, CMRR, noise, bandwidth and input range at your supply and gain. [1]

05

Input stage, RG, difference stage, reference pin and supply

All five rows are read from the INA828 datasheet; on a pin-selected part such as the ISL2853x, logic pins G0 and G1 take the place of the gain resistor. [1] [4]

06

Gain, offset, CMRR and supply values to quote against

Apart from the gain and CMRR rows, which add the Renesas pin-selected family, every value in this table comes from the INA828 datasheet.

07

Send the signal, accuracy and supply data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Signal: [source; mV range; frequency]
Gain: [G]
Accuracy: [offset; drift; CMRR]
Noise/bandwidth: [nV/√Hz; kHz]
Inputs: [common-mode range; protection]
Supply: [V; µA]
Temp/package: [°C; package]
Quantity: [units]

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