Operational amplifier (op amp)

01

Check gain-bandwidth and slew rate against the signal

Texas Instruments' application report Understanding Operational Amplifier Specifications (SLOA011B, 2021) defines the gain-bandwidth product as open-loop gain × frequency (GBW = AVD × f) and unity-gain bandwidth as the frequency where open-loop gain is 1. It describes slew rate as the rate of change of output voltage for a step input, notes that the maximum output-swing bandwidth is limited by slew rate, and explains that op amps trade power consumption for noise and speed. [1]

TI's OPAx171 datasheet lists a 3 MHz gain bandwidth and a slew rate of 1.5 V/µs at G = 1, with a supply range of 2.7 V to 36 V, rail-to-rail output and stability with capacitive loads up to 300 pF. [2] Its LM358B datasheet lists 1.2 MHz gain bandwidth and 0.5 V/µs slew rate. [3]

Editorial calculation: in a non-inverting gain of 100, a 3 MHz GBW leaves about 3 MHz / 100 = 30 kHz of bandwidth, and 1.2 MHz leaves about 12 kHz. A 10 V-peak sine at 20 kHz has a maximum slope of 2π × 20,000 × 10 ≈ 1.26 V/µs, within 1.5 V/µs but well above 0.5 V/µs, so the slower part would distort it. Gain, frequency and amplitude are assumed examples. [1] [2] [3]

Give the supplier the circuit function (gain stage, filter, buffer, integrator), gain and bandwidth, largest output swing and frequency, supply rails, input common-mode range, source impedance, load resistance and capacitance, DC accuracy and drift budget, noise needs, temperature range, package and channel count, and any automotive qualification.

02

Budget DC error from offset, drift and bias current

SLOA011B defines input offset voltage as the DC voltage that must be applied between the inputs to bring the output to zero, its temperature coefficient as the expected offset drift over temperature, and notes that normal ageing changes offset over time; offset matters whenever DC precision is required. It describes CMRR and supply rejection (PSRR) as DC parameters that fall off as frequency rises. [1]

The OPAx171 lists input offset of 0.25 mV (typ.) and ±1.8 mV maximum, drift of 0.3 µV/°C (typ.) and ±2 µV/°C maximum from −40 to +125 °C, input bias current of ±8 pA (typ.) and ±15 pA maximum, and 120 dB CMRR (typ.). [2] The LM358B family lists offset maxima of 2 to 3 mV at 25 °C depending on version, drift of ±3.5 µV/°C (typ.), and input bias current in the tens of nanoamperes. [3]

Editorial calculation: at a gain of 100, ±1.8 mV maximum offset appears as up to ±180 mV at the output, and a further ±2 µV/°C over a 100 °C span adds up to ±0.2 mV at the input (±20 mV at the output). Through a 100 kΩ source, 15 pA of bias current adds only 1.5 µV, whereas 35 nA would add 3.5 mV. Gain, temperature span and source resistance are assumed examples. [2] [3]

03

Use a zero-drift op amp when microvolt offsets matter

Renesas' ISL28134 datasheet (FN6957 Rev. 6.03, October 2022) describes a single auto-zeroing rail-to-rail op amp for sensor front ends such as pressure, temperature, strain-gauge, weigh-scale, thermocouple and inertial sensors. Auto-zero circuitry gives a maximum input offset of 2.5 µV and a maximum offset drift of 15 nV/°C, and pushes the 1/f noise corner below 0.1 Hz, with input noise of 10 nV/√Hz at 1 kHz and 250 nV peak-to-peak from 0.1 to 10 Hz. It runs from a single 2.25–6.0 V supply or ±1.125 to ±3.0 V, draws 675 µA (typ.), has a 3.5 MHz gain-bandwidth, CMRR of 135 dB (typ.) with the inputs 0.1 V beyond the supplies, output swing within 10 mV of the rails, and comes in SOIC and SOT-23 for −40 to +85 °C or −40 to +125 °C. [4]

Editorial calculation: at a gain of 100, the 2.5 µV maximum offset becomes 0.25 mV at the output, against up to 180 mV from the ±1.8 mV general-purpose part above, and 15 nV/°C over a 100 °C span adds 1.5 µV at the input (0.15 mV at the output) against 200 µV (20 mV) at ±2 µV/°C. The trade-off is the 6 V supply limit against 36 V for the OPAx171, so a zero-drift part suits low-voltage sensor amplifiers rather than ±15 V industrial signal paths. [4] [2]

04

Single-supply and rail-to-rail parts in the one confirmed listing

The listing reviewed covers single-supply, rail-to-rail and precision operational amplifiers among a broad range of analogue ICs. Ask for datasheet values at your supply voltage and temperature, and for qualification data.

05

Input stage, compensated gain stage and output stage

The compensation capacitor in the gain stage is what fixes GBW and slew rate, the two limits worked through at the top of this page. [1]

06

Where the OPAx171, LM358B and ISL28134 sit on each datasheet line

Rows with values set the three datasheets side by side; output current has no published figure here, so ask for it at your load.

07

Send the circuit, signal and supply data together

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

Circuit: [function; gain; bandwidth]
Signal: [largest swing; frequency]
Supply: [rails; current budget]
DC accuracy: [offset; drift; bias current budget]
Load: [resistance; capacitance]
Environment: [temperature range; qualification]
Package: [type; channels]

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