Industrial and medical design continually push to improve product accuracy and speed. The analog integrated circuit industry has generally kept up with speed requirements, but it is falling behind on accuracy demands. There is a march toward 1 ppm accurate systems, especially now that 1 ppm linear ADCs are becoming common.
Accuracy is about numbers: how closely a system works to intended numerical value. Precision is about the depth of the numerical value in terms of digits. Here we use accuracy as a term that includes all limitations to system measurements, such as noise, offset, gain error, and nonlinearity.
Many op amps have some error terms at ppm levels, but none have all the errors at the ppm level. For instance, chopper amplifiers can provide ppm-level offset voltages, DC linearity, and low frequency noise, but they have problematic input bias currents and linearity at frequency. Bipolar amplifiers can provide low wideband noise and good linearity, but their input currents can still cause in-circuit errors. MOS amplifiers have excellent bias currents but are generally deficient in the low frequency noise and linearity areas.
This article from Analog Devices presents op amp accuracy limitations, how to choose the few op amps that have a chance of 1 ppm accuracy, as well as a few application improvements for existing op amp limitations.
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