We have an input noise voltage V NOISE with wideband and 1/f spectral content. While there are many op amp topologies, the error sources shown apply to them all.įigure 1. The topology is a single-pole amplifier with an input g m that drives a gain node that is buffered as the output. Op Amp Error Sourcesįigure 1 shows a simplified op amp block diagram with ac and dc error sources added. They support –100 dBc distortion but usually not when heavily loaded. They typically have a 1 mV offset and microvolts of 1/f noise. Then we have the modern general-purpose amplifiers. Current feedback amplifiers also have a thermal drift that extends fine settling times greatly. Their input stage has a mess of error sources, and they do not have much gain nor input nor supply rejections. This application space sees more like –80 dBc to –100 dBc performance, and ppm performance is not practical with these op amps.Ĭurrent feedback amplifiers also cannot support deep linearity nor even modest accuracy, no matter how wideband nor huge their slew rates may be. These are usually bipolar throughout and have large input bias currents and 1/f noise. There are op amps meant to support MHz signals linearly. They also cannot deliver distortion beyond perhaps 10 kHz. However, they are not designed for and do not offer good offsets nor good 1/f noise. They are fairly cheap, and their distortions can be very good. There is the audio amplifier class of op amps. Then there are the cheap amplifiers, new or old, that cannot best –100 dBc when loaded more heavily than 10 kΩ. The next category is older classic op amp designs, such as OP-07, that may have high gain, CMRR, and PSRR, and good offsets and noise, but that cannot achieve better than –100 dBc distortion, especially into a 1 kΩ or heavier load. Expected accuracies are 0.3% to 0.1% at dc, although the ac distortion can be from –55 dBc to –90 dBc (2000 ppm to 30 ppm linearity). These are wideband amplifiers with terrible dc accuracies: offsets in the several millivolts and bias currents in the 1 ♚ to 50 ♚ range, and usually with poor 1/f noise. #How to calculate ppm error relative error driver#The least linearity is found in so-called video or line driver amplifiers. Let’s discuss the types of amplifiers we reject as not highly linear. In this article we will use the rough equivalency of 1 ppm nonlinearity in the transfer function as –120 dBc distortion in harmonic distortion. MOS amplifiers have excellent bias currents but are generally deficient in the low frequency noise and linearity areas. Bipolar amplifiers can provide low wideband noise and good linearity, but their input currents can still cause in-circuit errors (we will hence use the term application for in-circuit). 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. Many op amps have some error terms at ppm levels, but none have all the errors at the ppm level. In this article we will use accuracy as a term that includes all limitations to system measurements, such as noise, offset, gain error, and nonlinearity. Precision is about the depth of the numerical value in terms of digits. We will also discuss a few application improvements to existing op amp limitations.Īccuracy is about numbers: how closely a system works to intended numerical value. #How to calculate ppm error relative error how to#This article presents op amp accuracy limitations and how to choose the few op amps that have a chance of 1 ppm accuracy. There is a march toward 1 ppm accurate systems, especially now that 1 ppm linear ADCs are becoming common. The analog integrated circuit industry has generally kept up with speed requirements, but it is falling behind on accuracy demands. Industrial and medical design continually push to improve product accuracy and speed. Can You Really Get ppm Accuracies from Op Amps?
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