ADC INL Errors—Greatest Match Line, Whole Unadjusted Error, Absolute and Relative Accuracy

Integral nonlinearity (INL) an essential specification that permits us to characterize the static linearity efficiency of an A/D (analog-to-digital) converter. The INL error quantifies the deviation of the transition factors of the particular switch perform from the best values, which are obtained from a reference straight line. Nevertheless, totally different INL definitions use totally different reference strains.

Beforehand, we checked out a few of these definitions, just like the endpoint-based definitions. As a refresher, the reference line for the commonest INL definition is the road that goes via the primary and final code transitions (the road going via factors A and B in Determine 1).

Determine 1. Instance reference line INL definition.

The above INL definition is categorized as an endpoint technique as a result of it solely makes use of the primary and final codes for deriving the reference line. On this article, we’ll cowl one other technique for outlining the INL error, one of the best match line technique. On this case, a straight line match via all codes is used because the reference line.

Transition Greatest Match INL—Endpoint Technique vs. Greatest Match Technique

No matter utilizing the endpoint or greatest match technique, the static switch attribute of an ADC could be outlined when it comes to the code facilities or the transition factors. The reference line for the transition-based greatest match INL definition is the straight line that greatest represents all transition factors of the attribute curve. Let’s think about the non-ideal response in Determine 1, which is reproduced in Determine 2.

Determine 2. Instance of a non-ideal response.

In Determine 2, the dots signify the transition factors of the attribute curve, and the inexperienced line is the endpoint line that goes via the primary and final transition factors. Clearly, a straight line can’t undergo all these transition factors. Nevertheless, we will discover the straight line that “most closely fits” our information factors (the purple line within the determine). The least-squares technique is used to search out the road of greatest match.

The least-squares technique is a statistical process that matches a line to the information by minimizing the sum of the offsets (or “residuals”) of the factors from the straight line. The calculations concerned within the least-squares technique are tedious, and oftentimes, a spreadsheet or a pc program is used to do these calculations.

As an instance, illustrated in Determine 2, calculating the deviations of the transition factors from one of the best match line results in the next INL plot.

Determine 3. Instance INL plot from calculating the transition level deviations from one of the best match line.

For this hypothetical ADC, making use of the endpoint technique results in a most INL error of +0.5 least important bits (LSBs), as proven in Determine 1. With one of the best match technique, nevertheless, absolutely the worth of the INL error is lower than 0.3 LSBs, which is sort of half that of the endpoint technique. Determine 4 ought to aid you higher visualize how one of the best match technique makes a given attribute curve appear to be extra linear.

Determine 4. Examples displaying the endpoint technique (left) and greatest straight-line technique (proper). Picture used courtesy of Analog Devices [PDF]

In Determine 4, the stable curve represents the nonlinear ADC response. As you may see, one of the best match technique inherently chooses a reference line that minimizes the utmost INL error and tends to cover the small print of linearity efficiency. Consequently, one of the best match technique doesn’t appear to be actually helpful when analyzing the error price range of a measurement system. It is because, for error price range evaluation, we have to calculate the deviations from the best switch attribute, not from some arbitrary “greatest match.”

Whereas the endpoint technique higher fits measurement and management functions, one of the best match technique may present a greater prediction of distortion in AC functions. Nevertheless, even for AC functions, we usually choose to make use of specs similar to harmonic distortion and spurious-free dynamic vary (SFDR) as a result of these metrics can characterize the dynamic linearity of the system.

Subsequently, it’s hardly ever vital to make use of one of the best match INL specification. It’s good to be conversant in it as you would possibly often run into devices which are characterised utilizing a greatest match technique.

Code Heart Greatest Match INL

For the sake of completeness, the code center-based INL definition can also be proven under.

Determine 5. Instance of a code center-based INL. Picture used courtesy of K. Lundberg [PDF]

On this determine, the dashed line is the linear mannequin of the ADC, and the dotted line is the road that matches the precise code facilities. The INL is outlined as the space of the code facilities from the reference line. For the endpoint technique, the reference line is the dashed line. For one of the best match technique, nevertheless, the reference line is the dotted line. Once more, one of the best match technique inherently hides the system’s nonlinearity and may produce a a lot smaller INL error than the endpoint technique.

Now that we’re conversant in the INL specification, we will talk about how the accuracy of an ADC is outlined. Within the context of ADCs, accuracy shouldn’t be really a well-defined parameter, and there are some inconsistencies that we’ll talk about shortly. “Absolute accuracy,” “relative accuracy,” and “whole unadjusted error” are three widespread strategies of describing the switch perform accuracy in ADCs.

ADC Absolute Accuracy

This TI document defines the “absolute accuracy” of an ADC at a given code as the utmost distinction between the analog enter that produces that exact code within the precise response from the best code heart. That is greatest illustrated graphically in Determine 6 (which is reproduced from the article I discussed above).

Determine 6. Plot displaying absolutely the accuracy of an ADC. 

On this instance, an analog enter of 4.25 LSBs (which corresponds to level A) is the smallest worth that produces code 110. The analog equal of code 110 is 6 LSBs. Which means, for an enter of 4.25 LSBs, the ADC outputs 6 LSBs, resulting in an absolute accuracy error of 1.75 LSBs. As you may see, this error could be discovered by calculating the distinction between level A and the best code heart (level B). Determine 7 reveals one other instance of making use of this accuracy definition.

Determine 7. A further instance of making use of absolute accuracy. Picture used courtesy of TI [PDF]

With the above “absolute accuracy” definition, 4 totally different error sources, specifically offset error, acquire error, INL error, and quantization error, are included. Nevertheless, remember the fact that generally the absolute accuracy definition supplied in sure references doesn’t embody the quantization error. For instance, the favored textbook “Analog Integrated Circuit Design” defines absolute accuracy because the distinction between the anticipated and precise switch responses. The e-book additional elaborates that absolute accuracy contains offset, acquire, and linearity errors.

Moreover, the e-book “The Data Conversion Handbook” additionally doesn’t embody the quantization error and defines absolute accuracy because the distinction between the precise and superb code facilities. For instance, if the values within the vary 5V ±1.2 mV produce a given code within the superb response and the precise converter produces that code for inputs from 4.997 V to 4.999 V, then absolutely the error is obtained as:

[Absolute text Error=frac12big(4.997+4.999)-5=-2 text mV]

This definition is much like the code center-based INL definition; nevertheless, the distinction is that it calculates the error with out nullifying the offset and acquire errors.

ADC Relative Accuracy

“Relative accuracy” is another term for the maximum INL error. Subsequently, the relative accuracy expresses the ADC accuracy after its offset and acquire errors are calibrated out.

ADC Accuracy—Whole Unadjusted Error (TUE)

TUE is one other option to specific the entire accuracy of an ADC. The TUE can also be the mixed impact of offset, acquire, and INL errors. Nevertheless, it’s not calculated as a summation of those errors. The utmost TUE is often calculated because the root-sum-square (RSS) of the utmost values of the offset, acquire, and INL errors: 

[TUE=sqrt(Offset text Error)^2+(Gain text Error)^2+ INL^2]

The TUE can be used to specific the accuracy of the entire acquisition system. On this case, the offset and acquire errors from different blocks, such because the enter driver, voltage reference, and so forth., are additionally included.

Key Takeaways on ADC INL Errors

• The INL error quantifies the deviation of the particular switch perform from the best response. Totally different INL definitions use totally different reference strains to outline the best response. INL definitions could be categorized as endpoint and greatest match strategies.
• Whereas the endpoint technique makes use of the primary and final codes for deriving the reference line, one of the best match technique makes use of a line of greatest match as its reference.
• No matter utilizing the endpoint or greatest match technique, the static switch attribute of an ADC could be outlined when it comes to the code facilities or the transition factors (for INL calculations).
• The accuracy of an ADC is often expressed utilizing three specs: absolute accuracy, relative accuracy, and TUE. 

Featured picture used courtesy of Adobe Stock

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