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Accuracy

Many people confuse resolution with accuracy. Just because you have a converter with 16 bits of resolution doesn’t mean you always have 16 bits (or 15 ppm) of accuracy. Recall that a data converter requires a reference, and accuracy is the degree to which the result conforms to the correct value measured against a standard or reference. If you put exactly 1 V into an ADC and could resolve exactly that value with 16 bits, but the ADC tells you that it’s 1.2 V, that’s only 20 percent accuracy – a far cry from 15 ppm.

At a system level, the accuracy of the reference voltage will be one of the primary factors in the overall accuracy of the system. But the data converter itself contributes some errors. Some data converters will express an overall accuracy specification – often called total unadjusted error – but it is more common to see specifications for offset error, gain error and linearity.

The ideal transfer function for a data converter is shown in Figure 4, as a blue line. It appears as a staircase because the quantizer can only represent a range of voltages with a single code. The location of the transition point from one code to another is key to describing the converter's accuracy.

Offset error is the difference between the actual transition point and the ideal transition point – resulting in a translation of the transfer function left or right. Although this error is measured around a near-zero voltage input or output, the error continues throughout the entire transfer function from zero to a full-scale (FS) voltage. Consequently, the offset error near zero is the same as the offset error with a near FS voltage.

Gain error is the difference between the actual last transition point and the ideal last transition point – resulting in a rotation of the transfer function. It is the difference between the ideal slope of the transfer function and the actual slope between the measured zero point to FS, minus the converter’s offset error. This is actually measured by looking at where the last transition point at full scale occurs, as shown in Figure 5.

As you might imagine from the use of the term “error,” smaller gain and offset errors imply a more accurate converter. Gain and offset errors can often be corrected for in the digital domain, using the system microcontroller or DSP.

Linearity errors , on the other hand, are much more difficult to correct. While the transfer functions in Figure 4 and Figure 5 show steps that are exactly the same size, in actual converters the width of these steps will vary. Figure 6 shows how differential nonlinearity (DNL) is measured. DNL is the deviation in code width from the ideal 1LSB code width.

A DNL error less than –1 LSB can cause an entire code to “disappear,” resulting in what is called a missing code . The smaller the range of the DNL specification, the better.

Integral nonlinearity (Figure 7) is the cumulative effect of all of the differential nonlinearity errors, and is the maximum deviation between the actual code transition points and the corresponding ideal transition points after gain and offset error have been removed. The smaller the INL specification, the more accurate the data converter will be.

Speed, resolution and accuracy are important considerations in selecting a data converter, and you will often find what you need to know about these on the front page of the converter data sheet. But other important factors, such as the details of the digital interface and mechanical drawings of the package, are further inside the data sheet. You need to read the entire data sheet to effectively use a data converter.

Choosing the right data converter

A multitude of data converters are available for your design. While the three main specifications of speed, resolution and accuracy will help in the final selection process, narrowing down the options to choose from is the first step.

When selecting an ADC for a particular application, a good first selection criteria is to look at the topology of the ADC. Figure 8 is a simple comparison of ADC architectures that can help you find the right place to start when selecting a converter for your application. Delta-sigma converters are most suitable for higher-resolution tasks, while successive-approximation-register and pipeline architectures are the ones to look to for higher-speed applications.

Digital-to-analog converters have similar trade-offs in architecture: delta-sigma DACs tend to be slower, while R-2R, string and multiplying DACs all provide good general-purpose performance. High-frequency, fast-settling DACs are generally for current-steering architectures.

Knowing the speed and resolution requirements of your application, you can anticipate which architecture of converter will most likely suit your needs. Then you can look more closely for the converter that matches the accuracy requirements and other features that the application demands.

Table 1 lists some applications where data converters are used, as well as the selection criteria and some examples of actual converter products suitable for those applications.

Application Required sample rate Required resolution Architecture Example part Comments
Weigh scale <100 sps 18-24 bits Delta-sigma ADS1211, ADS1258 High accuracy
Temperature measurement <10 sps 8-18 bits Delta-sigma ADS1146 High accuracy
Waveform analysis/synthesis <100 Msps 8-16 bits Pipeline ADS6445, DAC2902 High speed and good linearity (low distortion) required
Test and measurement <1 Msps 12-24 bits SAR ADC, multiplying DAC ADS7824, ADS8326, DAC8820 High accuracy and throughput, multiple channels
Ultrasonic imager <100 Msps 12-14 bits Pipeline ADS6445 High speed, good resolution
Software-defined radio <500 Msps 12-14 bits Pipeline ADS5474, ADS41B49 High speed, good resolution
Motor control and positioning <500 ksps 12-18 bits SAR ADC, multiplying DAC ADS8361, DAC8811 High accuracy and throughput, multiple channels

Questions & Answers

where we get a research paper on Nano chemistry....?
Maira Reply
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
learn
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
learn
Google
da
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Bhagvanji
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
revolt
da
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
Damian
yes that's correct
Professor
I think
Professor
Nasa has use it in the 60's, copper as water purification in the moon travel.
Alexandre
nanocopper obvius
Alexandre
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
if virus is killing to make ARTIFICIAL DNA OF GRAPHENE FOR KILLED THE VIRUS .THIS IS OUR ASSUMPTION
Anam
analytical skills graphene is prepared to kill any type viruses .
Anam
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
Hafiz
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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