Dynamic Range of an ADC

Question

I am tasked with a situation where i have to analyse different ADC for a medical instrument (abstracted on purpose just to emphasize that the input is sensitive and results can endanger someone's life if incorrect).

I am to analyze the reliability and simplicity of different ADC in order to suggest the best one for the situation. I was a bit confused on the parameter of dynamic range and how that affects these two design goals. Is having a wide dynamic range preferred to provide reliable results from an ADC? Does it add much in the way of circuitry to the ADC's design?

Are there other components of an ADC that can be analysed for reliability? For example the resolution can be looked upon as the higher the res the more reliable the output (ie closer to the analog input)

Thanks

Answer 1

This should be answered formally by someone who you are paying and who signs his opinion and can be held accountable for it - or ata least who specifically states their qualifications, competence and degree of culpability in the event of subsequent "issues".

There is more to this than can be answered without a deeper level of understanding than your abstraction will allow and than can be even partially answered well, without many to and fro q&As, in this sort of forum. We'll try regardless :-).

" ... the more reliable the output (ie closer to the analog input). ...

Your use of the term "reliable" is non standard and demonstrates a lack of understanding of the process you are attempting to monitor. This is not meant to be rude - just factual. And important.

If you are making this instrument then somebody with a good degree of technical understanding needs to go over the proposal and design in detail. If you are buying it then the name on the nameplate is more important than the specs of the ADC. eg you can be about certain that if it say "Agilent" (and is genuine) then it will do the job well and be 'reliable enough' [tm]. If it says "eg "Golden Sparrow" you may wish to look elsewhere.

ADC resolution affects the potential accuracy of the result. As the required resolution increases the other factors which affect achieved accuracy and resolution become increasingly important.

• Example: I have a well built but low cost digital scale with a full scale reading of 500 grams and a resolution of 0.01 gram (10 mg). I can obtain about 300 mg of reading variation by holding my wristwatch about 30mm above it in one orientation and about half that when the watch is turned at 90 degrees. An electric fan heater with coiled heating element of about 150mm dia, when operated within a 100 mm of the scales turns them into a random number generator).

ADC technology influences susceptibility to external interference. Successive approximation, Flash, dual slope, ... all have pros and cons. A dual slope system will reject 50 Hz superbly if designed to do so but may fail miserably in a 60 Hz mains environment. RF interference (cellphones, mobile phones, pagers, wristwatches !!!, other instruments, ...) MAY cause issues. The ADC proper is part of this but the overall design needs to address the actual requirement.

Potentially more may be said if better information on actual requirement becomes available.

What country?

Answer 2

Dynamic range is usually not something quoted for A/Ds, and it's ambiguous what exactly it might mean. A/Ds have a fixed word width they convert the analog signal to. With N bits, there can only be 2^N diffent output values. You could say that "dynamic range" is therefore (2^N):1, but that is somewhat artificial.

You are correct, however, when you say you are confused. Reading a good general introduction to what A/Ds are and how they are usually specified would be a good start. Another way to learn this is to look at some datasheets from a sampling of manufacturers. After a few you will see what is specified in what way.

You say the design goals are reliability and simplicity. Reliability without quantification is not a design goal or specification. It is one parameter that can possibly be traded off with cost, size, power consumption, and other parameters. Without knowing the relative values of these other things, simply stating you want reliability is not meaningful.

There are several different overall methods of converting a analog signal into a digital value. Some examples include successive approximation, delta-sigma, tracking, and flash. Each method has its own set of advantages and disadvantages. For example, delta-sigma can have very high resolution (20 bits or more) but is slow. Flash is very fast but usually more expensive, power hungry, and often suffers from larger non-linearity errors. You have to start with the characteristics of the signal you want to measure and how you want to use those measurements before you can pick a A/D.

This is not for you

On a separate topic, you have no business designing something where "the results can endanger someone's life" (your words) if you have to ask very basic questions here. It may be legal for anyone to work on such a product, but eventually there will be testing done with strict legal limits that need to be met. These include safety features such as minimum isolation distance, maximum leakage current, and various others. And that's only the beginning if this product can endanger someone not just thru shock, but by providing incorrect readings and the like.

You don't say where you are located (that's really for us, not you, so it's kind of rude to leave it off), but here in the US the FDA regulates such things. Not only does your product have to conform to numerous regulations, but so does your design process. If you don't know what a "510K" is, then you really really don't belong in this situation. You can assist a senior engineer and maybe next time you can take on a more responsible role yourself, but do yourself and everyone else a favor and bail out of this trap now.

Answer 3

caveat: I do not work with medical devices

I am to analyze the reliability and simplicity of different ADC in order to suggest the best one for the situation. I was a bit confused on the parameter of dynamic range and how that affects these two design goals. Is having a wide dynamic range preferred to provide reliable results from an ADC? Does it add much in the way of circuitry to the ADC's design?

Usually when we talk about reliability in electronics, we're talking about the likelihood of failures in use and lifetime in the field. It is often measured in terms of mean time between failures or MTBF.

Reliabilty models for ICs often assume that each transistor in an IC has a certain MTBF (maybe dependent on the operating temperature, fabrication technology, etc.), and so that if we use some statistical analysis we can count up the number of transistors in the IC and calculate the MTBF for the whole device. This motivates selecting a "simple" IC for a high-reliability application, if you define "simple" to mean "containing very few transistors". Many datasheets (especially for parts meant for high-reliability apps?) will actually have a line item specifying the number of transistors in the chip.

Dynamic range is often defined for an ADC, as the other answers have suggested, by the number of bits in the digital result. There may be other factors (distortion, noise) that make this definition technically invalid, but nonetheless it's a common usage. This is essentially unrelated to reliability and simplicity, except in the sense that a high-precision (more bits, higher dynamic range) likely requires more transistors to implement.

My suggestion is don't worry about the dynamic range specified for each ADC. Check the datasheet for the transistor count. If it's not listed in the datasheet, call up the vendor and ask them. In any case, most ICs are not authorized for use in medical devices without written approval from the manufacturer, so you will need to contact them to discuss your application before you can design with their part anyway.

Note: your reliability engineer might be able to suggest other factors besides transistor count that ought to be considered when evaluating IC reliability.