This thread points out an error that needs to be corrected.

 

Hi, im researching flash ADC's for school presentation, via this website's link on the subject - however the following statement confuses me...



"Unfortunately, it is the most component-intensive for any given number of output bits. This three-bit flash ADC requires eight comparators. A four-bit version would require 16 comparators. With each additional output bit, the number of required comparators doubles. Considering that eight bits is generally considered the minimum necessary for any practical ADC (256 comparators needed!), the flash methodology quickly shows its weakness. "



http://www.allaboutcircuits.com/vol_4/chpt_13/4.html



I thought that 2n-1 comparators were needed, so shouldnt a 4 bit require 15 comparators (not 16) and a 8 bit - 255 comparators? - and a 3 bit should require 7?



Thanks for any help.

I don't see it, it looks OK to me.

The top comparitor is to measure the overvoltage condition, which is mentioned in the text.

Or are you refering to somewhere else in the text? To correct the text we need to pin it down.

 

I don't see it, it looks OK to me.

The top comparitor is to measure the overvoltage condition, which is mentioned in the text.

Or are you refering to somewhere else in the text? To correct the text we need to pin it down.

Bill, I must be blind. Please point me to the text that explains the function of the top comparator.
Below the schematic that you posted, there is a 7-comparator version, but the text implies that 2^n comparators are required.

 

Vref is a stable reference voltage provided by a precision voltage regulator as part of the converter circuit, not shown in the schematic. As the analog input voltage exceeds the reference voltage at each comparator, the comparator outputs will sequentially saturate to a high state. The priority encoder generates a binary number based on the highest-order active input, ignoring all other active inputs.

They don't mention the top comparator specifically, but it does the same job. If the input voltage exceeds Vref then the 8th digit is on, the maximum number. Binary 000 - 111, which is eight permutations. Matches 8 comparators nicely.

 

They don't mention the top comparator specifically, but it does the same job. If the input voltage exceeds Vref then the 8th digit is on, the maximum number. Binary 000 - 111, which is eight permutations. Matches 8 comparators nicely.

But only 7 comparators are required to get all 8 codes. I agree that the 8th comparator will give some information about overvoltage, but if that's the intent, then it should be stated explicitly in the e-book, because it's a fact that only 2^n-1 comparators are required for an n-bit converter.

 

So how are you going to get 111 if you eliminate the top comparator? 8 possible numbers, 000-111, require 8 inputs. You can not have 000-111 with 7 inputs, it becomes either 001-111 or 000-110.

Is the assumption if they are all zero then you have 000? I'm thinking of a mirror to the CD4512.

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OK, I can see it. The author was making the same assumption I was, 8 inputs required for a mod3 output. This section is going to require a complete rewrite, I think.

What he is showing is valid, but you can remove one comparator with the assumption of all zeros in equal all zeros out. Since removal of a component is more or less industry standard, I can see the confusion.

 

Another thing, look at the first 2 pictures, compared to the 3rd. It is correct on the 3rd illustration. Good catch.

 

Thanks Ron and Bill for sorting this out.

The corrections have been made to the master copy at ibiblio.org

Ron H@allaboutcircuits.com is credited in the contributors page appendix-A-2 at ibiblio
If you want to be credited under your real name, send me a private message.

D Crunkilton