No. of Comparators in a Flash ADC.

Thread Starter

Denny1234

Joined Feb 17, 2008
27
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.
 

Ron H

Joined Apr 14, 2005
7,012
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.
You are correct.
 

Wendy

Joined Mar 24, 2008
21,840
Ron, I have responded to your post in the Feedback and Suggestions forum.
 

Thread Starter

Denny1234

Joined Feb 17, 2008
27
Thanks for your help.

Just could I ask about one thing I cant quite understand?

The following statement..

Comparators -
"They are designed for low voltage offset, such that the input offset of each comparator is smaller than a LSB of the ADC. Otherwise, the comparator's offset could falsely trip the comparator, resulting in a digital output code not representative of a thermometer code

When it says smaller than a LSB of the ADC - is that refering to the voltage formed by each potential divider (of the analouge imput Voltage) that is to be 'compared' to Vref?
 

Ron H

Joined Apr 14, 2005
7,012
Thanks for your help.

Just could I ask about one thing I cant quite understand?

The following statement..

Comparators -
"They are designed for low voltage offset, such that the input offset of each comparator is smaller than a LSB of the ADC. Otherwise, the comparator's offset could falsely trip the comparator, resulting in a digital output code not representative of a thermometer code

When it says smaller than a LSB of the ADC - is that refering to the voltage formed by each potential divider (of the analouge imput Voltage) that is to be 'compared' to Vref?
Yes, the voltage represented by an LSB change is equal the the voltage across any single resistor in the reference divider chain, providing that all resistors are of equal value.
Note that Vref is applied to the divider chain, not Vin.
 
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