flash adc & input voltage of preamplifier

[akbarza]

hi
we know that in in a flash ADC there is a resistor ladder and comperators(=preamplifier+ latch)
my question: for each resistor in ladder there is a preamplifier.
with note that each resistor in ladder produce different voltage than other resistores,then each preamp will have different condition.
what is different for each preamplifier? the dc bias or common mode voltage or else?
please explain.
thanks.

 

[betwixt]

They are normally just comparators although the symbol is the same as a preamplifier. They should all be the same, each having a slightly different reference voltage from the divider chain but all having the same signal voltage applied to them. The outputs of the comparators then goes to a logic encoder to produce the binary output. It may be that there are some slight design differences in the comparators at the top and bottom of the divider chain where they operate at near rail input voltages but functionally they should be identical.

Brian.

 

[danadakk]

Many SARs now C based -

Guide to Understanding Successive Approximation Registers (S

Find info on the basics of successive-approximation-register (SAR) analog-to-digital converters (ADCs). Review SAR ADC with pipeline, flash, and sigma-delta ADCs.

www.maximintegrated.com

Regards, Dana.

 

[sutapanaki]

Reference voltage of the comparators in a flash is not necessarily the same as the supply voltage. So, depending on the full scale range of the ADC, that is the ref voltage, the comparators may be the same. Which means the preamps have to be able to handle the full range of input voltages without essentially changing their characterisitcs, so that the latches following them trigger with nominally the same threshold, noise, etc.

 

[KlausST]

Many SARs now C based -

Hi,

But the question was about "flash ADC"....

For Flash ADCs I agree with the above:
One amplifier should be sufficient.... then many comparators with different threshold levels.... then the logic...

Klaus

 

[danadakk]

Thanks Klaus for picking up my error.

Flash ADC - Wikipedia

en.wikipedia.org

Here is interesting approach with no comparators, just scaled threshold inverters, attached


Regards, Dana.

 

[akbarza]

They are normally just comparators although the symbol is the same as a preamplifier. They should all be the same, each having a slightly different reference voltage from the divider chain but all having the same signal voltage applied to them. The outputs of the comparators then goes to a logic encoder to produce the binary output. It may be that there are some slight design differences in the comparators at the top and bottom of the divider chain where they operate at near rail input voltages but functionally they should be identical.

Brian.

--- Updated Apr 14, 2021 ---

hi
you said:It may be that there are some slight design differences in the comparators at the top and bottom
can you explain more?
thanks