[help] preamplifier before the comparator

[skyeaglemm]

question: a 6 bit 2 GSamples/s flash A/D converter with Vref = 1V. It uses an array of preamplifiers followed by a comparator array.
(a)The comparator can resolve a 50mV difference at its input to full logic levels at its output. What is the gain required in the preamplifiers?

soution:VLSB=Vref/64=15.6mV,minimum input to preamplifiers=VLSB/2,which must be amplify to 50mV. So gain=6.4

i am confused why minimum input to preamplifiers=VLSB/2. if vin=15mv, the input to the first preamplifiers is just 15.6-15=0.6mv<<15.6/2 mv,right? [/img]

 

[yibinhsieh]

The minimum requirement for the ADC to be solved is 1/2LSB. In your case, the 1LSB=15.6mV. So the mininum vlotage difference that can be recognized is 7.8mV. When your input is 15mV, the correct output is 6'b000001. Because the input pass the first criteria of the first comparator in Flash ADC. For the second comparator to be output by 1. your input needs to be larger than 7.8+15.6 mV.


Yibin.