Hi all,
I'm designing a full flash adc,5 bit, 4 GS/s.
I have a big doubt about the choise of Vref. in many papers i saw that there are Vref+ and Vref-, why?
for example if the input is a sinewave Asin(2*pi*fin*t) with A=1V, power supply is 1.2 V, How can i choose Vref+ and Vref-?
In this example 1 LSB is equal to???
If you consider an wave with middle tension equal to zero, then you need to have + vref and - vref to convert all the wave. If you have the continuos component, then depending of its value, you only need the +vref.
The +vref -vref is relative to your common-mode voltage. If your vss=0V and vdd=1.2V, your common-mode voltage might be somewhere between, say, 450mV and 650mV (depending on how you design your comparator). Let's say 500mV for sake of discussion. Your +vref could be 800mV and your -vref coud then be 200mV. It really depends on what your THA can deliver to the comparators.