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Some questions about flash adc

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Junior Member level 2
Sep 10, 2009
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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.


ok, but if the wave doesn't have negative value,I have to use onlly Vref?

in the case the wave has this form, how can I choose Vref+ and Vref-?
Which is the value of 1 LSB if the resolution is 5 bit?

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.

so Vref+ and -vref are indipendent by the input signal? they depend only by the common mode voltage,Vss and Vdd?

The input signal must be within +vref and -vref, of course!

yes sure!Thanks

and about the value of 1 LSB? 1 Lsb= (+VREF - VREF-)/2^N, ?

sorry but I have other questions:

In this scheme what does mean Vin+ and vin-?
Can you explain me with an example?

i'm simulating the Track and Hold with pspice student,how can I calculate the HD3 and SNR with Pspice?

thanks all

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