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Design >> Mixed-Signal Design >> Applying sinusoidal voltage to flash??
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Message started by VINAY RAO on Jan 1^st, 2010, 10:48pm

Title: Applying sinusoidal voltage to flash??
Post by VINAY RAO on Jan 1^st, 2010, 10:48pm

hi,
To apply sinusoidal centered around zero (DC point zero) to conventional flash ADC whose supply is (0,vdd)., is it has to be DC shifted in positive side?.?? bcz if we apply directly then -ve cycles will be interpreted as same digital values.
if input signal is in range (-vss,vdd) (ex:-3.3 and +3.3) then which would be hvng advantage --DC shifting the input sinusoidal or providing supply of
(-vss,vdd)..??

Title: Re: Applying sinusoidal voltage to flash??
Post by bala on Jan 8^th, 2010, 3:24pm

Is this a real ADC or are you just trying to simulate it?

In a real ADC, you will run into some problems if you try to overdrive the input.

First, if its an IC, you run the risk of oxide breakdown in some transistor in the worst case.
If there are any ESD circuits present, then the voltage would be strapped to Vdd and what you apply to the ADC would not be the sine wave.
Get an ADC that can support your input range, that would be your best shot.
If not, and you are stuck with this one, then tie the -Vss of sine to gnd! of the ADC, then add a resistive divider and get the sine wave within range of the ADC. you will loose some SNR, but better than degrade SNDR.
Or chopper it.
Anything that works for a real circuit will work for a simulation as well.

Good luck!

Title: Re: Applying sinusoidal voltage to flash??
Post by VINAY RAO on Jan 12^th, 2010, 8:04pm

HI,
Thanks for your reply. I didn't understand clearly about what u said, to tie -vss to gnd , and then to add a resistor devider.Even after using resistor dividers and making to connect -vss to gnd,how negative peaks will be shifted above the origin so that only positive variations will be present and faithfully digitized??.