thanks all.
the main problem is usb voltage which is variable from one PC to another .
and with bypassing Caps and ... the problem still remain.
I have used AD7655 before and i got to 40 count noise in 64K that is OK for me .
but I have used a 8v adapter and the ADC analog pins(5v) came from 7805 regulator with ferrite bead and decoupling capacitors which are necessary.
My recommendaton: Use a 3V3 ADC and a LDO.
my new duty is supplying this boards with USB without changing ICs.
because of that I cant use another ADC :|
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...another (convoluted) method: If your USB can output sufficient current, you could begin the circuit with a voltage doubler subcircuit, then follow that with a 5V regulator, to power the ADC and op amp, and if the ADC/op amp are low power enough any old 5V linear regulator would suffice unless an LDO were preferred for some reason.
Otherwise you need a SEPIC or buck/boost swtiching regulator to ensure proper power supply.
So i have to use booster?!
It seems little impractical because my board must be as small as possible.
would you tell me about a simple and compact booster circuit which i can boost USB voltage for using LDO?
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If i forget about setting usb voltage to exactly 5 v and just confine to filtering USB voltage such as this :
https://andybrown.me.uk/2015/07/24/usb-filtering/
I will have to issues :
1.the USB voltage :
I think its allitle impossible for USB voltage to exceed from this range :4.75V ... 5.25
what about you?
2.the ADC analog pin's noise...
do you think the filtered USB voltage (in the website) can be as well as 7805's output?!
this way is very compact and easy!! and it's tempting me