Comparator at low-voltage

[thanhtri_pc]

Hi all,
I want to design a comparator which can compare two input at verry low-voltage, For example, I want to compare Vi+ = 20mV with Vi- = 30mV. I just used CMOS two-states comparator structure, but I realized that it can compare only if 450mV <= Vi+ and Vi- <= 1.7V (with voltage supply are Vdd = 2V, Vss = 0V).
As you know, if the input voltage is out of range, the comparator does not work properly.
In that situation, we must use another architecture.
Could you show me what the architecture I can use and how to design it?
Many thanks,
Tri Vo

 

[jackym]

you can use a two-stages comparator with PMOS input

 

[thanhtri_pc]

Hi,
Thanks for your suggestion. I just tried to use the 2-state comparator with P driver, but the problem seems not to be resolved. I am designing flash ADC 6bit. The analog signal input is in range 0->2V. So, Vstep is 2/64 = 31.25mV and I want to design a comparator that can compare at low-voltage level (Vi < 500mV)

 

[leehying]

Hi,
Use a source follower as a level shift of the input signal whose CM voltage is beyond your comp..
By the way, may I know the bandwidth or frequency of your input signal.
I feel that you have not got the best resolution.
LEE

 

[thanhtri_pc]

Hi,
I plan to design a Flash ADC 6bit with sample clock is about 200MHz, using process TSMS 0.18um. Input signal amplitude is from 0V to 2V. I also use a singal voltagr supply, Vsup=2V
Could you give me more detail about your recently comment "Use a source follower as a level shift of the input signal whose CM voltage is beyond your comp..".
You know, I am new in anlaog design.
Many thanks