4 bit resistor ladder flash adc

[tompham]

I have a question in designing 4 bit flash adc.
How to choose resistor value in resistor ladder? When I change resistor value I see output of compartor has wrong comparation. The strange thing here is if I run compator alone it works well. Does value of resistor ladder affect comparation?
Thanks

 

[BradtheRad]

There is a chance the comparators are 'loading' the resistor ladder. This can alter volt levels between the resistors.

Suppose a comparator has 1M input resistance. The resistors should be a much lower value, maybe 1k. Then the comparators will have only a small influence.

 

[dick_freebird]

Input "kickback" can perturb the network, and high ladder
impedance may push the settling time from that, onto the
next cycle. Input and kickback settling time along with plain
supply current concerns define your "box" pretty much.

Now, not seeing your schematic, I couldn't say whether
there might be some other issue like changing only one of
the 16 or so resistors' values, when you meant to change
them all.

 

[tompham]

Hi BradtheRad and dick_freebird

Thanks for your comment. You are all correct, the large resistor value make comparator got incorrect result. I change to small register and adding the cap (show in picture) make reference level stable.