why latches are used for cmos comparator design

cmos comparator design

flash adc is achieved using high speed comparator..

it has 3stages; preamplifier, latch/buffer...

preamplifier is for gain(least required) and freq response(mostly)...

why do we use latches/flipflops in the following stage..

cmos latch comparator design

uhh.. to store the bits until the next clock cycle or until all the data is transfered out?

which latch is used mostly in digital

one is to store data and another is that latch will increase the speed of the comparator after the initial preamplification is done.......

comparator design cmos

If we go to the definition of an comparator, it should be able to give a digital data for an error signal (e). Now to do this it need some time. An op-amp in open loop can do this, but at much higher cost of circuit/power and also takes more time. Latch, if we look its operation, is again an amplifier basically. The difference is that, it works in re-generative mode. Thus its feeds its own output to itself and re-generates unless it saturates at the digital output voltage. Again, this is done at lower transistor count and power consumption. This is the reason why re-generayive amplifiers (the latch) are popular in comparator that simple linear amplifiers. The other reson (to store data) mentioned above is also true once the data is in digital domain. Thus here, latch serves both the purpose.

For reference, you can go through a review paper by Razavi (in JSSC) , also availble in this forum.

sankudey

design of cmos regenerative latch

if positive feedback is used it is called regenerative latch... the actual reason behind the use of latch is that for power saving the comparator is generally switched off during one phase mostly by using a MOS controlled by the clocks which open circuits the comparator from Vdd or Gnd... so if the output goes to common mode voltage or supply rails during this phase then there would be problem in the next stage (if there is any) due to delay of arrival of valid input especially for the common mode voltage case where it may lead to metastable state... so latch is used to hold the output when the comparator is disconnected....

reason to use latch in cmos comparator

For the hign speed. Preamplifier is used for kickback noise,Latch will provide high speed.

Of course, latches can increase speed and sometimes it is required to add another switch between two outputs for recovery time.

preamplifier is used to reduce offset....