effect of non uniform preamplifier gain on comparator speed

[hacksgen]

calculate comparator gain

Hi everyone,

How does the gain of the preamplifer affect the maximum speed of operation of the comparator. For example assuming that the preamplifer has constant gain of 28dB till 1MHz and thereafter it decreases to 21dB for 10MHz. does it matter if i use the comparator (latch is clocked at 10MHz and input signals are coming in at frequency of 10MHz to the preamp) at this frequency of 10MHz.

will the non uniform gain affect the operation in anyway at all like low frequency noise components of the circuits.

this preamp+latch type comparator is intended to used in a SAR ADC.

Thanks

 

[lladnar23]

comparator speed

OK - you need to consider: What is the purpose of the preamp in the first place?

I can think of at least two important reasons to have it:
1 - to provide an amplified input signal to the latch. We know that the delay of the latch is proportional to -tau*ln(Vin) where tau is the regeneration constant and Vin is the input voltage. So for very low Vin, the delay gets quite large. The preamp helps to provide a larger input voltage. If the preamp has low bandwidth, the rise time of the latch input will not be very fast, and you may not get all of the benefit!

2- Isolation from the kickback noise of the latch. Again, if you have low preamp bandwidth, it is going to be hard to surpress the high frequencies present in the kickback noise.

 

[hacksgen]

pre amplifier comparator

Well what you said is right. But from my simulation results of a 3 stage preamp followed by latch , it seems that the use of comparator at 10MHz has no effect at all on the conversion.
the signal is anyhow amplified to a level much greater than the offset of the latch. the amplification is more than 60dB from the 3 stages. the combined delay of the preamp stages is not that large either and well below the clock cycle. How does the bandwidth affect the rise time of the latch. How did you come the equation -tau*ln(Vin). Can you point me to where I can find more about this in detail.

Thanks for your help