+ Post New Thread
Results 1 to 5 of 5
  1. #1
    Member level 3
    Points: 1,723, Level: 9

    Join Date
    Nov 2004
    Posts
    56
    Helped
    8 / 8
    Points
    1,723
    Level
    9

    latched comparators by ken martin

    In my opinion, the main contribution of the preamplifier is its prevention from the kickback effect generated by the following latch during its transition from track mode to latch mode [ref: David Johns & Ken Martin]. Although the preamplifier CAN attenuate the input offset of the latch, it has offset itself, which needs also extra circuits to reduce it. Therefore I think acturely the preamplifier contributes nothing in the aspect of offset-cancellation.
    Am I right?

    •   AltAdvertisment

        
       

  2. #2
    Member level 5
    Points: 1,853, Level: 10

    Join Date
    Mar 2004
    Posts
    90
    Helped
    8 / 8
    Points
    1,853
    Level
    10

    Re: About the preamplifier in the comparator design.

    I think the effect of the preamp on reducing input offset is dependent on its gain.However, since GBW is fixed, preamp will actually not play this role in high-speed applications.Other ways for cancelling offset should be used.



    •   AltAdvertisment

        
       

  3. #3
    Full Member level 6
    Points: 3,806, Level: 14

    Join Date
    Nov 2004
    Location
    Russia
    Posts
    340
    Helped
    73 / 73
    Points
    3,806
    Level
    14

    Re: About the preamplifier in the comparator design.

    Let's consider the comparator consisting of the preamplifier and latch.
    The latch functions in a large signal mode with output logic levels and usually it should provide the maximal speed. Hence sizes of transistors of latch should be minimal and consequently the latch will have the large input offset.
    The amplifier functions in a small signal mode at the comparison time. Its gain gets out small from 5 up to 20. Therefore amplifier can have the large sizes of transistors and large enough GBW and small input offset. Besides the amplifier reduces kickback signal, it is very important.


    1 members found this post helpful.

    •   AltAdvertisment

        
       

  4. #4
    Full Member level 1
    Points: 1,931, Level: 10

    Join Date
    Nov 2004
    Posts
    118
    Helped
    5 / 5
    Points
    1,931
    Level
    10

    Re: About the preamplifier in the comparator design.

    Besidez, offeset, speed, noise is also one factor. Preamplifier acts as amplifier and buffer as well.



  5. #5
    Newbie level 5
    Points: 1,453, Level: 8

    Join Date
    Jan 2005
    Posts
    8
    Helped
    0 / 0
    Points
    1,453
    Level
    8

    Re: About the preamplifier in the comparator design.

    If we use two stages of amplifiers with autozero caps in between, then theoretically the offset from the 1st preamp will be totally cancelled out, and only the offset from the 2nd preamp presents at the output. Therefore the 1st preamp can live with small input devices, while the 2nd preamp needs not to have too much offset. A strategy is to use minimal channel length in the 1st preamp to boost the gain there, but not in the 2nd preamp to avoid otherwise high offset.



--[[ ]]--