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    Create a long delay with using inverter gate?

    In my asynchronous SAR ADC, I need a delay of hundreds of nanoseconds for the capacitive DAC to settle. I realize that it is difficult to use the inverter chain to create such a big delay and it results in big power consumption and area. Is there any effective way to create such a long delay? Thanks in advance.

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    Re: Create a long delay with using inverter gate?

    as with most analog designs today, they have a digital wrapper. you can control time externally and let the DAC settle very easily. counting clock cycles would work.
    Really, I am not Sam.


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    Re: Create a long delay with using inverter gate?

    It is not practical to build a delay that large, not to mention its variations, out of inverters. In the case of a SAR it is best to keep things internal to the SAR and not go outside, especially if the SAR is asynchronous. To build your timer maybe it is a good idea to think about some sort of a replica circuit that tracks the dac delays, not just using a brute force approach as with the chain of inverters delay.


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    Re: Create a long delay with using inverter gate?

    Long delays made with leisurely ramps tend to pick up
    a lot of jitter. Aperture jitter is a problem in applications
    where you are trying to use the ADC data stream to
    do frequency domain "stuff", it adds frequency spurii.

    A timing ramp and comparator can at least address the
    power problem better than a bazillion inverters in a string.
    Another approach could be a PLL / DLL working from the
    master clock and producing derivative clock phases.
    But jitter / phase noise may remain an issue to be solved.


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    Re: Create a long delay with using inverter gate?

    Quote Originally Posted by ThisIsNotSam View Post
    as with most analog designs today, they have a digital wrapper. you can control time externally and let the DAC settle very easily. counting clock cycles would work.
    Click image for larger version. 

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    Can you tell me some keywords related to this technique, The settling time of my DAC look very slow compared to other works although I tried to reduce the on resistance of the switch.



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    Re: Create a long delay with using inverter gate?

    You would have to learn a bit of digital design, I can't teach that in a forum post.
    Really, I am not Sam.



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    Re: Create a long delay with using inverter gate?

    I realize that it is difficult to use the inverter chain ...
    One RC coupled with a simple inverter gate can be used to introduce a fixed delay. You can also use a diode (reverse biased) to discharge the capacitor (in case it is needed).



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    Re: Create a long delay with using inverter gate?

    Quote Originally Posted by c_mitra View Post
    One RC coupled with a simple inverter gate can be used to introduce a fixed delay. You can also use a diode (reverse biased) to discharge the capacitor (in case it is needed).
    But as @dick_freebird said, won't a slow ramp pick up noise and jitter?



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    Re: Create a long delay with using inverter gate?

    But as @dick_freebird said, won't a slow ramp pick up noise and jitter?
    That depends; what is the time interval or frequency range you are interested? IMHO, noise and jitter has nothing to do with whether it is a fast or slow ramp.



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    Re: Create a long delay with using inverter gate?

    The ramp dV/dt transforms dV (supply noise and noise
    riding on the ramp from whatever source) to dt (jitter).
    Slower ramp makes higher time-deviation for the same
    amplitude of noise.

    Slow enough ramps also bring the possibility of "chatter"
    unless you have hysteresis greater than input-referred
    noise amplitude.



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    Re: Create a long delay with using inverter gate?

    Jitter in this case hardly matters. The delay done with inverters, or even a ramp (unless one goes through lot of trouble making exact ramp ) will have variations with temperature and supply that will be more severe than jitter.



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