Random jitter specification for ADCMP600 comparator

I am considering using the ADCMP600 high speed comparator to detect pulses from a photo diode. The output will be fed to a time to digital converter (TDC) chip and will be part of a laser ranging system.  To maximise the timing precision of the system I need to minimise the random jitter in the comparator.  Unfortunately the data sheet for the ADCMP600 only gives values for propagation delay dispersion due to variation in overdrive and common-mode voltage.  There is no mention of random jitter.  I would like to be sure that the random jitter added by the comparator I select will be lower than the maximum allowed by the laser ranging system, however I can't find any TTL output comparators that specify their random jitter contribution.  This parameter only seems to be specified for comparators with ECL/PECL/LVDS etc differential outputs.

I suspect that the random jitter of the ADCMP600 might be good enough (I'd like it to be < 1ns rms) given that in comparators that do have a spec for random jitter it is typically much lower than both the overdrive and propagation delay dispersion (eg for ADCMP561/562 random jitter is 1ps, OD dispersion is 75ps and CMV dispersion is 10ps).  If I assume a similar relationship for the ADCMP600 which has OD dispersion 1.2ns and CMV dispersion 200ps then random jitter would be something like 20ps rms.  But is this a valid assumption?

Can anyone help?

Cheers

Josh

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  • Hi Andy,

    Thank you for the reply.  It helps somewhat, but what I really would like to know is the actual jitter performance of the ADCMP600/1/2/3, rather than just knowing that the higher speed devices are suitable for my application.  The implication is that the ADCMP600/1/2/3 is not suitable but it seems that the apps engineers haven't measured its jitter and so can't definitively say whether or not it would be suitable.

    The problem I have is that I need a 3.3V TTL output from the comparator.  So if I were to use either the ADCMP604/5 (LVDS output) or ADCMP606/7 (CML output) I would then need to add a translator to convert the output to TTL.  This translator would also add random jitter to the signal, and while I expect I could find a device with a low enough jitter specification to meet my requirements I would prefer to use a single device, hence my interest in the ADCMP600/1/2/3.  So it would be very helpful if your apps engineers could complete the comparision you mentioned by measuring the jitter produced by an ADCMP600/1/2/3.  Of course if it turns out to be greater than my requirement I will have to use one of the higher speed comparators with a translator, but I would prefer to make a fully informed decision.  I should note that I haven't found any manufacturers of high speed comparators that specify the random jitter of TTL output devices, which I suppose might be an indication that they assume that if designers are concerned about jitter they will be using one of the the high speed logic standards (LVDS, CML, ECL or PECL) and so wouldn't be interested in the slower TTL devices.  I do realise that this may well be the case in general.

    On a different matter, I found that there are some discrepencies in the datasheet for the ADCMP6004/5 that you might like to pass on to whomever is responsible for making corrections.  The general description on the first page states

    "The devices offer 1.5 ns propagation delays with 1 ps rms random jitter (RJ). Overdrive and slew rate dispersion are typically less than 50 ps."

     

    The specifications section of the datasheet does not list random jitter but assuming the figure of 10ps quoted by your apps engineers is correct the datasheet description of 1ps is incorrect.  Also the figures given in the general description for propagation delay and overdrive and slew rate dispersion do not agree with the figures in the specifications table.

    Thanks again for your help with this, I do appreciate it.

    Kind regards,

    Josh

Reply
  • Hi Andy,

    Thank you for the reply.  It helps somewhat, but what I really would like to know is the actual jitter performance of the ADCMP600/1/2/3, rather than just knowing that the higher speed devices are suitable for my application.  The implication is that the ADCMP600/1/2/3 is not suitable but it seems that the apps engineers haven't measured its jitter and so can't definitively say whether or not it would be suitable.

    The problem I have is that I need a 3.3V TTL output from the comparator.  So if I were to use either the ADCMP604/5 (LVDS output) or ADCMP606/7 (CML output) I would then need to add a translator to convert the output to TTL.  This translator would also add random jitter to the signal, and while I expect I could find a device with a low enough jitter specification to meet my requirements I would prefer to use a single device, hence my interest in the ADCMP600/1/2/3.  So it would be very helpful if your apps engineers could complete the comparision you mentioned by measuring the jitter produced by an ADCMP600/1/2/3.  Of course if it turns out to be greater than my requirement I will have to use one of the higher speed comparators with a translator, but I would prefer to make a fully informed decision.  I should note that I haven't found any manufacturers of high speed comparators that specify the random jitter of TTL output devices, which I suppose might be an indication that they assume that if designers are concerned about jitter they will be using one of the the high speed logic standards (LVDS, CML, ECL or PECL) and so wouldn't be interested in the slower TTL devices.  I do realise that this may well be the case in general.

    On a different matter, I found that there are some discrepencies in the datasheet for the ADCMP6004/5 that you might like to pass on to whomever is responsible for making corrections.  The general description on the first page states

    "The devices offer 1.5 ns propagation delays with 1 ps rms random jitter (RJ). Overdrive and slew rate dispersion are typically less than 50 ps."

     

    The specifications section of the datasheet does not list random jitter but assuming the figure of 10ps quoted by your apps engineers is correct the datasheet description of 1ps is incorrect.  Also the figures given in the general description for propagation delay and overdrive and slew rate dispersion do not agree with the figures in the specifications table.

    Thanks again for your help with this, I do appreciate it.

    Kind regards,

    Josh

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