Groups > sci.electronics.design > #453046

Re: bad, better delay circuits

From	JM <dontreplytothis173@gmail.com>
Newsgroups	sci.electronics.design
Subject	Re: bad, better delay circuits
Date	2017-03-22 16:02 +0000
Organization	A noiseless patient Spider
Message-ID	<oau70n$hu2$1@dont-email.me> (permalink)
References	<1roqcclua35sdrn6jduc9q0folr3fg8oqu@4ax.com> <0275dcp7j0qgf9chfq8vboanahoodf3hde@4ax.com>

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On 22/03/2017 15:54, John Larkin wrote:
> On Sat, 18 Mar 2017 10:04:42 -0700, John Larkin
> <jjlarkin@highlandtechnology.com> wrote:
>
>>
>>
>> We want to trim the rising edge times of some pulses. The obvious
>> circuit is a current source + capacitor linear ramp followed by a
>> comparator.
>>
>> The first circuit here was the initial idea, seems simple, until I
>> started thinking harder.
>>
>> https://dl.dropboxusercontent.com/u/53724080/Circuits/Timing/Delay_Bad_Good.JPG
>>
>> BSS123 is about the best compromise between capacitance and ON
>> resistance, and we have it. The two problems here are
>>
>> the g-d capacitance of Q1 shoots an ugly negative spike into C1 at
>> ramp start and
>>
>> the drain capacitance of Q2 is nonlinear, which is maybe OK, but the
>> drain voltage can wander all over the place. That modulates the ramp
>> slope, and the body diode could conduct at ramp discharge time. Fixes
>> for that are variations of ugly.
>>
>> E-Phemts would be better than mosfets, but that would multiply the
>> parts cost by 5x or so. Open-drain cmos gates could be interesting,
>> but we don't have any in stock
>>
>>
>> The second circuit looks better. The mux switches in C2, or keeps it
>> grounded. These "bus switch" parts are actually killer analog
>> multiplexers at a fraction of the price of things sold as analog
>> mux's.
>>
>> The comparator is of course an LVDS line receiver, another example of
>> using a part off-label and saving a bunch of dollars and nanoseconds.
>>
>> Another bus switch could be used to discharge the cap, instead of the
>> diode, but sadly their control-input delays are high. That's the
>> pattern in mux's: fast signal path, slow switching.
>
>
> One of my guys is arguing that we should use an ATF-55143 ephemt to
> discharge the caps. That would slam them hard all the way to ground
> and eliminate the diode tempco effect on timing. Those phemts have
> absurdly small Cg-d, so wouldn't pump the ramp down at startup.
>
> He's right of course. I hate it when other people are right.
>

Sack him! Get in a yes man... ;-)

> I think we could keep the analog mux/bus switch thing to switch the
> big cap in and out.
>
>

Thread

bad, better delay circuits John Larkin <jjlarkin@highlandtechnology.com> - 2017-03-18 10:04 -0700
  Re: bad, better delay circuits Winfield Hill <hill@rowland.harvard.edu> - 2017-03-18 12:29 -0700
    Re: bad, better delay circuits John Larkin <jjlarkin@highlandtechnology.com> - 2017-03-18 16:15 -0700
      Re: bad, better delay circuits Winfield Hill <hill@rowland.harvard.edu> - 2017-03-18 19:08 -0700
  Re: bad, better delay circuits "Tim Williams" <tiwill@seventransistorlabs.com> - 2017-03-18 17:02 -0500
    Re: bad, better delay circuits John Larkin <jjlarkin@highlandtechnology.com> - 2017-03-18 16:25 -0700
      Re: bad, better delay circuits rickman <gnuarm@gmail.com> - 2017-03-18 19:40 -0400
        Re: bad, better delay circuits dagmargoodboat@yahoo.com - 2017-03-19 10:42 -0700
          Re: bad, better delay circuits John Larkin <jjlarkin@highlandtechnology.com> - 2017-03-19 15:37 -0700
            Re: bad, better delay circuits "Tim Williams" <tiwill@seventransistorlabs.com> - 2017-03-19 22:24 -0500
              Re: bad, better delay circuits dagmargoodboat@yahoo.com - 2017-03-20 08:58 -0700
                Re: bad, better delay circuits John Larkin <jjlarkin@highlandtechnology.com> - 2017-03-20 09:42 -0700
                Re: bad, better delay circuits Robert Baer <robertbaer@localnet.com> - 2017-03-20 23:55 -0800
                Re: bad, better delay circuits "Tim Williams" <tiwill@seventransistorlabs.com> - 2017-03-21 00:46 -0500
              Re: bad, better delay circuits bill.sloman@ieee.org - 2017-03-21 22:54 -0700
                Re: bad, better delay circuits "Tim Williams" <tiwill@seventransistorlabs.com> - 2017-03-22 02:10 -0500
          Re: bad, better delay circuits rickman <gnuarm@gmail.com> - 2017-03-19 18:47 -0400
            Re: bad, better delay circuits Clifford Heath <no.spam@please.net> - 2017-03-20 09:55 +1100
              Re: bad, better delay circuits John Larkin <jjlarkin@highlandtechnology.com> - 2017-03-19 16:12 -0700
      Re: bad, better delay circuits "Tim Williams" <tiwill@seventransistorlabs.com> - 2017-03-19 10:32 -0500
  Re: bad, better delay circuits Clifford Heath <no.spam@please.net> - 2017-03-19 13:36 +1100
    Re: bad, better delay circuits John Larkin <jjlarkin@highlandtechnology.com> - 2017-03-18 19:44 -0700
      Re: bad, better delay circuits Clifford Heath <no.spam@please.net> - 2017-03-19 15:58 +1100
        Re: bad, better delay circuits John Larkin <jjlarkin@highlandtechnology.com> - 2017-03-19 15:43 -0700
          Re: bad, better delay circuits Winfield Hill <hill@rowland.harvard.edu> - 2017-03-19 17:54 -0700
            Re: bad, better delay circuits Clifford Heath <no.spam@please.net> - 2017-03-20 12:44 +1100
              Re: bad, better delay circuits Winfield Hill <hill@rowland.harvard.edu> - 2017-03-20 12:22 -0700
                Re: bad, better delay circuits John Larkin <jjlarkin@highlandtechnology.com> - 2017-03-20 15:49 -0700
  Re: bad, better delay circuits whit3rd <whit3rd@gmail.com> - 2017-03-18 23:51 -0700
    Re: bad, better delay circuits John Larkin <jjlarkin@highlandtechnology.com> - 2017-03-19 08:22 -0700
  Re: bad, better delay circuits George Herold <gherold@teachspin.com> - 2017-03-20 17:55 -0700
  Re: bad, better delay circuits John Larkin <jjlarkin@highlandtechnology.com> - 2017-03-22 08:54 -0700
    Re: bad, better delay circuits JM <dontreplytothis173@gmail.com> - 2017-03-22 16:02 +0000
      Re: bad, better delay circuits John Larkin <jjlarkin@highlandtechnology.com> - 2017-03-22 09:47 -0700
    Re: bad, better delay circuits Winfield Hill <hill@rowland.harvard.edu> - 2017-03-22 10:31 -0700
      Re: bad, better delay circuits John Larkin <jjlarkin@highlandtechnology.com> - 2017-03-22 10:52 -0700

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