Hi One very simple question - how good would it do if you just did it all with logic gates? Tri-state buffers and things like that….
Now that you are up to a 100 to 200 ns long pulse, a lot of the fiddly stuff about "can't get a 2 ns pulse through it" goes away. I'm not suggesting you tear up what you have. It's just something else to try and compare. Bob On Dec 22, 2012, at 6:00 PM, Fabio Eboli <[email protected]> wrote: > Hello, Bruce > >> Using saturated transistors as switches in the current source and >> elsewhere isn't conducive to fast switching. >> The traditional arrangement using current mode switches is much >> faster and more predictable. > > This is something I'd like to understand better. > > I'm referring to this schematic here: > http://www.flickr.com/photos/14336723@N08/8293076065/ > Q2 and Q5 are saturating toward the end of the > ramp pulse, when the ramp capacitor C1 starts > to go up. > I was prepared to see the circuit I designed > fail miserably on switch time, but it seem > to be working, as far as I could see on the DSO. > As far I can understand, the fact that Q2 and Q6 > don't saturate, saves the circuit, since > at the end of the ramp, when Q1 and Q5 are > into saturation, Q6 is able to steer the > current to ground, and reverse bias BE (and CB) > of Q5. Is this correct, or I was only > lucky with the specific parts I used? > >> Buffering the ramp with an opamp requires that the opamp settling >> time be known so that the opamp has fully settled before a sample is >> taken. With a charge redistribution ADC that has a sampling switch >> connected to a capacitor array a buffer isnt usually necessary. >> >> Bruce >> > > I was planning to read the voltage with a microcontroller's ADC. > I will set a fixed delay from the PPS rising edge and start > sampling there. To do so I need that the voltage on integrating > capacitor to stay reasonably stable during the delay. > > Fabio > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
