Moin, I do understand the advantages of having differential input. But my problem is that we are really sensitive to delay differences between channels. And I mean _really_ sensitive.
The TDCs will have a total usefull range of about 100-200ps. The input pulses will arrive within this time frame. There is some provision in there to calibrate delay differnces between different paths, but it's not much. If there is an electronic component in the path, that has a delay variation between different devices of 1ns (like the ADN4661 that Bruce recommended), this would mean we would need somehow extend our measurement range to at least 2ns to be able to compensate for this delay variations. This >10 fold increase in measurement range means that we need more circuitry after the TDC, ie the digital circuitry would increase by more than a factor of 40 (grows with approximately n+n*log(n)) and that will cause us all kinds of other headaches. This sensitivity to delay variations also means that we have to minimize the number of components in the signal path. Hence things like having the option of selecting single-ended or differential is would degrade our signal path too much. The multiplexer alone would add too much delay variation. As the signal is distributed to multiple boards and the number of connections we need for that is already large enough, i want to keep the signal going between the boards single-ended. This also makes it easier for us to find cables that are withing a few ps (a non-representative sampling of cables of same type i had at hand gave me <30ps variation between cables) On Tue, 28 Mar 2017 02:00:11 +0200 Gerhard Hoffmann <[email protected]> wrote: > Somewhere you'll have to produce sharp differential edges and this > semi-analogue stuff is probably hard on a chip shared with thousands of > other gates and a digital process. Going differentially into the chip will > reduce the effects of ground bounce etc a lot. It's a full custom ASIC, with only the circuitry to support the TDC around it. The digital logic after the TDC are only there to convert the thermometer code we get from the TDC into binary. There is also a bit of arming logic and various other small bits, but nothing substantial. We are also going to split the power supplies on the chip for the TDC and the digital part, to isolate them even further from eachother. > This is what you can expect from an ADCMP580 on a homebrew > board and soldered-in semi rigid cable: I was actually thinking about the ADCMP572. The datasheet unfortunately does not specify a variation of delay between devices, not even a maximum propagation delay. Guessing from the typical propagation delay of 150ps, the 35ps rise/fall time and the 10ps deterministic jitter, it is probably safe to assume that the device to device variation is in the order of 20 to 100ps. This means, in best case this device will work, in worst case we would need to enlarge our measurement range to at least 400ps. I am now trying to figure out whether we have any other posibilites to get path delay variations down. > Some will cry: eeek - a comparator, and an ECL or CML one at that, but > in the end > it is just a differential amplifier like that Wenzel design, made by > people who > know their SiGe process. I've seen worse :-) We are currently designing CML in a pure digital, Si 65nm process (nothing fancy, not even SOI). So, having a CML chip as input source isn't going to kill us, quite to the contrary. > If you have a slowly rising source, you'll need multi-stage slope > amplification > and low pass filtering in Collins style anyway which will be safer to do > off-chip. Slope is reasonably fast. I was thinking about using an LMK00101 into an 50R load (~10cm of coax). This should provide us with <500ps of rise/fall time. > If you can specify the interface, go for something speedy, then > the guys who do the the signal source have the hard time and > can be pointed at. :-)) The "guys who do the signal source", that's me :-) As I am the only one in the group with significant experience in both analog and digital electronics, I have to not only come up with a feasible overall design, but also figure out solution for all these pesky details. Attila Kinali -- It is upon moral qualities that a society is ultimately founded. All the prosperity and technological sophistication in the world is of no use without that foundation. -- Miss Matheson, The Diamond Age, Neil Stephenson _______________________________________________ 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.
