Good day!
I am Fulcrum, a new member here at the EEVblog. I've been following Dave Jones' youtube channel for some years, and have learned lots, but now I need a bit of help!
So at the core of this problem lies the
ADCMP553, a low voltage positive emitter coupled logic (LVPECL) comparator. I have made pulse stretchers from comparators before, following
this handy guide. Works a charm.
However with the ADCMP553, it's not so trivial.
First of all, I am unsure of how the latch inputs work on an ECL device. The datasheet says (page 3) that the latch enable inputs will draw anything from -150µA to 150µA. I find this hard to understand. Is it simply not possible to say if the inputs will source or sink current?
Now, what I thought I could do, is get rid of the discharge resistor R1 in the circuit diagram in the article I linked. The datasheet tells me that the latch inputs have internal pullup resistors, and that they source/sink 150µA(Or do they?). So the latch inputs will only latch the output as long as they have some current to draw/sink from/to the capacitors. When the capacitors have been depleted, the latch inputs will technically be floating. The datasheet tells me that floating latch inputs default to their unlatched state. All well and good! So I will simply have to choose capacitor values that give me a latched comparator for the time I need, which is about 50ns. Another way to think of it is that the capacitor couples the AC components of the comparator output to the latch input, while blocking DC.
However, I don't like building circuits without performing simulations to give me confidence that they will actually work as intended. And since there are no simulation models available for the ADCMP553, that's not so easy! Therefore I would very much like to hear your opinions on this. And I can't choose the obvious, easy solution, of replacing the ADCMP553 with a different comparator. The pulses I am going to detect are ~1ns long, and most non-ECL comparators have minimum pulse widths of 2-3ns at best.