rohare

Analog and Pseudo-Analog in Papilio

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Hello. I'm trying to solve a problem and I'm wondering if this is the solution.

 

I need voltage comparators. Lots of them. I've gotten to the point where dual and quad comparator IC's would take up multiple desks if I built out the designs I'm experimenting with. I've been trying to keep it 100% analog, but space has become too much of a problem.

 

What I need to know is, can I simulate the action of a single voltage comparator, both input and output? I know that LVDS reciever blocks can act much like a comparator, but I'm sketchy on the details. Can I have an LVDS reciever block with one input (inverting or non-inverting) tied to a voltage rail or ground and the other input attached to either an external pin or the output of another LVDS reciever block? And then, can the resulting output be sent to another LVDS reciever or to another external pin? Or is there a DAC block that would be more appropriate for this?

 

What I'm trying to accomplish are some pretty complex voltage sensing and signaling circuits. One of three different voltages should be produced based on different voltages sensed. This therefore requires that the LVDS blocks should also be able to output different voltages base on how they're "wired". Most comparator IC's have open-collector outputs so you can wire them to have any voltage output. Can I set an LVDS block to output the voltage of one rail, but set a different LVDS block to output a different voltage? In other words, am I confined to only two voltage values, "on" and "off"? I need three, even if one of them is ground.

 

Thanks much for your help.

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I think I remember Alvie or Hamster experimenting with using LVDS for analog input... Maybe they can shed some more light? I know it is possible but I don't remember what the results were, whether it was reliable/practical.

 

Here is something from Hamster's site:

http://hamsterworks.co.nz/mediawiki/index.php/Cheap_Analogue_Input

 

Jack.

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Even if you can make it work, I'm skeptical that there will be significant savings. FPGAs have perhaps a few dozen LVDS pairs on average, and if you were to use small SMD comparitor ICs  you could fit quite a few comparators on a small board.

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