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High speed comparators are rather expensive and speed is what FPGAs are very good at. On the other hand, FPGAs (in my case: XC3S400) have paired differential pins in each bank that their voltages are compared ( At least I think so !). They also have Vref for single ended standards that may act as a comparator.

I want to know if I can use those differential I/O pair pins as a comparator -and if so- how should I do that ( Should I connect a vref and use single ended standards or simply connect two voltages to differential I/O pins ?)

EDITION: I tried it and works excellent !!!

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  • \$\begingroup\$ If you stay within the documented voltage limits it ought to work after a fashion - precision may be more like 50-100mv than 1mv. How precise do you need? \$\endgroup\$ – Brian Drummond Nov 10 '13 at 11:24
  • \$\begingroup\$ @BrianDrummond precision is not a big concern, just converting a sine wave to logic level square wave. \$\endgroup\$ – Aug Nov 10 '13 at 12:56
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    \$\begingroup\$ @Aug, that is how all the ADCs in the world work! \$\endgroup\$ – FarhadA Nov 11 '13 at 15:10
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Yes you can. There are some applications notes using the differential pairs inside an FPGA as a low cost ADC.

There is a very good document describing this that you can use for your design:

Analysis on Digital Implementation of Sigma-Delta ADC with Passive Analog Components

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  • \$\begingroup\$ the link does not work for me, do you have an alternative? \$\endgroup\$ – David Nov 10 '13 at 21:41
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    \$\begingroup\$ Thank you for your guidance! I tried it and works nice. Now I have a very high speed comparator at no more cost ! \$\endgroup\$ – Aug Nov 20 '13 at 14:02
  • \$\begingroup\$ Glad it worked, I used it long time ago and was surprised how easy it was to implement a simple ADC inside the FPGA without any active components. I saved a lot of time and money doing so :) \$\endgroup\$ – FarhadA Nov 20 '13 at 21:36
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    \$\begingroup\$ That is total BS Jon, I have been using it for over a decade. Just becasue a smart ass decided to patent it does not mean it is their design, I can show you at least 3 industrial application on the market for the past 10 years or so that uses this simple ADC idea. \$\endgroup\$ – FarhadA Mar 30 '16 at 7:16

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