Arty Board and SMA Connector

[Fields]

Greetings!

I am using an Arty board (xc7a35ticsg324-1L), and I want to receive an external 
clock signal which comes from a signal generator.

To receive this signal, the Arty board needs a SMA connector, and I recall the
Pmod AD5 has two of them:

http://store.digilentinc.com/pmod-ad5-4-channel-4-8-khz-24-bit-a-d-converter/

Could the Pmod AD5 be a good choice for my purpose? What would you recommend me?

Thanks
Fields

[D@n]

@Fields,

Is your clock between 0 and 3.3V with respect to the Arty's power and ground?  If so ... why not just wire the clock straight into the port without using the PMod at all?

Dan

[jpeyron]

Hi @Fields,

If your needs for sample rate and resolution fall in line with the PmodAD5's specs then it would be able to do what you want. Unfortunately, we currently do not have a demo/tutorial/library or IP core for this Pmod. The PmodAD5's resource page is here. We currently have not had the time to create a demo/library/tutorial or IP core for the PmodAD5. If you were to use the PmodAD5 it would fall on you to design the hdl or IP core to use the PmodAD5. If you decide this is what you want we will be here to help you with this process. If you can do as @D@n suggests this would be a much quicker/easier process. We do have a XADC project for the Arty here that should help as well. 

cheers,

Jon

[Fields]

Thank you both for the advice,

At first I was looking for a SMA connector to the FPGA. Actually that is the only reason why I was considering using that PMod to receive the signal, but using it could make my design unnecessarily dificult.

I much prefer to wire the clock straight into the port without using the Pmod as @D@n suggests. But the Arty board hasn´t any SMA connectors and I need one to receive the clock signal.

Is there any way to input the signal straight into the Arty without using a PMod with a SMA connector?

Thanks

Fields

[D@n]

@Fields,

Sure!  Take your knife, cut off the end of the SMA cable and then separate the sheath from the center of the SMA cable.  Wire the two into opposite ends of a differential connector.  Alternatively, you could do the same sort of thing with an SMA end, such as this one.  The result of that would be that you can then plug (or unplug) your device without tearing a cord up.  Alternatively, you could create a small device/circuit board that connects an SMA connector to a PMod.  Either way, you'll have a touch of soldering to do.  If this sounds scary to you (it does to me ... I'm a software type), I'm sure you could find individuals on this forum who'd be glad to do it for you for cash. 

Dan

[Fields]

@D@n,

Impressive! That is a great notice, I was really worried about that, but it looks like very different now.

By the way, should I worry about the output (and input) impedance between my signal generator and the arty board?

The output impedance of my signal generator is 50 ohm, and I can´t find the input impedance of the arty board in the Arty Reference Manual
   https://reference.digilentinc.com/reference/programmable-logic/arty/reference-manual

I don´t want to damage the board. Should I be worried about the input impedance of the arty board?

Thanks

Fields

[D@n]

@Fields,

Should you be worried about impedence?  Yes.  How much?  Well, an impedence mismatch can send a reflection of your waveform back to the Arty.  If your power is low enough, this shouldn't be too big of a problem, but ... it might corrupt what you are doing somewhat.

This is really getting outside of my comfort zone, though, so I'd have to depend upon others, @jpeyron maybe?, to give you an official answer.

Dan

P.S.  I'm thinking of starting up a project that would demonstrate how to create an arbitrary clock signal from FPGA logic.  Judging by the Artix-7 data sheet, it should be possible to use logic to generate anything between about 19MHz and 160MHz with 1.25ns precision on the clock edges, although it would cost a PLL or two ... should be a fun project.

[jpeyron]

Hi @D@n and @Fields,

I will do some research tomorrow to better answer you question regarding input impedance and potential damage to the board.

thank you,

Jon

[jpeyron]

Hi @D@n and @Fields,

As long as you follow the voltage requirements for the inputs you will not damage the board. In this case impedance has to deal with reflections. Depending on what frequencies you are trying to input into the arty it can cause issues with signal integrity. I linked a couple forum threads that might be useful if you are trying to input higher frequencies here and here.

cheers,

Jon