High frequency digital inputs to FPGA cannot be detected correct

[jack-Lee]

Hi Billingsley,

Thanks for your patient replay and discussion. 

Sorry, I don't ask my question clearly.

If we have 1-bit signal input parallel with a 50 ohm resistor in bread board, we can get correct result. So we think this might be an impendance matching issue.

I try to figure out the difference between PCIe-7842R and FlexRIO. I have read the adapter datasheet (http://sine.ni.com/nips/cds/view/p/lang/zht/nid/209099). The matching impendence is 50 ohm or 100 ohm according to different module while the input imdendence of PCIe-7842R is high impendence. The FPGA chip both are Vertix-5.

For impendence matching, we try to have PCLK be parallel to a 50 ohm. While we think the signal will drop down to half, we have 2x amplifier circuit to compensate it. But it's still wrong.  Is there anything I lost for PCIe-7842R to make this impendance maching issue still go wrong?

Thanks.

[Ting1224 · Accepted solution]

The difference between PCIe-7842R and FlexRIO are not only the impedence.

The front end module of FlexRIO has it's own hardware clock and it has it's own signal conditioning circuit.

If you are building your own circuit to maximize the accuracy up to 20 MHz, there is really no suggested way to do it.

Also, I think it's beyond the application spec regard the R series according to the NI R&Ds.

I am sorry that I couldn't give you any further suggestions regard your application.

[cychang0829]

Hello Ting,

Thank you for your kind patience for this long discussion. If there is a special circuit for FlexRIO while PCIe-7842 doesnt have. We have to think about some other ways to build our application, maybe considering a new FlexRIO hardware.

anyway. Thanks all the people in this discussion again.