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USB-8451 SPI transmission distance.

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What is the maximum distance that the SPI bus can transmit and receive data from a slave sensor? It must depend on a few things like the clock frequency and types of buffer. What buffer do you recommend at the slave end?

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Hi Peter

 

You may find the following links useful-

http://www.ni.com/white-paper/9119/en/

http://www.ni.com/white-paper/6975/en/

http://www.ni.com/product-documentation/7448/en/

 

I hope this helps!

 

Thanks,

 

Sarah

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Okay, but I can find no discussion of how long a cable can be run between the ADC in a sensor and the USB-8451.

For example:

http://www.ti.com/lit/an/slyt441/slyt441.pdf

 

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Although I can find no definite length, as you said, it will depend on multiple things, it seems that if people are using an un-shielded cable, then they start having problems around 3ft long, and shielded you can have the cable a bit longer. However this is very dependent on environment, which is why we can make no guarantees

 

Thanks

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Okay Sarah, so probably safe at 0.5m. Which may be okay for some initial testing and develop a longer distance solution based on line drivers, clock feedback etc as in the TI document. The NI part could then be an FPGA card as we have many sensors.

 

Peter

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Based upon the TI information: SPI running at 100KHz should have around 100 ohms series termination along with a 1nF capacitance to ground.  This would imply that the desired frequency should be about:

Transmission Rate: ~ { 100 * [ 1 / (R * C) ] }

For the longest possible cable; keep to a transmission medium that has a controlled impedance matched to the R value to help control any line reflections. 

 

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