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AD7792 parallel connection on one SPI BUS

Question asked by ricky on Dec 18, 2016
Latest reply on Jan 4, 2017 by JellenieR

Hi All,

One of my customer uses 12ea of AD7792 in his new design.

The 12ea of AD7792 are connected on one SPI BUS line.

In this configuration one or two AD7792s are not be reset (or initialized at power-on) properly. Pull-up resister is 10kohm to +5V rail now.

He once tried to cut the SDO terminals of the device from the bus line and connected to MCU directly, in this test, the two devices be reset satisfactory.

His question is as next;

  • Is the 12ea of AD7792 on one SPI bus too many?
  • Are there any solution with this (12ea AD7792 on one bus) configuration?
  • What do you recommend if the 12ea of AD7792 on one SPI bus configuration is not adequate?

 

AN-1248 says that “In this case, to avoid electrical issues, the recommendation is to not connect the SDO pin to the bus. Alternatively, if it is possible to disable the SDO pin, place a serial resistance with the SDO pin to minimize electrical problems in the first transmission and disable the SDO pin at the beginning.

 

It says the placing of serial resistance between SDO terminals to SDO line.

What resistance value do you recommend?

Do you have reference design circuit diagram which shows AN-1248 recommendation?

 

Thank you for your help.

Ricky

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