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LTC6811-2 isoSPI not working

I have managed to communicate successfully with the LTC6811-2 over 4 wire SPI on a custom PCB. However, when I attempt to communicate over isoSPI with a custom PCB that has the LTC6820, I do not get a response from the chip. Below are the schematics for the custom PCB with the 6811

On the board with the LTC6820, this is my schematic - 

I get the following signal on the IPA and IMA of the LTC6811 when using a very large timescale of 100ms and voltage scale of 2V- 

When I zoom into the rising edge of one of those to a timescale of 50us and 1V I see this-

That is valid data coming from the LTC6820 as far as I am able to discern. The DC 5V pulse followed by a slow decay that I can only assume is happening due to the leakage of the C_IPMA capacitor occurs on both the IPA and IMA lines which results in no pulse being coupled onto the isolated side of the communications

Urgent help would be greatly appreciated



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[edited by: sindagi2 at 10:35 PM (GMT 0) on 22 Mar 2019]
  • Hi Chai,

    Can you confirm if your schematic is similar to what is mentioned here?

    This image is taken from page 44 of 6811 datasheet.

    The pulses generated by the 6820 board are roughly around 1V. From your responses, I can only understand that the capacitor might be damaged or there is some irregularity in the connections. Have you tried changing the capacitor and measuring it again?



  • I will swap out the components and try with fresh parts in case the old ones were damaged. I even tried the circuitry on page 79 which describes multi-drop connections as I wasn't sure if that applied only to multi-drop connections which this isn't because right now there are only 2 nodes on the bus, or if it applied more generally to the LTC6811-2 chip. In either case, that did not work either. I will have a new board with many different variations of the circuit present and try out all variations to see which one works. The signal amplitudes are all accurate, I am just at a loss to understand why there is a 5V common-mode pulse on the IPA/IMA lines as shown in the scope captures above



  • Hello sindagi2, we face exactly the same issue with an DC2260A Evalboard  trying to communicate with an DC1941D Evalboard. Standard SPI is working correctly, but with IsoSPI we measure 5V common mode voltages on IPA/IMA lines. Is this a bug in the LTC6811-2?

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