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Share SPI bus between 16 AD7771

mtx4
on Feb 25, 2021

Hello

other question about my new project with 16 AD7771 in a single board. 

I need to interface 16 ADCs with a FPGA saving lines. The sampling process will be simoultaneus than I have to read data from ADCs.

I'll use SPI bus also for read back sigma delta data. Two possible solutions, but I don't know if applicable:

1) Share CS and SCLK lines and dedicated SDI/SDO lines for each ADC -> It is possible share CS line? This is the only concern about this solution.

2) Share SDI/SDO/SCLK lines and dedicated CS line for each ADC -> In this solution the FPGA must write and read in series 8 channels from 16 ADCs, but if this process is slower than conversion time, unread data should be corrupted and lost.

Any advice about this solutions? I'd like the first if applicable, the reading process should be simultaneous.

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  • +1
    •  Analog Employees 
    on Feb 25, 2021 1:31 PM 1 month ago

    Hello,

    Option 1 seems feasable. Option 2 is a normal SPI shared bus, but that would make the read back much slower. You will need 16*8ch*32b=4096 serial clocks. At max fsclk we are talking 136us, i.e. limiting your ODR to 7kSPS. If that´s ok, you can take that approach. If not, you will need option 1.

    Given you can handle several SDO lines, is using DOUT interface not possible in your application?

    Regards,

    Lluis.

  • 0 on Feb 26, 2021 10:05 AM 1 month ago in reply to Lluis

    Hello, 

    thanks for the reply. How can DOUT interface help me to save more lines ?

    I still need SPI for control, in PIN mode I can't set PGA gain with internal reference.

  • 0
    •  Analog Employees 
    on Feb 27, 2021 6:42 PM 1 month ago in reply to mtx4

    Hello,

    I did not mean to save line, but to speed the data transmission. Because you can use several lines from the same ADC. It was just a suggestion.

    Also, do not get confused by PIN mode and DOUT interface. You can be in SPI mode, so you use SPI to write to the part but read the ADC data through DOUT. SPI mode does not imply you have to read the ADC data from the SPI. See table 13.

    Regards,

    Lluis.

  • 0 on Mar 4, 2021 12:49 PM 1 month ago in reply to Lluis

    Hello, thanks but I need to save lines so I prefer to not use DOUT interface.

    I know I can read data from DOUT even if in SPI mode, but SPI mode also allows to read data from the same SPI bus, with the right setup of the Register 0x013. This is the better choice for my design.

    Just last question, totally different: exposed pad conncection.

    The datasheet suggest to connect pad to AVSSx, but I'm using a bipolar +/- 1.65V supply, so this way it will be connected to -1.65V, not proper a good thing. Maybe datasheet refers to the case of unipolar supply.

    I would connect exposed pad to digital ground, it is right?

  • 0
    •  Analog Employees 
    on Mar 4, 2021 1:16 PM 1 month ago in reply to mtx4

    Actually not, the recommendation is to tie the exposed pad to AVSS because that´s to the analog lower level, connected to the substrate. The digital ground would be isolated from the exposed pad.

  • 0 on Mar 4, 2021 1:22 PM 1 month ago in reply to Lluis

    Thank you so much, maybe the datasheet should be clearer, few words about internal connection of the pad.

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