Connecting two acquisition boards

AgustinSAgustinS
in Other Platforms
Hi!
I am working in my own design of an acquision board similar to openBCI. I am thinking about daisy chaining up to two boards, but those boards are identical. One will work as master and the another as slave, but the working will be similar to the openBCI.

I want to ask for recommendations about the interconnection hardware. What kind of wires and connectors should I use for sharing power, SPI signals and other digital signals between the two boards?

For reference, the gUSBamp model of g.tec uses a similar set-up.

Comments

  • biomurphbiomurph Brooklyn, NY
    It's hard to give you any recommendations without knowing more regarding your system.
    Can you share more details about your design? Design files, etc?
  • AgustinSAgustinS
    Sorry for giving poor information.
    I don't have the design files yet but I could describe the system:

    The board won't be portable but it will be connected via USB with the computer. The power source will be provided with that USB connection and it will be also used for data transmision. Power and data lines will be isolated according to the recommendations for medical devices (IEC60601).

    The main processor will be a 32 bits microcontroler (LPC1769). It will control a ADS1299 chip and the communication with the PC.

    Right know I am trying to figure out how to implement the communication between two identical boards. I don't want to design a different slave board.
    When I connect two boards, in some way one of them should be configured as Master and its processor should control both ADS1299 chips, and the other board should be configured as slave and its processor should be in a "sleeping" or low activity mode.

    I want to connect up to two boards (16 differential inputs configuration).

    I have to decide how I will configure one of the boards as master and the other as slave. The other issue is how to fisically conect the two boards. What kind of connector and wires. I know I have to share power (DVCC, AVCC, GND, as I use unipolar power source) and digital signals: MISO, MOSI, SCLK, CLK, RESET, etc).
    The boards will be put in a plastic case (maybe stackable) so I should consider a easy way to inter-connect two systems.


    *(The system I am designing wont be a commercial product, it is for research)
  • biomurphbiomurph Brooklyn, NY
    The way we have connected the ADS chips is to have the daisy module receive it's clock signal from the main board ADS so that they are on the same clock.
    We reserve a CS pin for each ADS, so that we can set each of the 16 channel settings individually. This provides more flexibility for the user to apply OpenBCI to their purposes.
  • AgustinSAgustinS
    Sure biomurph, I followed your design, I am thinking mine in a similar way.


     
  • AgustinSAgustinS
    What about AVDD?
    If I share power supply voltages with a cable, between the two boards, some noise could affect AVDD and/or DVDD.
    I have read that AVDD should be really quiet.

    How could I protect the connection in order to share a good AVDD signal?
  • biomurphbiomurph Brooklyn, NY
    well, if you must use a cableto connect AVDD and AGND, it would be best to shield it.
  • AgustinSAgustinS
    I am thinking in sharing in one shielded cable the power supplies: AVDD, AGND, DVDD, DGND and de BIASINV signal.
    In another cable I will share the digital signals: SCLK, MOSI, MISO, CS/, RESET/ and CLK (ads1299 clock).
  • JakeStewJakeStew
    Probably best to not be using cables in the first place.  Just stack the boards with headers connecting them.
  • AgustinSAgustinS
    You are right. The problem is that my boards will be each one in its own plastic case and this make it difficult to simply stack the boards.

    The project leader has suggested to share the non-isolated 5V power supply and isolate and regulate it in each board (the boards are identical). I am working in it righ now. I think that I should algo share the isolated digital and analog grounds and digital signals.

    I think the design is becoming a bit complex.
  • biomurphbiomurph Brooklyn, NY
    If you can share the digital power, or as you say, the 5V supply, and then isolate and regulate the analog power supply on both boards, that would be much better than trying to connect the analog supplies with wires.
    All of the digital signals can be wired, no problem, but best to shield, so they don't act like antennas.
  • AgustinSAgustinS
    Thanks Biomurph!
    I will go ahead and I let you know about the results.

    My next steps are working in the pcb and routing, and thinking about firmware.
  • AgustinSAgustinS
    Thanks to @JakeStew also!
  • JakeStewJakeStew
    One guy on a website I came across was working with the ADS1299.  His idea was to use SATA cables as connectors.  The idea is that these are a good cheap source of tightly specd wiring and the conenctors are readily available.

    You might try something along those lines.
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