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ADS1299 SRB1 and SRB2 functionality in multiple devices

manu mohan
Intellectual 410 points
Other Parts Discussed in Thread: ADS1299

Hi

We are using 3 x ADS1299 for capturing EEG and ECG signal.

I have to know how i can effectively use the SRB1 and SRB2 in multi devices,we are configured the ADS1299 in diasy chanin mode.

  • 0
    TI__Mastermind 38745 points
    Hello Manu,

    The SRB1 or SRB2 can be used if there is a common input reference voltage to which all other input voltages will me measured. If that voltage is meant to be shared across devices, then that reference voltage can be sent to the respective SRB pins on all of those devices. A helpful diagram that shows the functionality of these pins is Figure 19 on page 16 of the datasheet. How were you thinking of using them?

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Intellectual 410 points

    Hello Brian

    We are giving common reference from forehead  for EEG and given to all non inverting pins of the channels.

    Right now we are not planning to give SRB1 and SRB2.

    I want to know whether it is make a problem in my circuit. 

  • 0 in reply to manu mohan
    TI__Mastermind 38745 points
    Hey Manu,

    That shouldn't be a problem. There is no requirement to use the SRB pins for data acquisition.

    Brian
  • 0 in reply to Brian Pisani
    Intellectual 410 points

    Hello Brian,

    Thank you for your support......

    Also i have shared a single oscillator for three ADS1299 devices.

    In this case what are the points i look into it, is it able to drive a single oscillator these three devices.

    May i have to take care in the routing side ?

  • 0 in reply to manu mohan
    TI__Mastermind 38745 points
    Hey Manu,

    In general, we recommend making the clock trace as short as possible and avoiding noisy lines such that noise does not couple on to the clock trace. Driving the clock pin will require the drive strength that is specified for all digital input pins: +/- 10 uA per device. If you were to drive three clock pins from one oscillator, be sure that that oscillator is capable of driving at least +/- 30 uA.

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Intellectual 410 points

    Hi Brian

    Thank you for replay...
    I will take care the clock drive capability in my design.

    I have one more question regarding the multiple device in daisy chain configuration.
    We configured three devices in daisy chain and that time it is challenging to meet the proper setup and hold for all devices.

    The more devices in the chain, the more challenging it could become to adhere to setup and hold times. A star-pattern connection of SCLK to all devices, minimizing DOUT length, and other printed circuit board (PCB) layout techniques helps. Placing delay circuits (such as buffers) between DOUT and DAISY_IN are ways to mitigate this challenge.One other option is to insert a D flip-flop between DOUT and DAISY_IN clocked on an inverted SCLK .


    Above paragraph i copied from the datasheet page number 34.
    Is it mandatory add d flip-flop between dout and daisy in .Can i achieve the setup and hold time by proper placement and software.
    Please give me your suggestion on this case.

  • 0 in reply to manu mohan
    TI__Mastermind 38745 points
    Hey Manu,

    Such a configuration would only be required if you felt that the board level propagation delays were so large that the setup and hold times for the ADS1299 which are specified on page 8 on the datasheet would be violated. By keeping the distances from DOUT to DAISYIN small and ensuring that the distances from the master SCLK out to the SCLK in for all the devices are short and similar, then violation of the setup and hold times should not be an issue. The larger the frequency of SCLK, the closer the edges are together and the more of a factor propagation delay could play into the logic.

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Intellectual 410 points

    Hi Brian

    Hope you are doing well.....

    I have one doubt regarding the bias configuration in multiple devices.

    In our multiple device diasy chain configuration

    BIAS REF pin from three devices connected together and gave option for external reference.

    BIAS INV pins from three devices connected together.Also each BIAS INV pin connected to the BIAS OUT pin of same device through external filter.

    BIAS IN pin of all the device connected to the BIAS OUT  of corresponding  IC.

    And BIAS OUT from one device connected to the RLD of ECG and EEG connectors.

    Is this configuration of BIAS is correct ? 

  • 0 in reply to manu mohan
    TI__Mastermind 38745 points
    Hey Manu,

    The answer depends on what you are trying to do. Are you just trying to provide common-mode cancellation using all the channel inputs as feedback? If that is the case I would look at Figure 51 on the datasheet as an example and use the internal mid-supply reference for device 1.

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Intellectual 410 points

    Hi Biran,

    Hope you are doing well...............

    Last time i discussed you we have project with 3 ADS1299 chip connected daisy chain for EEG and ECG acquisition.

    We received the board and tested internally.Now we sent 3 boards to customer.

    My customer need electrode impedance measurement .

    I tried to connect resistor in range 8K to the channel my input is single ended.And tried to capture the voltage by setting L_OFF register and Config4 register.But i couldn't read the value.You have any suggestion. 

  • 0 in reply to manu mohan
    TI__Mastermind 38745 points
    Hey Manu,

    For making rough measurements of impedance, you're going to want to use AC lead-off detection. Details for that are specified on page 51 of the datasheet. The excitation signal may be in-band for single measurements or out-of-band for continuous measurement.

    Brian Pisani