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ADS1299: Daisy chain questions

Husein Gonzalez
Prodigy 30 points
Part Number: ADS1299

Hi there,

I am designing a platform that might daisy chain up to 4 ADS1299 for EEG capture. My plan was to use one single CS to rule them all, and I have some questions regarding the connections:

1) At page 65 of the datasheet you have the following statement: 

The BIAS driver cannot be shared among the multiple devices and an external clock must be used because the individual devices cannot be programmed when sharing a common DIN

But I read at "9.3.2.4.5.1 Bias Configuration with Multiple Devices" that : 

Use the PD_BIAS bit to powerdown all but one of the bias amplifiers when daisy-chaining multiple ADS1299-x devices.

If I connect the 4 IC in daisy chain how can I turn off only one of the BIAS amplifiers? What should I do with the unused BIASOUT signals?

2) should I use the internal clock of one of the IC to provide a clock signal to all of the ADS1299?

3) what should I do with SRB1 and SRB2?, should I short them all or leave independently?

Thank you for your help

Best regards

  • 0
    TI__Guru 62196 points

    Hello Husein,

    Welcome to our forum!

    1. That comment on page 65 seems a bit unclear to me as well. In order to power down all but one BIAS amplifier, you will need a separate GPIO to control the other device /CS pins. If only one device will be different from the rest, perhaps you only need two GPIOs: one for the first device (i.e. with BIAS turned on) and another for the remaining three devices (i.e. with BIAS turned off). For each device which needs unique register settings, you will need a separate GPIO to control that device /CS pin. When reading data, however, you could enable all /CS pins simultaneously and read the data serially through a single DOUT pin.
    2. In order to synchronize all four ADS1299 devices, you will need to use a single master clock source. One way to do this is to output the internal clock from one device and connect it as an input to the other three, just as you described. Alternatively, you can use a single external clock source for all four devices.
    3. If SRB1 and SRB2 are unused, you can either leave them floating or tie them to AVSS. Internally, they will not be connected to anything unless you configure the MUX to use those pins.

    Best Regards,

  • 0 in reply to Ryan Andrews
    Prodigy 30 points

    Hi Ryan,

    Thank you for your answer!

    Should I treat the BIAS Drive as an unused OpAmp and connect the output to the inverting input? Or should I just leave the Bias Out floating?

    Best regards.