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ADS1299: ADS1299 connection

Alwyn DSouza
Intellectual 360 points
Part Number: ADS1299

Hi,

I have a few questions regarding how to connect the ADS1299

1. What are the pro's and cons of Referential montage vs Sequential montage for the EEG leads?

2. What are the pros and cons of Cascade connection and Daisy connection of the SPI connections?

3. What is the function of the BIASIN pin, is it meant just for measurement of the bias signal level going to the patient or does it have an additional function.

4. The function of the SRB1 pin is show in the datasheets for referential montage where SRB1 becomes a common reference electrode? However no such function is shown for the SRB2 pin, under what conditions can we use the SRB2 pin are there any application examples for that?

5. I have have multiple chips on a single board for example 2 ADS1299 how does the Biasing connections work, I would like to know how to combine the biasing of both the ADS1299 chips to a common bias electrode that is connected to the patient?

Hope this is not exhaustive, I just needed to get my mind clear for the ADS1299 circuitry before make a PCB.

Regards,

  • 0
    TI__Guru**** 168445 points
    Hi Alwyn!

    Thank you for the great ADS1299 questions! We're on Easter break till Monday the 22nd. One of our Bio-Potential experts will be in touch with you after the holiday weekend. In the meantime, please do have a look at some of the frequently asked ADS129x questions here:
    e2e.ti.com/.../772058
  • 0
    TI__Guru 57296 points

    Hi Alwyn,

    Thank you for your post. I will try to address each of your questions briefly to help you get started:

    1. To the ADC itself, the only performance difference I can think of is that sequential montage will have better common-mode rejection. This is because each signal will see the same signal path from electrode to PGA input pin. Other than that, how you measure biopotential signals depends on the application and the relevant information that you're looking for. We cannot give much advice in this area as we are not the medical experts.

    2. Both methods allow you to reduce the number of connections required between the ADS1299 device and the host MCU. Cascade interface allows you to share all SPI connections except for /CS. You must use a separate /CS signal for each ADS1299 so that only one device is actively communicating with the host at a time. Daisy-chain allows you to read data from multiple ADS1299 devices without using any additional pins.

    3. Please read sections 9.3.2.4.1 and 9.3.2.4.2 in the ADS1299 datasheet

    4. The original intention behind the SRB2 pin was to allow you to dynamically select the desired signal to be your reference for the other channels. The signal on INxP can be connected to the SRB2 pin on a channel-by-channel basis. Other users have chosen to use this pin as a way to do some channels in referential montage and other channels in sequential montage as opposed to using SRB1, which forces all channels to be referential montage.

    5. Please see Figure 39 in section 9.3.2.4.5.1 of the datasheet.


    Best regards,

  • 0 in reply to Ryan Andrews
    Intellectual 360 points

    Many thanks for the comprehensive answer Ryan! Highly appreciated!!

    I'm going thru each point you have listed and getting to digest it.

    I came across one more point, if I have a system that has multiple ADS1299, how do I get the bias circuit to work.

    I have read section 9.3.2.4.5.1 in the datasheet."Bias Configuration with Multiple Devices". which shows to connect BiasINV pin to all the ADS1299 and power down all except one Bias amp inside the ADS1299, So apart from tying all the BiasINV pins together, is the Bias generation circuit similar to  Fig. 68. inthe Datasheet.

    Many Thanks in advance!

  • 0 in reply to Alwyn DSouza
    TI__Guru 57296 points

    Hello Alwyn,

    Yes. However, keep in mind that it is likely not necessary, nor practical, to select every electrode. Each additional electrode that you enable and connect to BIASINV for common-mode derivation will also increase the gain of the BIAS amplifier. Eventually, you might saturate the amplifier's output. You can always make the connection in hardware with traces between all BIASINV pins and decide later which electrodes to connect using each device's register settings.

    Regards,

  • 0 in reply to Ryan Andrews
    Intellectual 360 points
    Hi Ryan,
    I guess I didn't express myself correctly there.
    My question is regarding generating a bias out signal when we are using multiple ADS1299 devices in either daisy or cascaded mode.

    With reference to that

    I have read section 9.3.2.4.5.1 in the datasheet."Bias Configuration with Multiple Devices". which shows to connect BiasINV pin to all the ADS1299 and power down all except one Bias amp inside the ADS1299, So apart from tying all the BiasINV pins together, is the Bias generation circuit similar to Fig. 68. inthe Datasheet.

    Do I need to combine Fig. 68 of the Datasheet and the one in section 9.3.2.4.5.1 to get the final Bias out signal that would be connected to the bias electrode?
  • 0 in reply to Alwyn DSouza
    TI__Guru 57296 points

    Hello Alwyn,

    Yes, sorry for not understanding your initial question.

    The circuitry shown in Figure 68 is only needed for the BIAS amplifier which is powered-on. This is would be the BIAS amplifier in Device 1 in Figure 39.

    The BIASINV pins of the remaining devices are shared with the BIASINV pin of Device 1 to increase the number of possible PGA outputs connected to the summing junction. Remember that not all PGA outputs can be selected at the same time; otherwise, the gain of the BIAS amplifier in Device 1 will be too high and the output will saturate.

    Best regards,