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ADS1299: Data settling time of ADS1299

Nikint Kshirsagar
Prodigy 50 points
Part Number: ADS1299

Hi TC,

We are testing input referred  noise performance by shorting all electrode to SRB1.
While we are taking 1st reading(for 40sec, with 500SPS), we are not getting proper output (For all channels).
Refer below image for reference. We are getting result like this when we are taking 1st reading after board power ON. We got this observation always after Power ON the device.


If we take reading after 1st reading(without power OFF) , we are getting proper data from 2nd reading onward.
Image attached in below.
I have few queries on this -
1. Does ADS1299 takes some time to settle data to the output after power ON?

Thank you.

Regards,

Nikint

  • 0
    TI__Genius 11550 points

    Hi Nikint,

    Thanks for the post.

    There will be some settling time for the internal reference after the device is power up. The actual settling time of the internal reference after power-up will depend on the RC time constant formed by the internal resistor R1 (see Figure 25 in the datasheet) and the value of the external VCAP1 bypass capacitor.

    Thanks

    -TC

  • 0 in reply to TCT
    Prodigy 50 points

    Hi TC,

    Thanks for quick reply. 

    We have already implemented a power-up sequence procedure as mentioned in datasheet section number 11.1. Also, implemented the following logic in FW (firmware) for signal measurement -

    1. After board power-up, FW performs ADS1299's power up sequence. (In power up sequence, we put a massive 3 seconds delay to reach the VCAP1 (100uF) voltage to 1.1v as mentioned in power up sequence procedure) 

    2. After power up sequence complete, it put ADS1299 into standby mode.

    3. When user starts EEG measurement, FW wakes up ADS1299 and configures the registers. (As mentioned in datasheet start-up time for internal reference is 150ms, which we have added in FW)

    4. After register configuration, measurement starts.

    5. When user stops measurement, FW puts ADS1299 back in standby mode.

    6. For next EEG measurement cycle, steps 3 to 5 are followed.

    So my queries are -

    1. Does there is delay required after wake-up ADS1299?

    2. What can be another cause for initial settling time in signal?

    Thank you.

    Regards,

    Nikint

  • 0 in reply to Nikint Kshirsagar
    TI__Genius 11550 points

    Hi Nikint,

    The internal reference circuitry for the device is still active when the device is in standby mode. Hence, there is no need for the delay after the ADS1299 is wake up from standby mode. The device wakes up from standby in about 32us typically.

    You can isolate the data settling time due to the device's internal reference settling time by reducing the VCAP1 to 470pF and decoupling capacitors for VREF to 330nF. The data should settle a lot faster with these new test conditions. However, if the data settling remains the same, the settling time is most likely due to other factors. Another debug is to probe the VCAPx and VREP voltages on the oscilloscope and observe the voltages during the AVDD startup.

    Can you also short all the input channels to the common-mode voltage by setting the MUXn[2:0] to 0b001 and disconnecting SRB1 for this test?

    Thanks

    -TC