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ADS1299: -3dB Bandwidth vs Input-Referred Noise

Jejomar Ildefonso
Genius 9490 points
Part Number: ADS1299

Hi Team,

Can you help me interpret the -3db bandwidth indicated within Tables 2-4 of the ADS1299 datasheet?

Based on the electrical characteristics (DC Channel Performance), the input-referred noise of the device is at 1 μVPP at the 0.01-70Hz range (Gain=24).

While the input-referred noise of the device (transmitting at 1000SPS for example) on Table 4 is specified at 1.97 μVPP at 262Hz (-3db bandwidth).

Setting these values as an example, can you help me identify the allowable frequency range in which the input-referred noise stays within the 1.97 μVPP value with respect to the -3db bandwidth given?

Thank you so much in advance.

Kind Regards,

Jejomar

  • 0
    TI__Guru**** 180961 points

    Hello Poster!

    Our support staff is taking time with their families for the Independence Day holiday here in the US.  It may be July 8th before we can get back to you with help on your query.  We apologize in advance for the delay and will get back to you as soon as possible.

  • 0 in reply to Tom Hendrick
    TI__Guru 66346 points

    Hello Jejomar,

    Thanks for your post.

    The -3-dB bandwidth is set by the digital filter's sinc3 response, which scales with the output data rate. In the Electrical Characteristics table, the 0.01 Hz to 70 Hz noise data was collected at 250 SPS as indicated at the top of the page. This is nearly the same result as shown in Table 4 with Gain = 24 V/V and data rate = 250 SPS. The only difference in the data sets is that the "10-second" data contains more samples (2500 vs. 1000). I'm not sure why we specified the upper bandwidth limit as 70 Hz in the Electrical Characteristics table - perhaps some additional post-processing was done to band-limit the data further. However, the -3-dB bandwidth is still 65.5 Hz in both cases as set by the digital filter in the ADS1299.

    You can use Tables 1-4 to find the data rate vs. gain options that will give you < 2 uVpp of noise.

    Best regards,

  • 0 in reply to Ryan Andrews
    TI__Genius 9490 points

    Hello Ryan,

    Thank you so much for the valuable response. Just to have an exact proper figure, when I choose to transmit at 1000 SPS at Gain =24, does this mean that the appropriate frequency range to have the constant noise will also be from 0.01Hz to 262Hz?

    Kind Regards,

    Jejomar

  • 0 in reply to Jejomar Ildefonso
    TI__Guru 66346 points

    Hello Jejomar,

    The upper frequency limit is correct - that is, you will have 262 Hz of -3 dB bandwidth before the sinc3 filter rolls-off. Theoretically, you can see frequency content all the way down to DC. We showed a plot in the data sheet for 10 seconds of data to include at least one full period of a 0.1-Hz signal. This demonstrates the peak-to-peak noise amplitude was still less than 1 uV.

    Best regards,

  • 0 in reply to Ryan Andrews
    TI__Genius 9490 points

    Hello Ryan,

    The 10 seconds of data indicated in the datasheet was extracted from a 250SPS operation. Would this be similar with a 1000SPS trasmission rate assuming all other operating conditions were the same?

    Kind Regards,

    Jejomar

  • 0 in reply to Jejomar Ildefonso
    TI__Guru 66346 points

    Jejomar - at 1000 SPS, the -3 dB bandwidth is 4x wider. The peak-to-peak noise will be higher due to higher-frequency noise content. However, if data were collected for 10 seconds, the contribution of 0.1 Hz noise to the total noise would not be any different.

    Regards,

  • 0 in reply to Ryan Andrews
    TI__Genius 9490 points

    Hello Ryan,

    Thanks a lot for the help!


    Kind Regards,

    Jejomar