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ADS1148: 50 HZ filter

Derek Xie
Genius 12255 points
Part Number: ADS1148
Other Parts Discussed in Thread: ADS114S06, ADS124S06

Hi Team:

My customer using ADS1148. They set the sample rate is 40SPS.

Input 0.67V DC and inject 0.33Vpp 50HZ. They verify the output will have 0.066V offset. And the 50HZ CMRR is about 14dB. But the datasheet show much higher rejection ratio. Any idea about it? Could you help me to check it? Thanks. 

  • 0
    TI__Guru* 98025 points

    Derek,

    Using the ADS1148EVM, I get about -14dB also. However, I believe this is the expected result.

    If you look at the frequency response at 40SPS, you'll find this plot:

    The first two frequencies where the magnitude goes toward 0 are at 40Hz and 80z. It's hard to tell on this graph, but 50Hz is definitely above -20dB. If 50Hz filtering is needed, 20SPS will give good rejection of both 50Hz and 60Hz signals.

    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Guru* 98025 points
    Derek,


    After re-reading this post, I gave the normal mode rejection for 50Hz signal using 40SPS, not the common-mode rejection. Are they looking for the common mode rejection (CMRR)? The CMRR should be much better than -14dB, especially if the normal mode rejection is at that level.


    Joseph Wu
  • 0 in reply to Joseph Wu
    TI__Genius 12255 points

    Hi Joseph:

    This is the customer test method. So it should not be the CMRR. It depend on the pure 50Hz filter.

  • 0 in reply to Derek Xie
    TI__Genius 12255 points
    Could you help me to check the 50HZ performance at the 40SPS?
    Why the datasheet show the good 50Hz performance? Thanks.
  • 0 in reply to Derek Xie
    TI__Genius 12255 points
    Hi joseph:
    If ADS1148 can't meet the 40dB 50Hz rejection at the 40SPS, any part can replace it? Thanks.
  • 0 in reply to Derek Xie
    TI__Guru* 98025 points
    Derek,


    From my first post, the -14dB result was actually from data that I took from the ADS1148EVM. It matches the customer's results that they had taken. The datasheet shows good 50 Hz rejection on Table 6 on page 22 of the datasheet. I would note that the good rejection comes only at 5, 10, and 20 SPS data rates. As I mentioned in my first post, a 20 SPS data rate will give good rejection at 50 Hz and 60 Hz.

    If your customer cannot use the 20 SPS data rate, the ADS124S06 (or the soon-to-be-introduced ADS114S06) has a 50 Hz data rate which will have good 50 Hz rejection. However with a 50 SPS data rate, the 60 Hz rejection will not be good.

    It might be helpful to tell me what your customer needs for data rate and rejection. Let me know if you need be to take more data for the ADS1148 or the ADS114S06.


    Joseph Wu
  • 0 in reply to Derek Xie
    TI__Guru* 98025 points
    Derek,


    Additionally, can you please translate the customer's diagram? I cannot read Chinese. Because this is a picture, I cannot cut and paste the Chinese characters to translate them.


    Joseph Wu
  • 0 in reply to Joseph Wu
    TI__Genius 12255 points

    Hi Joseph:

    Thanks for your quick reply.

    Please see the trnaslate one.

    Is it possible to show the datasheet data at the 40SPS 50Hz rejection data? Like 20SPS more clear picture.

    And whether you use the same test condition? Thanks.

  • 0 in reply to Derek Xie
    TI__Genius 12255 points
    Joseph:
    Customer need at least 40dB rejecion at the 50Hz at the 40SPS data rate. Any method can improve the chip(ADS1148) 50Hz rejection ratio?
    Could you share the ADS124S06 and ADS124S06 at the 50Hz rejection data at the 40SPS to me?
    I can't find the data in the datasheet. Thanks.
  • 0 in reply to Derek Xie
    TI__Guru* 98025 points

    Derek,

    My setup for testing the ADS1148 normal mode rejection is a simple one. I use an input signal of ±100mV and the negative input of the ADC is set to AVDD/2. Here is a basic diagram.

    For the ADS1148, 50Hz rejection requires a lower data rate. Again, data rates of 5, 10, and 20SPS will have good rejection.

    For the ADS114S06, you will have slightly more options, but there is no 40SPS data rate. Line cycle rejection is shown in Section 9.3.6.4 50-Hz and 60-Hz Line Cycle Rejection in the datasheet on page 43.

    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Guru* 98025 points
    Derek,


    I would also point out that for the Low-Latency filter with a data rate of 50SPS, the 50Hz rejection is greater than -40dB if the clock frequency or the 50-Hz noise is within 1% of the frequency. Table 16 shows a -33.8dB rejection of 50Hz±1Hz noise at 50SPS data rate using the low latency filter. The rejection becomes -39.9dB if the noise is 50Hz±0.5Hz.

    If you need to specific numbers, you can use the ADS1x4S0x calculator. You can find a link here:

    www.ti.com/.../ads124s08-excel-calc-tool


    Joseph Wu