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ADS1292R: Signal noise occurs because of the SMPS.

Part Number: ADS1292R
Other Parts Discussed in Thread: ADS1292, ADS1298R, ADS1298RECGFE-PDK

Hello TI experts,

My customer uses ADS1292R for their own product, and they have a problem.

They use P4(IN1P) and P31(RESP_MODP/IN3P) to measure human body impedance, and they connect 50ohm between these 2 pins when the device is not used.

the problem is, because of the SMPS noise, this 50 ohm is measured little different. (from 48 to 54 ohm)

do you have any idea to eliminate this noise by adding additional schematic on ADS1292R?

I attach a schematic for your convenience.

(they tested 64KHz and 32KHz on ADS1292R, and applied 7Hz low pass filter.-> does not changed.)

(also they tested PWM 100KHz and PFC 100KHz and 90KHz -> does not changed.)

Please check this issue. Thanks.

Best regards,

Chase

  • Hi,

    In the title, you mentions "...because of the SMPS".

    If it's surly from SMPS, probably customer need to solve/attenuate the SMPS noises first. And, make sure the power are clean and move high speed switching components and/or SMPS away from the ADS1292R as well as all the analog lines.

    May I ask dose the customer happen to have/use ADS1292(8)R evaluation kit/board(EVM)? 

    And, do they have a ECG simulator that can simulate the RESP signal?

    Have they tried this experiment using the EVM? How was the outcome?

    Regarding to this, 

    "They use P4(IN1P) and P31(RESP_MODP/IN3P) to measure human body impedance, and they connect 50ohm between these 2 pins when the device is not used.

    the problem is, because of the SMPS noise, this 50 ohm is measured little different. (from 48 to 54 ohm)

    do you have any idea to eliminate this noise by adding additional schematic on ADS1292R?"

    Some suggestions

    0. there are many parasitic and/or non-parasitic components along the path that could all affect the RESP measurement.

    1. please take a look of the ADS1298R EVM user guide, there are design schematics for the RESP and section 5 Evaluation of the ADS1298R Respiration Function

    2. Suggest them try larger resistor, e.g. 330 ohm, 1k, 10k, 100k, 1M to observe and compare the difference.

    3. RESP function is not to be used for exact impedance measurement, instead, it's used to measure impedance changing during/due to respiration. For example, could they put a 330 ohm resistor as a baseline resistance and a 1M ohm in parallel with switch and do the tests? 

    4. They may want to try different combinations of settings for the "Respiration Control Register works in conjunction with Configuration Register 4 (CONFIG4)." to get closer to the outcome they want. Please take a look of section 5.2.2 Software Configuration

    5. They may also need to use some digital Low Pass Filter to improve the outcome, Please take a look of section 5.2.2 Software Configuration Figure 38.

    6. user guide 5.3 Testing with Patient Simulator is also a good reference.

    in the meantime, they can also take a look at -

    https://e2e.ti.com/support/data-converters-group/data-converters/f/data-converters-forum/842528/faq-does-ti-have-an-application-note-that-covers-how-respiration-rate-is-measured/3115352#3115352

    Regarding to 

    "

    (they tested 64KHz and 32KHz on ADS1292R, and applied 7Hz low pass filter.-> does not changed.)

    "

    Where did they apply the 7Hz low pass filter?  Is it analog or digital? 

    User guide uses a 2Hz LPF - "... using a data rate of 500SPS. To obtain better resolution, these results must be low-pass filtered. Figure 38 shows the result after the signal has been processed through a 2Hz low-pass filter."

    "

    (also they tested PWM 100KHz and PFC 100KHz and 90KHz -> does not changed.)

    "

    What does this mean? 

    In the video, are you trying to show the bottom right corner resistance varies between 50 to 54 ohm? Please take a look of the user guide 5.2.2 Software Configuration and the related figures to see how it can achieve. User guide uses a 2Hz LPF.

  • Dear ChienChun,

    Thank you for your support.

    My customer now tests in several ways. Could you check about more questions below?

    1. I checked ADS1298R EVM what you mentioned, especially section 5.

    Could you check the details of 2Hz low pass filter/ (gain, phase, and other details)

    2. in the bottom of page 42, they applied 2Hz low pass filter, but I checked that 4Hz low pass filter is applied on figure 38. Which one is right?

    3. My customer now tests with gain 12 and phase 157.5. Could you check that is there any problem with this setting?

    Best regards,

    Chase

  • Hi,

    1.Could you check the details of 2Hz low pass filter/ (gain, phase, and other details)

    For this product, TI does not provide the detail or support for the digital LPF design. However, there is software source code available in this link -

    www.ti.com/.../ADS1298RECGFE-PDK

    2. in the bottom of page 42, they applied 2Hz low pass filter, but I checked that 4Hz low pass filter is applied on figure 38. Which one is right?

    4Hz shown in Figure 38 might be correct, but customer could try both settings.

    3. My customer now tests with gain 12 and phase 157.5. Could you check that is there any problem with this setting?

    Does customer test this with the EVM board? If yes, in that settings/cases, could they see the respiration impedance or voltage change?

    Did the customer try the settings in user guide section 5.2.2 and/or 5.3.2?

    Gain and Phase are the parameters combination that customers need to try&test&V&V as it heavily depends on the products designs, electrodes materials, wires/cables, skin conditions, environments, etc.

  • Dear Chienchun,

    Thank you for your support.

    my customer installed ADS1298RECGFE-PDK, but we don't have any idea for implementing digital LPF design, because there are many source codes.

    Could you help me where should we refer to make digital LPF, among the code list below?

    Best regards,

    Chase

  • Hi,

    digital LPF design for the EVM was designed & implemented in LabView software GUI not in the firmware c code.

    Customer may need to look into the LabView project folder.

    But, the LabView project is in very old version.

    Thanks

  • Hi,
    Since I did not hear back from you, I believe my suggestions answered your questions.
    I will close this post and if you have any pending questions, feel free to post them here or open a new thread.
    Thanks and have a great day!