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ADS1292R: Respiration circuitry Modulation clock

Maharajan Maruthupandi
Prodigy 80 points

Part Number: ADS1292R

I am using ADS1292R for Patient ECG and Impedance measurement. For impedance measurement, I am using external clock for Modulator/Demodulator.

For that  I am using GPIO1 and GPIO2 which are operated at same 25Khz frequency with 50% duty cycle but in a delay of 10usec (90 degrees phase delay).

Is that violating the spec of that modulator/demodulator circuitry?

Regards

Maharajan

  • 0
    TI__Guru 59882 points

    Hello Maharajan,

    Thank you for your post and welcome to the forums!

    Yes, 25kHz violates the specification for the impedance measurement circuitry. This can be see on page 4 of the datasheet under respiration. For an external source, using either 32kHz or 64kHz is recommended. This is discussed in length on page 56-59 of the ADS1292R datasheet.

  • 0 in reply to Alexander Smith
    Prodigy 80 points
    Hi Alex,

    Thanks for the reply.
    But in our design we use the same configuration as I mentioned before. It is working well and we could able to perform the impedance measurement. If it violates, how it will impact on our measurements?
    Also if I use 32KHz frequency, can I use the same frequency for both Modulation clock as well as the Blocking signal with a phase delay of 90degree?

    Regards
    Maharajan
  • 0 in reply to Maharajan Maruthupandi
    TI__Guru 59882 points
    Hi Maharajan,

    Happy to assist!

    If it is working for you, great! The only thing to keep in mind is that we only guarantee performance for the specs that are listed in the datasheet.

    As you can see from figure 58. & figure 59 of the ADS1292R datasheet, the noise vs. impedance curve changes based on the frequency of the modulation & respiration clock frequency. These curves being accurate are important when calculating your normalized resistance.

    Figure 55. and Table 16. show the relationship between the modulation clock and blocking signal.
  • 0 in reply to Alexander Smith
    Prodigy 80 points
    Hi Alex,

    Thanks for the response.
    From the datasheet, what I understood is that the frequency of the blocking signal should be twice that of the modulation clock.
    Suppose if I give 32KHz modulation clock, I should give the blocking signal as 64KHz with the phase of 168.75degree max. Please correct if I am wrong.
    Also please confirm that the phase delay which is referred in the Table16 is actually the duty cycle of the blocking signal.

    Regards
    Maharajan
  • 0
    Prodigy 80 points

    Hi Alex,

    I little bit confused on your last statement.
    I referred TI's App note "Respiration Rate Measurement Based on Impedance Pneumography" (SBAA181–February 2011) for understanding the concept "Demodulation with Blocking" which is explained in the Section3.4 in the App note.

    From that app note I understood that the frequency of the Block signal should be twice (always) that of the Modulation clock irrespective of the Phase (Duty cycle). Please refer "Figure 9. Square Wave Demodulation with Blocking " in the app note.

    Regards
    Maharajan

  • 0 in reply to Maharajan Maruthupandi
    TI__Guru 59882 points
    Hello Maharajan,

    You are justifiably confused by my last statement, as it is incorrect. Please disregard my previous post and I apologize, I was confused in regards to phase delay vs duty cycle.

    You are correct that the blocking signal should be twice the frequency of the modulation clock.

    As a gentle reminder: if 25kHz is working for you that's great - but none of the other specs can be guaranteed when using a frequency out of spec.