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ADS1298 Writing Registers with Daisy-Chain

Mes
Intellectual 330 points
Other Parts Discussed in Thread: ADS1294R, ADS1298, TPS7A89, TPS7A87, TPS7A88

Dear Alex,

Thank you very much for your continuous support. What is the possible input for CLCKSEL, RLDIN, RLDREF pins?? If on the RLDINV pin of say ADS1294R is shorted to the RLDINV pin of the ads1298, are the rest of the RLD pins (RLDIN, RLDREF and RLDOUT) pins of the ads1294r floating inputs with no connection at all?

Internal respiration circutary with internal clock (RESP_CTRL = 10b):
First question; is the respiration clock generator the same to the ads1294R internal clock generator??

Second , for cascade connection external clock source is inevitable. And hence the internal clock seems inactive. In this configuration, does the internal respiration clock generator inactive too? Is it a must that the GPIO pins should be configured to as a clock input for respiration??

Again I know it is impossible to configure the individual ADS129X if Daisy Chain is used. Only the Device is configured and device register read back (RREG) doesn't work on the rest. Then how do you configure the two devices?? For example how can I configure ADS1294R for impedance measurement if ADS1298 is Device 0 in the Daisy Chain configuration?

If cascade configuration is used, how is the two devices configured independently? For example, one for ECG and the other for impedance measurement?



Kind regards!!

  • 0
    TI__Guru 55322 points
    Hi Mes,

    Happy to help!
    Since we were straying off topic in the old thread I created a new one for us.

    Have you had a chance to search the forum for the answers to these questions?

    1) As posted in the pin function section of the datasheet, RLDIN can be tied to AVDD if not being used. Additionally the RLD amp can be powered down to save power. The rest of the pins can float if unused.
    e2e.ti.com/.../367241

    2) It is a separate clock. Respiration clock frequency can be seen on page 15 of the datasheet, 32-64kHz is reccomended.

    3) Using an external clock is not necessary. It is possible to use the internal clock of one device and route to the next device in the chain.
    e2e.ti.com/.../662475

    4) How to configure daisy chained devices:
    e2e.ti.com/.../665600
    e2e.ti.com/.../611270

    Other threads that may be helpful:
    e2e.ti.com/.../612447
    e2e.ti.com/.../456679
  • 0 in reply to Alexander Smith
    Intellectual 330 points

    Dear Alex,

    Again thank you and so sorry for deviating from the original idea.

    Thank you for the links for the thread, they are very informative. 

    From your answers, I picked up the following points. 

    1. Both cascade and daisy chain connection is possible using the internal clock. Correct me if I am wrong, but in one of the treads, I kind of read that an external clock is necessary for cascade connection (The internal clock of one of the device cannot be used to control the other device). 

    2. The internal oscillator used in the breathing circuit has a different clock from the internal clock of the ads129xr. Hence even if the ads129xr is configured as a slave, it is possible to use the internal ICG circuit for respiration. 

    3. RLDINV can be tied to AVDD. 

    Could you confirm  the above points are correct. 

    Then I have still the following questions:

    1. I am using ads1298 as the master device in the daisy chain / cascade connection (ads1298 as the source of the CLK signal). I intend to use theRLD drive and the internal reference signal. From the data sheet RLDINV and RLDOUT are shunted via 1M Resistor and 1.5nF cap. While the RLDIN is shorted to the RLDOUT. Then what is the value of the RLDREF?? Altium generates error when it is left open and the data sheet doesn't say anything about this pin. 

    2. I also read that

    "RLD signal generated from channels 1, 2, and 3 routed to the N-side of channel 8. Use this feature to dynamically change the electrode that is used as the reference signal to drive the patient body. The corresponding channel cannot be used and can be powered down. 

    So what exactly is the importance of RLDIN pin? For the RLD circutary to work, is it a must to feed the RLDOUT signal to any of the channels? And if the RLDOUT signal is fed to any of the channels and if that particular channel can not be used, how is the ads1298 a 12 leads ECG?? 

    3. Assume the ads1294R is the slave in the daisy chain. If it is  merely used for impedance measurement, all unused pins to should be connected to AVDD directly or through current limiting resistors (say 100K)??

    4. RLDINV of ads1294r is shorted to the RLDINV of ads1298, based on the datasheet. My guess is the RLD of the ads1298 is used to drive both devices. Then, is that OK to leave RLDIN, RLDOUT and RLDREF pins of the ads1294r unconnected? Altium is generating error. 

    Again sorry if the questions are repetitive and unrelated to the specific thread. 

    Kind regards!!

  • 0 in reply to Mes
    TI__Guru 55322 points
    Hi Mes,

    Happy to help as always and no worries, we just do this to make the threads easier to search!

    1) You are correct, I missed that cascade was being used in this setup. For the cascade configuration an external clock must be used.
    2) Correct
    3) RLDIN can be tied to AVDD, not RLDINV. In the pin section of the datasheet, the pins that should be tied to AVDD have a (1) next to them.

    1) From the pinout section of the datasheet, RLDREF is the RLD noninverting input and RLDIN is the RLD inverting input. You connect RLDIN to RLDOUT such that the inverted signal is applied to the patient, thus canceling the noise and improving the signal quality. RLDREF is there to be used as a reference if you would like to see the RLD signal. Altium will always generate a warning when pins are left unconnected, but this does not mean that it is unacceptable.

    2) It is not required to use one of the input channels for the RLDOUT signal - this is simply extra feature that is available.

    3) Connected directly to AVDD is acceptable.

    4) Correct!

    Don't be sorry! These devices are very complicated and I'm happy that you're asking now before you make a board!

    Let me know if all of this makes sense and don't be afraid to reply if you need clarification.
  • 0 in reply to Alexander Smith
    Intellectual 330 points

    Dear Alex,

    Really appreciate your kindness and prompt response. 

    I thank you very much, your points are crystal clear. Just out of curiosity and I know it is possible but I want to use your expertise. What do you think about 3.3 analogue and 2.5 digital voltage supplies? Or do you recommend me of any dual output LDO rated at 1.8V and 3.0V?

    Kind regards!!

  • 0 in reply to Mes
    TI__Guru 55322 points
    Hello Mes,

    I don't see anything wrong with using 3.3V and 2.5V for your supplies. However I would do my best to make sure that my ADC digital supply is the same magnitude as the other digital supplies (Clock, flipflop, MCU, etc) in the system to avoid the need for level translating your SPI communication lines.

    For 500mA to 2A I would recommend TPS7A87, TPS7A88, TPS7A89.
  • 0 in reply to Alexander Smith
    Intellectual 330 points

    Dear Alex,

    I thank you very much. I was planning to use level shifting. But I think I could make 3.3V digital supply and 5V analog supply. 

    Kind regards!!