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ADS1298 WCT external capacitance

Other Parts Discussed in Thread: ADS1298

We are making an own ECG board , and currently we have some noise issues on the CHEST channels.

As an error search we go around the elements which are connected to the WCT, chest channels, power supply, etc.

What i found is that the noise almost disappears if i took out then external 100pF capacitance.

I attach some pictures about the external test signal we connect on the board.

Did you have some experiences with noise regarding this external cap?

It might happened that the noise is coming from the AVSS, as this cap is connected to that net.

A bigger resolution :

And the situation without the external cap. on WCT:

Thank you.

Best Regards,

Lou

  • Hey Lou,

    I have never heard of any issues regarding the capacitor at the output of the WCT amplifier. That capacitor band limits the output of the WCT amp so it's important that you try to keep it in the design. I suppose it is possible that some AVSS noise is causing the interference that you are seeing. Can you tell me at what frequency that interference occurs? Are there any digital signals routed near the AVSS terminal of that capacitor?

    Regards,
    Brian Pisani
  • Dear Mr. Pisani!

    Thank you for these information.

    I will re-check the board design, and the AVSS routing.

    Currently we cant see in which freq. are the noises, we need to add a save option in our desktop app.

    Thanks!

    Best Regards,

    Lou

  • Hi Mr. Pisani!

    It looks like that we had a some problem in a 2.048MHz CLK buffer IC,when i take out that buffer IC and driver the CLK direct to the ADS, the noise gets to minimum value.

    It looks like the buffer IC  generates some noise on the AVSS (as it is also powered from the AVSS).

    So its looks like, we have to solve this electrical problem.

    I have an other question.

    On our channels we put also filters as it is on your PDK (22k47p and 10k 47p).

    On our first demo board on the RLD line we dont leave place for the 22k43p+10k43p, but only for the patient safety resistor (100k), with which i put manually a 25nF capacitor.

    I make 2 tests, : RLD line with only 100k||25nF, and i place manually after this 100k part 22k47p and 10k47p, as it is on your PDK( but with this additional 100k||25nF).

    As i saw from the resouts (curretly still graphics reading), the signal is more noisy when this additional 22k||47p + 10k||47p is in the line.

    A picture if only 100k+25nF in RLD line:

    A picture if 100k||25n + 22k||47p + 10k||47p:

    I'm thinking can this change be as a results of the different resistance value?  I mean in the first version between the ADS RL leg ad the patient is only the 100k resistor, but in the second version the value is 100k+22k+10k= 132k...

    So it looks like we get better signals if we leave out this "anti aliasing filter".

    My second question is, why is this filter recommended for the RL line?

    Best Regards

    Lou

  • Hey Lou,

    There is really no need for an anti-aliasing filter on that line. That was an oversight. In fact, the shunt capacitance could create problems for the amplifier's stability, so I recommend removing it. You may choose to keep the resistor for patient protection.

    Regards,
    Brian Pisani
  • Dear Mr. Pisani!

    Thank you for the information!

    Than i will keep in the RLD line only the 100k patient protection resistor with 25nF in parallel , and between the RLDINV and RLDOUT pins the 390k||0.01uF .

    Best Regards

    Lou

  • Hey Lou,

    Why did you choose that combination for the RLD feedback (390k||0.01uF)?

    Brian
  • Hi Mr. Pisani!

    Actually its a mistake in the design.... i should put 392kOhm(as it is in the sbau171d.pdf).. but by mistake it is 390KOhm(typo error)..so our current board is with this 390kOhm value. I will change it on the next version PCB on 392kOhm.

    Than you!

    Maybe I never spot this error if you don't mention it...

    Best Regards,

    Lou

  • Hey Lou,

    Are you planning on setting any of the bits in the BIAS_SENSP or BIAS_SENSN registers to provide input feedback for the RLD amplifier? If so I might chose a different RC value than what is in the EVM since that cutoff will not allow for very good cancellation at the utility frequency and thus not allow for the RLD amplifier to provide any help to common-mode rejection at that frequency.

    Regards,
    Brian
  • Hi Mr. Pisani!

    I dont found the BIAS_SENSP, BIAS_SENSN registers... did you mean on RLD_SENSP, RLD_SENSN (0Dh, 0Eh registers).

    If we are talking about this, than yes, i will set a few bits.

    As the HW config for the channel input order is same as on the EVM, eg. the LA is IN2P, the RA is IN2N and IN3N, the LL is IN3P ,  we want to set these bits in the 0Dh, 0Eh registers for the RLD.

    Or is it better to choose some other combination (to include the chest channels too?)?

    We plan to use the internal reference voltage for the RLD.

    We was thinking about the WCT to RLD option too, but unfortunately i dont found here a clear information about the differences in methods, and also why to use WCT as RLD.

    If in the RLD_SENSN, SENSP we set the  LL, LA, RA as RLD, why to use WCT as RLD (which is basically the same signals) , and as i read the WCT amplifier has a limited drive strength...

    But if i got a good information about the RLD (is it better to choose LA,RA,LL combination, or + the chest channels too..) maybe i will understood this issue.

    Thanks

    Best Regards

    Lou

  • Hey Lou,

    Yes I meant the RLD_SENS registers. If this is the case, you're definitely going to want to expand the closed loop bandwidth of the RLD to at least include the utility frequency.

    The reason you might consider using the WCT amplifier as an RLD source is because it is derived from the RA, LA, and LL electrodes just like you mentioned. However, to get common mode cancellation, you'd need to invert the phase of the WCT output. Further, you may actually want to use the chest electrode inputs as inputs to the RLD amplifier to improve CMRR, which you cannot do with the WCT amplifier.

    You may want to include the chest electrodes in your derivation of the RLD signal since presumably a signal that was common to those electrodes could produce a cancelling output on the RLD amplifier and provide higher common-mode rejection to your system.

    Regards,

    Brian Pisani

  • Hi Mr. Pisani!

    So if i good understood, the current setup is good if we want to use other electrodes also (chest). So wich combination of the resistor || capacitor will be the best for  expanding the closed loop bandwidth?( instead of the 392k||10nF).

    Also if i good understood the WCT as RLD  is not as good as "simple" RLD.

    But if we want to use the WCT as RLD than could we use the this config:

    Is this gain enough? Or we need other?

    Also is the cut frequency Ok, or we need some other value, to get better/as noiseless signals?

    Best Regards

    Lou

  • Lou,

    The datasheet recommends 1 M || 1.5 nF which sets the bandwidth at 1/(2*pi*R*C) = 106 Hz. This is wide enough to allow for 60 Hz cancellation.

    The image you show has an inverting amplifier connected to the output of the RLD instead of the WCT. If you want to use the WCT as RLD, then you need to connect the amplifier to the WCT output. In addition, are you using bipolar analog supplies for the ADS1298? If not you're going to have to connect the non-inverting input of that op amp to analog mid-supply rather than the board ground.

    The cutoff for the feedback loop of the op-amp does not include the utility frequency, which it must if you want to be able to cancel that interference with the RLD amplifier. Finally, the gain is probably too large. A gain of 4 or 5 should probably be sufficient.

    Regards,
    Brian Pisani
  • Hi Mr. Pisani!

    Just to clear, than in the sbau171d -ADSD1298EVM-PDk the values for R8 (392k) and C20 (0.01uF), are also incorrect regarding the 50Hz, and theese should be 1M and 1.5nF as it is in the ADS1298 data sheet.

    So the values in the ADS1298 are the correct values, and not whats are on the PDK.

    So according to this the element values around the external operational amplifier-inverter should be changed also on 1M and 1.5nF (or some other combaniation which the bandwidth is around 105Hz).

    But whith this external am,plifier-inverter connection I'm a little confused after you information that we need to connect this on the WCT leg... so what will happened with the chest channels negative inputs?

    And also in the ADS1298 datasheet i found exactly the same config i draw it :

    I dont found in the data sheet that we should connect for this option the external amplifier on the WCT leg...

    We use +2.5, -2.5V power supply fro the ADS and thats why i put the inverter positive input on the middle point ,GND_E in our case.

    Please correct me if i dow saw , or miss some detail.

    HBest Regards

    Lou

  • Hey Lou,

    You're right that the EVM user's guide shows the values that you used in your design. I am almost certain that in a previous version of that user's guide that the component values were the same as in the datasheet... Sorry about that! In any case, you will probably want to use something like what is in the datasheet because of the bandwidth limitation I mentioned.

    Regarding the WCT connections, I was imagining another scenario where instead of using the WCT_TO_RLD bit, you would connect an amplifier directly to the output of the WCT, but now that I think about it, that's probably not the best idea because you would need to buffer the WCT output since it has a very low short circuit current. Again, my mistake. You should do what is shown in the datasheet.

    Regards,
    Brian Pisani
  • Hi Mr. Pisani!

    Thank you for this information! I will use the values from the ADS1298 data sheet.

    I have one more question:

    On the EVM on every channel there is an 22k  + 10k .

    And we was talking about the 100k patient protection resistor on the RL line.

    My question is, did this 100k also needed on the other lines too, or it is only important on the RL line?

    Regarding the patient security will it be better if i put before the 22k an 100k resistor on every line?

    Or there should be totally 100k (eg: 22k+10k+68k? ), and i have to put only 68k before every 22k, (and a 100k on the RL line) ? 

     

    Or regarding the patient security only the RL line is the important as other lines comes from the patient?

     

    Thaks!

    Best Regards

    Lou

  • Hey Lou,

    Unfortunately I cannot speak to those values when it comes to patient safety. I believe you will be able to find information about patient protection resistors in any of the various medical device safety standards that are available. What I can say is that the more series resistance on a line that is connected the patient, the less current will flow in case a fault happens. However, you must also be mindful of the low-pass filter response you create with those resistor values.

    Regards,
    Brian Pisani
  • Hi Mr.Pisani!

    I accept that you cannot tell what is the best resistor value, and i aware of that it can also acts as a LP filter (but with choosing a good capacitance it can also act as  the first stage of the anti-aliasing filter)

    But please just tell, is this Rp needed on the all other lines  too as it is on the RL, or the 22k+10k is enough there.

    Because as i found in the ADS1298 data sheet in the Lead off section, that there is a 100k patient safety resistor on the other lines too, not just in the RL, so thats why i think you put it there for some reason.

    Best Regards

    Lou

  • Hey Lou,

    Patient protection of some kind is needed on all nets that will come into contact with a patient.

    Brian