This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS1298 WCT external capacitance

Intellectual 300 points

Replies: 18

Views: 2482

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

18 Replies

  • 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
  • In reply to 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

  • In reply to Brian Pisani:

    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

  • In reply to ECGlou:

    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
  • In reply to 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

  • In reply to ECGlou:

    Hey Lou,

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

    Brian
  • In reply to Brian Pisani:

    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

  • In reply to ECGlou:

    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
  • In reply to Brian Pisani:

    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

  • In reply to ECGlou:

    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

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.