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ADS1298: RLD Specifics

Peter Vander Jagt
Prodigy 40 points

Part Number: ADS1298

I have a very specific question on the ADS1298 that doesn’t appear to be answered by the data sheet.  On page 42 of the spec (http://www.ti.com/lit/ds/symlink/ads1298.pdf ), there is a drawing of the Right Leg Drive provisions of the ADS1298.  In short, 220Kohm resistors from each of the 8 channels are mixed and inverted by the RLD amplifier.  The gain of that amplifier is set externally by the ratio of an external  resistor (likely metal film) chosen by the designer and the 220K internal resistor (or 220K/channels mixed).  My question; Are those resistors ordinary silicon resistors (+/- 30% tolerance) or are they an improved configuration with less drift that the simplified drawing can’t show.  If the former case is true, the RLD amplifier will experience a significant amount of CMRR drift.  Would appreciate an answer.  Thanks.

 

Pete Vander Jagt

DornerWorks Ltd.

  • 0
    TI__Prodigy 290 points

    Part Number: ADS1298

    Hello

    I have a very specific question on the ADS1298 that doesn’t appear to be answered by the data sheet.  On page 42 of the spec (http://www.ti.com/lit/ds/symlink/ads1298.pdf ), there is a drawing of the Right Leg Drive provisions of the ADS1298.  In short, 220Kohm resistors from each of the 8 channels are mixed and inverted by the RLD amplifier.  The gain of that amplifier is set externally by the ratio of an external  resistor (likely metal film) chosen by the designer and the 220K internal resistor (or 220K/channel count).  My question; Are those resistors ordinary silicon resistors (+/- 30% tolerance) or are they an improved configuration that the simplified drawing can’t show.  If the former case is true, the RLD amplifier will experience a significant amount of CMRR drift.

  • 0 in reply to Manny Salazar
    TI__Mastermind 38745 points
    Peter, Manny,

    Are you referring to the resistor's absolute variation from the nominal 220k? Or are you asking about the matching between the resistors? In either case I will have to find out more information from some colleagues.

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Prodigy 40 points
    I would assume the internal resistor matching is good but the internal resistor value(s) change significantly with temperature UNLESS the designer somehow made provisions for RLD gain drift.
  • 0 in reply to Peter Vander Jagt
    TI__Mastermind 38745 points
    Hey Peter,

    The resistor values are probably close in value to one another, but the nominal value may vary from die to die. So you are correct in saying the gain of the RLD amplifier will very from chip to chip, but I don't think it will cause a significant issue. If the gain is high enough on the amplifier, CMRR will be good, even if it changes couple of dB from board to board. Have you tested this and seen larger variations that are causing an issue for your application?

    Brian
  • 0 in reply to Brian Pisani
    Prodigy 40 points
    What you're saying is that the resistors ARE going to vary in terms of original value and they ARE going to drift significantly relative to any external Rfb I select (if I attempt to create an anti-CM noise signal for CM noise cancellation purposes with the on-board RLD amplifier. What this means to my application is that I can't use the on-chip RLD amplifier.
  • 0 in reply to Peter Vander Jagt
    TI__Mastermind 38745 points
    Hey Peter,

    Why can't you use the on-chip RLD amplifier? If you set the gain high enough, I doubt there would be a significant measurable performance difference between boards. Can you explain in more detail? I can run some ideal circuit simulations to see what we can expect as a baseline of performance for different configurations.

    Brian
  • 0 in reply to Brian Pisani
    Prodigy 40 points
    In my application, I don't know (yet) what the magnitude of the CM noise or the DM noise will be or how many RLD feed channels will be used to create the "composite" anti-CM signal and I'm planning to operate from a 3.3v rail. I would have to walk a tight rope between having the worst case amount of gain available (and possibly creating an anti-CM signal that is so mismatched I am prone to oscillation) and clipping at the ADC. By the way, is the Labview code for the eval board available?
  • 0 in reply to Peter Vander Jagt
    TI__Mastermind 38745 points

    Hey Peter,

    I don't think you should be too concerned. Oscillation will only occur if the loop is driven unstable. This could happen if there is too much capacitive load at the output of RLD amplifier, but there are some simple ways to prevent this from occurring. Preventing the amp from clipping can be done via trail and error. You can always scale the gain with the external resistor. If you want some more information about using the RLD amplifier in an application, see this app note about it.

    The LabVIEW code can be found on the ADS1298ECG-FE PDK product in the "Order Now" table next to "ADS1298ECGFESW-SOURCE".

    Regards,

    Brian Pisani