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ADS1261: AC Excitation with IDAC

ka88
Intellectual 275 points
Part Number: ADS1261
Other Parts Discussed in Thread: ADS124S08

Hello,

I have a typical wheatstone bridge application with 4x active strain gage elements.  In my case I cannot use voltage excitation, and hence must use IDAC current excitation for the bridge (currently using 500uA).  Due to an additional external INA gain stage, I am looking to apply the AC excitation to remove this offset and drift.  However, I do not see any reference designs using AC excitation with the internal IDACs - is there anything preventing this?

If not, it would be great to see a diagram similar to Figure 97 in the datasheet, with IDAC excitation.

  • 0
    TI__Mastermind 38271 points

    Hi ka88,

    We have a couple of AC-excitation related resources that I can point you too, though none that directly deal with current excitation:

    However, in principle the design should be similar to using 4 switches as shown in Figure 97 in the ADS1261 datasheet. Instead of using a 5V excitation voltage, you would route one of the ADS1261's integrated IDACs to these two switches. The switch control would be handled by the drive circuitry inside the ADC, you would need to consider the common-mode voltage of the bridge output signal however (though your INA may take care of this depending on the device.

    Can I ask what the gain stage is for, since the ADS1261 already includes a low noise PGA?

    Also, can you let me know why you cannot use voltage excitation in your case? Just curious...

    -Bryan

  • 0 in reply to Bryan Lizon86
    Intellectual 275 points

    Hi Bryan,

    Thanks for the feedback on AC excitation with IDACs. 

    • I cannot use voltage excitation due to very high source resistance
    • Re-evaluating the benefits of an external gain stage.  Initially I wanted to maximize dynamic range given I need to resolve between 0.2uV - 8uV (with IDAC = 500uA) - however I don't think there is an external gain stage that can actual beat the integrated PGA in terms of input noise performance (assuming PGA is set to 1V/V, 20SPS).

    So given this, sticking with IDACs - I plan to add a resistor to the bottom of the bridge in order to meet the common mode requirements.  Do you see any problem with this? What is the ideal common-mode set point assuming I am using internal 2.5V VREF and AVDD = 3.0V?

    Also, what is the compliance voltage for the IDACs?

  • 0 in reply to ka88
    TI__Mastermind 38271 points

    Hi ka88,

    Thanks for the additional information.

    With such a high sensor impedance, I am wondering why you are using 0.5mA? That means you could only use a 10kohm bridge or lower before you start violating the ADC's input range. Will that be the case for this system? I would also guess that voltage excitation would be more appropriate for a high impedance sensor, since the current values will be very limited in this situation due to the ADC input voltage limitations.

    I agree about reevaluating the need for an external gain stage to maximize dynamic range, as this can come at the cost of noise performance. You can review our detailed training content on this matter here, specifically the modules covering ADC noise: https://training.ti.com/ti-precision-labs-adcs

    You can calculate the VCM range of the output signal using the nominal bridge impedance, the expected change in bridge impedance, and the excitation current you intend to use. If you want to provide this information, I can double check for you as well. You can also use the ADS1261 Excel calculator to help check the VCM range of your input signal. Note that the input must be in volts, which is why the resistances and currents are necessary. Link is here: https://www.ti.com/lit/zip/sbac200

    The ADS1261 IDAC compliance range is given in the Electrical Characteristics section on page 9. It is shown to be AVDD - 1.1 max, or typically 3.9V.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Intellectual 275 points

    Bryan,

    Thanks for the feedback, and documentation.

    After reading through the training content - regarding the Reference Noise section, your example with the ADS1261 uses the input noise datasheet values with the Internal 2.5V Reference as the 'ADC noise'.  Is it fair to assume that all TI 24-bit delta-sigma datasheet use the input short method for input noise measurements (thus removing the effect of Internal Vref noise)?  I ask to make sure not to double count noise from the Internal Reference combined with noise from an External Reference.

  • 0 in reply to ka88
    Intellectual 275 points

    And if it is the case that the datasheet input noise tables are with inputs shorted - shouldn't there be a chart that shows the Internal Reference spectral noise density so we can add that noise using the same method?

  • 0 in reply to ka88
    TI__Mastermind 38271 points

    Hi ka88,

    I cannot say with certainty that every 24-bit ADC uses the input short noise test, but I could say this is very likely true. Since thermal noise dominates in higher resolution ADCs, this is the best way to determine what that baseline / inherent noise is. And since the input short test is implemented with a 0V input signal, there is no reference noise in the output code (it is only ADC noise). So there would be no double counting of noise if you were to consider using an external reference.

    Some of our ADCs do include reference noise information in the datasheet - see ADS124S08 as an example. However, I cannot think of an ADC with integrated reference that includes the spectral density curve as it would be shown in a discrete reference's datasheet. This is something we are going to include in some future devices however, for the exact reason that you want that information.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Intellectual 275 points

    Bryan,

    Got it, I am glad that data will be coming in future devices to help make that internal vs external reference decision.  In terms of external reference, is it fair to say that output noise from the external reference can be ignored (i.e. does not need to be included in noise analysis) if the system uses a ratiometric configuration (external ref powers both the bridge and the ADC reference? If so, then the external reference parameters can be selected to optimize other specs - rather than output noise in the ENBW region.

  • 0 in reply to ka88
    TI__Mastermind 38271 points

    Hi ka88,

    The noise should, in theory, cancel out in a ratiometric reference configuration. In practice, board / component tolerances can certainly make this relationship not purely ratiometric. For example, if the noise gets filtered slightly differently through the input path versus the reference path, due to component mismatch / tolerances, the noise will not completely cancel out in the final measurement. The noise will still be significantly reduced, but it won't be zero. I would say that in most cases, the noise will still be insignificant, but I am sure there are some applications where this might need to be accounted for. Therefore, I leave it to you to determine if this can be ignored in your system.

    I would also argue that even if the relationship between VIN and VREF was not 100% ratiometric, this is still generally a lower noise solution compared to a discrete reference. The only downside of course is that many systems cannot benefit from a ratiometric reference since this only really applies to specific sensor types.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Intellectual 275 points

    Hello Bryan,

    For AC excitation, has there been consideration to the noise added by the MOSFET gate drive signal (for external AC Exc FETs) coupling into the bridge supply through the MOSFET capacitance(s)?  Given that the bridge is excited by an external reference, but the gate drive I/O are referenced to IOVDD/DVDD which is generally not supplied with a low noise source.

    Also - is there an email I can use to submit a schematic for review?

  • 0 in reply to ka88
    TI__Mastermind 37915 points

    Hi ka88,

    Bryan is out this week, please allow a delay for him to answer your particular questions until next Tuesday. Thanks.

    Regards,

    Dale

  • 0 in reply to ka88
    TI__Mastermind 38271 points

    Hi ka88,

    You can request me as a friend by hovering over my name and then selecting "Request Friendship". Then you can start a private message with me where a schematic can be shared

    What noise are you referring to that is being added by the MOSFETs? If this is a static noise then using a 6-wire (Kelvin-connected) implementation should help eliminate this noise via the ratiometric measurement.

    On the ADS1261, the GPIO signals that control the external switching are referenced to AVDD, not DVDD, for a variety of reasons. These are discussed in that application note I previously referenced (SBAA290A). In other words, they should be fairly clean signals.

    -Bryan