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ADS1262: AVDD Voltage Level

Neal Gallagher16
Prodigy 110 points
Part Number: ADS1262
Other Parts Discussed in Thread: ADS124S08EVM

Hello!

I am working on an ADS1262 based wheatstone bridge temperature sensor.  The ADC and all analog side component lie on the same custom board, the microcontroller that communicates with the ADC lies on another custom board. 

- The incoming power to the ADC board is ~4.7-4.8V and is supplied via USB power.  Currently, this is Vdvdd.

- There is a voltage reference 3.3V on the board.  This voltage excites the Wheatstone bridge, is used as VREF.  I also use it for Vavdd!!!

*The board has been built and the circuit seems to work quite well...
HOWEVER, Upon reviewing the datasheet, I see that Vavdd is supposed to be 4.75-5.25V which, of course, makes my design non-compliant.  The reason for my choice was that I wanted to use a less noisy source for the Analog side since I am not using the Internal Reference.  What problems could I expect from this?

How would it affect the noise performance to put the incoming USB ~4.7-4.8V to Vavdd?  I suspect that this is used to supply the amps on the PGA and supply the internal reference - am I missing something else? 

Thanks!

  • 0
    TI__Mastermind 44680 points
    Hi Neal,

    I would suspect that the ADC's performance, particularly the performance of the analog PGA could suffer.

    Another potential critical issue would be the AVDD supply's POR threshold. The ADS1262 is internally held in a reset state until the analog supply voltage exceeds the POR threshold. This threshold is around 3 to 3.5 V, so you might find that you won't be able to communicate with some devices at all!

    For powering devices off of USB, I would recommend using a boost converter followed by a 5-V LDO to ensure a stable and low-noise supply voltage. You can see an example of this circuit on the ADS124S08EVM. Refer to the EVM schematic near the end of the User's Guide: www.ti.com/.../sbau272

    Best regards,
    Chris
  • 0 in reply to Christopher Hall
    Prodigy 110 points
    Hello Chris.
    Thank you very much for your reply! What do you mean in your first statement: "I would suspect that the ADC's performance, particularly the performance of the analog PGA could suffer. "? Do you mean from a noise or from a resetting standpoint?

    I was surprised myself that we didn't run into issues with the ADC resetting itself but it hasn't happened yet (or doesn't with great regularity).

    Just to be crystal clear, I still intend to excited the Wheatstone and supply the reference, Vref with the 3.3V regulator. I would like to use the 4.7-4.8 directly into the Vavdd (instead of the +3.3). I will have a compliant Vavdd, the only concern I had is whether the noise will be worse...

    Cheers,
    Neal
  • 0 in reply to Neal Gallagher16
    TI__Mastermind 44680 points
    Hi Neal,

    It's hard to say what exactly the effects might be since we don't have data for this low of an AVDD supply. However, I would guess that noise, offset, gain error, linearity, etc. could all be degraded from the typical datasheet specifications. Outside of the recommended operating conditions we really can't guarantee any of the performance metrics.


    For the USB supply:

    Regarding whether or not a USB 5-V power supply will be clean enough for the ADS1262, is also difficult to predict. Every USB port will be a bit different and some may output a voltage less than 4.75 V (again outside of the ADS1262's specification). Generally speaking desktop computers will have better and cleaner USB supplies, while laptops may have nosier supplies.

    I think with a good PCB layout and good supply decoupling you could mitigate the effects of a noisy supply. However, if noise performance is truly a big concern for your application to the point where it is worth the extra cost of using an ADS1262, I think I would consider using a higher quality supply source in combination with the ADS1262; otherwise you might be better off going with a lower cost ADC.


    For the reference voltage:

    I don't see any issues with keeping the 3.3-V reference/bridge excitation voltage. However, the ADS1262 can operate off of an external 5V reference, so if you do decide to add a boost converter + LDO for AVDD, I might also consider upgrading to a 5-V reference source (which would require a ~5.3-V supply) to increase your signal amplitude for the best resolution.


    Best regards,
    Chris