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Original question:

ADS1146: Reference voltage range

ADS1146: VREFP/VREFN equal to AVDD/AVSS on ADS1146

Benjamin Vespone
Prodigy 120 points
Part Number: ADS1146
Other Parts Discussed in Thread: ADS114S06, ADS1120

Hello,

I am using the ADS1146 in a ratiometric configuration to measure a wheatstone bridge sensor. A unipolar supply of 3.3V is available, and I intended to configuration the circuit as follows:

AVDD - 3.3V

AVSS - GND

VREFP - 3.3V

VREFN - GND

bridge Vexc+ 3.3V

bridge Vexc- GND

It is based on this configuration - 

Although this configuration would meet the requirements of the absolute voltage levels on the VREF pins, it would exceed the differential maximum voltage of (AVDD - AVSS) - 1. 

I'm failing to understand why this limit exists and what issues I might encounter if I set VREF equal to AVDD. Any easy solution is to decrease the size of VREF by adding a voltage divider on the VREF input and increasing the gain on the PGA, but I don't understand why I should need to do that. 

As an aside, based on the common mode voltage requirements, VOUT (AVIN * Gain) will range from 770mV - 2.5V under normal circumstances.

V_cm = 1.65V

Vin = 110mV

Gain - 16x

Vout = 1.76V

Vout Range (abs) = 770mV - 2.53V

Thanks!

  • 0
    TI__Mastermind 23275 points
    Hi Benjamin,

    thanks a lot for reaching out with your question.

    Unfortunately the ADS1146 has this restriction with respect to the differential reference input voltage range. This is due to the internal circuit implementation. The ADS1146 is therefore not the ideal ADC for a ratiometric bridge measurement.
    The device will most likely work but you might see degraded DC performance (e.g. INL) when using it in this way.

    Newer similar ADCs such as the ADS114S06 or ADS1120 do not have this restriction anymore and would be preferred solutions for what you are trying to do.

    You could certainly use a resistor divider and feed the reduced excitation voltage into the reference pins. However this will most likely introduce additional errors (gain error) even if you use precision resistors for the divider.

    Regards,
  • 0 in reply to Joachim Wuerker
    Prodigy 120 points
    Hi Joachim,

    Thank you for that feedback.

    I choose the ADS1146 for the sensor fault detection feature. I see that the ASD114S06 also has this feature. I had not considered this chip previously because it has many more channels than needed, which adds to cost. Are you aware of any 1 or 2 ch. sigma-delta ADCs that are designed for bridge circuits and have the sensor fault detection feature?

    Regards,
    Ben
  • 0 in reply to Benjamin Vespone
    TI__Mastermind 23275 points
    Hi Benjamin,

    by sensor fault detection you are referring to the Burnout Current Sources I guess?
    ADS1120 offers those as well.

    We don't really develop any single channel sensor measurement ADCs these days anymore to be honest.
    Therefore ADS1120 would be my recommendation if you are cost constraint.

    Regards,
  • 0 in reply to Joachim Wuerker
    Prodigy 120 points

    Excellent!

    I didn't realize that the ADS1120 has the burnout source, as it wasn't listed in the features nor in the primary block diagram. I see the detail now on page 19. I will plan to use that instead of the ADS1146. Thank you for your help.

    As an aside, I would suggest that for the next revision of ads1120 to add the burnout source to the feature list. For safety critical sensor applications like mine, that was a huge bonus that made me choose TI over others. I had originally excluded ADS1120 because I thought it didn't have that feature.

    Regards,

    Ben

  • 0 in reply to Benjamin Vespone
    TI__Mastermind 23275 points
    Thanks for the feedback Benjamin.

    Let me see what I can do in the next datasheet revision.

    Regards,