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ADS1220: Scaling resistor

Viktorija Cecil
Mastermind 18491 points
Part Number: ADS1220

Team, 

My customer is working on a design with ADS1220, and needs help understanding the relationship between precision of the scaling resistor, and resolution of the ADC. 

Scaling resistor in the datasheet has a tight temp coefficient, and is hard to get. They are looking to loosen up that spec, but need to understand the impact on the resolution of the ADS1220.

Thanks

Viktorija

  • 0
    TI__Guru** 113765 points

    Hi Viktorija,

    The demand for resistors is high and supply is low regardless of the tempco of the resistor.  Some believe it will take a couple of years before the supply/demand equalizes.

    I believe the scaling resistor you are referring to is the reference resistor for the RTD measurements is that correct?  The measurement itself is ratiometric where the drift of the excitation source will not affect the result.  What will affect the result is the drift of the reference resistor.  If the resistance fluctuates over temperature, then a gain error will be seen as the value of the reference changes (IDAC * Rref). It does not affect the resolution, but rather the precision and accuracy of the measurement.

    With low tempco devices the precision of the resistor is also quite good, so the value selected will be very close to the rated value (0.1% or better) which for those that do not calibrate the system allows for much greater consistency from system to system.  A 0.1% resistor will often have a tempco of +/- 25ppm per deg C or better.  A 1% resistor may be +/- 100ppm per deg C.  A 5% resistor may be +/- 200ppm per deg C.

    Equation 24 in the ADS1220 datasheet shows:

    Code ∝ (Rrtd (at temperature) · Gain) / (2 · Rref)

    So the output code is directly proportional to the RTD resistance relative to the reference resistance.  If the value of the reference resistor changes considerably, then the output code will be skewed as the temperature changes for the ADS1220.  Obviously if the temperature doesn't change that much (like always operating at room temperature) there will not be as much error.  However, when the system may operate in a range of temperature (like from 0 to 65 deg C) there could be considerable error with a 200 ppm per deg C change. 

    If I did my quick math correctly, for a 65 deg C change the variation of the reference resistor value could be 1.3% at 200ppm, 0.16% at 25ppm and 0.065% at 10ppm.

    Best regards,

    Bob B