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XTR108: XTR108

SteveKK
Prodigy 150 points
Part Number: XTR108
Other Parts Discussed in Thread: XTR117

Can the XTR108 be easily ranged for offset and span at room temperature without doing a full thermal calibration?  For example when interfacing a sensor that is already compensated and you just want to range for 4-20mA.

  • 0
    TI__Expert 6250 points
    Hi Steve,
    This should be possible. The thermal calibration is used to calculate the linearization coefficient. If the external sensor is already temperature compensated, then a linearization gain of 0 can be selected, disabling it altogether. From there, you would need to go through the standard calibration process to calculate PGA gain and offset, skipping the linearization calculations.
  • 0 in reply to Alexander Davis
    Prodigy 150 points
    Hi Alex,
    Thanks for the tip. What happens when the sensor output is already amplified ( 0.5 to 4.5 Vdc ). Is it difficult or impossible to scale it for 4-20mA out?
  • 0 in reply to SteveKK
    TI__Expert 6250 points
    Hi Steve,
    If you are using a voltage input into the XTR108, you won't be able to use the Iset DACs as a gain control as you would for an RTD or bridge sensor, so you do lose some calibration flexibility there. Additionally, you'd need to add an attenuator to the input, since the XTR108's minimum gain is 6.5 and a 4.5V input will be out of range for that gain.

    Your gain calibration in this case would be set purely by your input voltage, divider ratio, PGA gain, and Riv selection. For this input mode the XTR108 would simply handle the 4-20mA conversion, similar to a XTR117 (or other loop transmitter) - except still giving you control over offset calibration. If you needed additional gain calibration, that would have to be handled by an additional PGA or MDAC ahead of the XTR108.