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ADS1248 Ratiometric question

James Davis
Prodigy 685 points
Other Parts Discussed in Thread: ADS1248

Can you ask the a/d guys something about the ADS1248.

There are current sources up to 1.5mA, and there is an internal 2.048V reference.  Is there any ratiometricity or proportionality between these?  Suppose, for example, the 2.048V internal reference dropped by 1% due to temperature effects.  Would the current source drop by the same amount as a direct result of the drop in internal reference?  This is not considering any separate effect temperature might have on the current source itself.

 The data sheet claims the 2.048V internal reference must be enabled in order to use the current sources.  This suggests they may be linked together somehow.

 The reason for this question is because we are using the ADS1248 to read strain gauges.  These use the 1.5mA current sources to excite the gauges and the internal 2.048 reference as reference for the a/d conversion itself.  Just wondering if I can consider this conversion to be ratiometric as described.

  • 0
    TI__Guru** 112705 points

    Hi James,

    The internal reference of the ADS1248 does establish the value of the IDAC current.  This may relate some to noise and the response of the IDACs to reference drift, but the initial output current the IDACs may have differences and the drift between them are also different.  So the question still remains as to what part of the measurement is ratiometric?  In other words how do you determine what the ratio is relative to the transfer function?

    It is possible given the right circumstances (load cell resistance, currents, etc.) to make the measurement truly ratiometric by establishing an external reference in a similar fashion as the RTD concept.  Here you must be careful to maintain the IDAC compliance voltage and common mode.

    Best regards,
    Bob B

  • 0 in reply to Bob Benjamin
    TI__Prodigy 685 points

    Thanks!

    It is understood that conversion noise increases as reference voltage decreases, but what is the exact relationship between the two in the ADS1248.  Linear? Exponential? 

  • 0 in reply to James Davis
    TI__Guru** 112705 points

    Hi James,

    The relationship appears to be linear over temperature, but we don't have enough data with respect to noise over the full-input range to make a solid claim.  There are a couple of interactions to consider.  From characterization data we see that the input referred noise level decreases as the reference voltage decreases.  It is the percentage of full-scale that increases as the reference decreases.  In other words, a smaller reference voltage has fewer bits of resolution.  This may not be a big factor as the actual LSB size is smaller for the smaller reference.

    The trade-off takes place relative to available gain.  The input referred noise lowers with increasing applied PGA gain.  This is similar to what I explained above where the number of bits of resolution decreases but the LSB size is smaller.

    Best regards,

    Bob B