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ads1248 Rcomp calculation

pazcal pazcal
Prodigy 90 points
Other Parts Discussed in Thread: ADS1248

Hi, 


I would like to use ADS1248 for monitoring a 3-wires PT100, in a measure range of  -200 to +250°C. And of course, I would like to get the best performances in term on accuracy.

In my design I would like to use the system described in SBAA201 Figure 10 page 11. Ratiometric configuration with RC filter. Furthermore, I would like to add a hardware compensation as described in chapter 4 of SBAA180.

So my question is about the Rcomp calculation. Should we consider the serial resistance of RC filter to calculate Rcomp? To compensate a PT100 (100Ohm@0°C) I would like to add into Ain0 a serial resistance of 100Ohm. Should I use another value?

Furthermore, the term of "ratiometric approach" is confusing for me. When using IDAC to biases my RTD is it a ratiometric approach?

Thank's in advance

Regards

  • 0
    TI__Guru* 92715 points

    Pazcal,


    If the filter resistors (and IDAC currents) are completely matched they will cancel each other in much the same way that Rcomp will cancel the voltage of the RTD at your base temperature. In that respect, you don't need to roll it into the Rcomp calculation.

    Look at Figure 3 on page 4 of SBAA180. Imagine the resistors for Line 2 and Line 3 are for the filter. If the same amount of current goes through each, then the voltage drop is the same and the ADC doesn't see the drop across them.

    However, point that I'd like to make though is that using an Rcomp may be problematic. It may give you more resolution by using the full bipolar range, but it may be less accurate. You need to ensure that the currents are completely matched in the IDAC. The part has a typical mismatch of 0.15%, but is not guaranteed for a max. Also you need to consider the drift of any resistances in the analog input path. That would include Rcomp and any filter resistances in the inputs. The filter resistances need to match exactly as their mismatch and drift will get gained up by the IDAC.

    For the definition of a ratiometric measurement, generally driving the IDAC through an RTD for the measurement and through a reference resistor for the reference is a ratiometric measurement. In this configuration the output code of the ADC divided by the full-scale code of the ADC (7FFFFF) is the same ratio of the RTD resistance divided by the known reference resistor value. That's why it's referred to as a ratiometric measurement. Note that regardless of the IDAC current, the ratio of the two resistances stay the same - and the ADC should give you the same output code.

    In contrast, if you use a voltage reference, either the on-board or external reference. You get a voltage measurement for your ADC output, which is not considered ratiometric.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 90 points

    Thank's a lot Joseph

    regards

    pazcal