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ADS1247: IDAC out and Vbias in, but i can't get my idea data

lance Chan
Prodigy 40 points
Part Number: ADS1247
Other Parts Discussed in Thread: ADS124S08

This is my major design and i want to measure the resistance sensor variable with a constant-current source.

fig1 show my hardware design

fig2 show my idea

i set the MUXCAL bit of the MUX1 register and insured device was working in Analog supply monitor. The Conversion result was right.

But i set the device with my idea and the conversion result was no right.

fig1 schdoc

fig2 myidea

Debug1

output format

debug1 result

Debug2

debug2 result

Debug3

debug3 result

  • 0
    Prodigy 40 points

    The Right result of Conversion in debug2 should is 0.1V(1mA×100Ω) but the result is 0.352V .

    And The Right result of Conversion in debug3 should is 0.051V(1mA×51Ω) but the result is 0.368V 

    I sure  the voltage between AIN0 and AIN1 is 0.1V and AIN2 and AIN3 is 0.051V with the ammeter of Fluke

  • 0 in reply to lance Chan
    TI__Guru** 113765 points

    Hi Lance,

    Welcome to the E2E forum!  There are a couple of very important considerations.  The first is the current path from the IDAC must flow to the AVSS (AGND).  So the current path through your sensor must complete a path to AGND.  The second consideration is the analog input range.  Due to the input restriction for the PGA, the sensor to be measured cannot be directly connected to AGND.  This would require a value of resistance in series with the sensor that will bias the input to the correct input range.  The input range is discussed on page 27 of the ADS1247 datasheet in section 9.3.2.1.  The third consideration is to make sure that the total voltage drop across the sensor and the bias resistor does not become so large that the IDAC compliance voltage is exceeded (see Figures 41 and 42 on page 19 of the datasheet).

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 40 points

    Thank you Mr. benjamin

    I think I get the point. Acturally i not real understand the bias resistance before your help.

    But i have aother question now. Should the both ends of the bias resistance(in series with the sensor) in parallel with the REFP pin and REFN pin?

    In my opinion, the potential both of ends of the bias resistance always high than Vref so does ADC work ?

  • 0 in reply to lance Chan
    TI__Guru** 113765 points

    Hi Lance,

    It would be helpful to know exactly what types of sensors you are wishing to use.  It appears that one sensor may be a 2-wire RTD (such as a PT100).  After reviewing the drawing again I see that you wanted to connect the temp sensor to the IDAC source and then to VBIAS.  This will be problematic for a couple of reasons.  One potential issue may be with compliance voltage when using the IDAC.  The second and bigger issue will be that the VBIAS is not meant to sink/source significant current.  The VBIAS is designed to bias sensors such as thermocouples.  If you attempt to drive a lot of current through the VBIAS switch you will see the error of the switch resistance in the measurement. 

    As I did not initially notice that you were using the VBIAS, I mentioned directly connecting the sensor to a bias resistor.  The bias resistor could also be used as a reference resistor for making a ratiometric measurement.  In this case the resistor would be in series with the sensor.  I would suggest taking a look at A Basic Guide to RTD Measurements:

    http://www.ti.com/lit/an/sbaa275/sbaa275.pdf

    You could also look at the following example for a 2-wire RTD:

    http://www.ti.com/lit/an/sbaa329/sbaa329.pdf

    The above example is specific to the ADS124S08 but can be adapted to the ADS1247.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 40 points

    Hi, Mr. Benjamin,

    Thanks for your instructions and i realize using Vbias is a error in my design. The character of sensor my used is similar to a 2-wire RTD, so following the example i understand now. 

    I try to consider the input range of ADC in my second question, but i think i go wrong.

    Best wish, (◠‿◠)

    Lance