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ADS1248 Bipolar power supply

Franz Lamers
Prodigy 40 points
Other Parts Discussed in Thread: ADS1248

I am using the ADS1248 with bipolar power supplies (+-2.5V). My schematic is based on the proposed schematic found here:

http://e2e.ti.com/support/data_converters/precision_data_converters/f/73/t/222313.aspx

Questions:

1) Do I have to enable the VBIAS when measuring thermocouples (Both grounded and ungrounded)? as VBIAS is midpoint between the 2 power supplies? Is using a single ended configuration better?

2) Is it ok to measure 3 wire RTD with the VREF resistor on the high side as in the schematic, or should I change the schematic to use the low side. Do I connect the ref resistor to ground or -2.5V?

You help is appreciated.

Regards,

Franz

  • 0
    TI__Guru** 113765 points

    Hi Franz,

    Welcome to the forum! The purpose of the VBIAS is to make sure the sensor is placed within the correct common mode input range.  It is not necessary to use the VBIAS when using bipolar supplies as mid-supply in this case is ground with +/-2.5V.

    The correct configuration for the RTD is shown by Luis farther down in the post you referenced.  The reference resistor establishes a ratiometric measurement, but also places the RTD in the correct common mode input range.  If the sensor is tied to AVSS this will violate common mode.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 40 points

    Thanks Bob,

    Clear on the VBIAS.

    Just want to make 100% on the following. I am using a single ended design with the negative of the TC connected to ground. Will this work for both grounded and ungrounded TC. Still using +-2.5V power supply?

    Also on the RTD. You mention that the ref resistor must not be connected to AVSS as this will violate the common mode. But just above the diagram Luis wrote the following:

    2)  The input bias current is in the order of ~10nA; and the input bias current mismatch is in the order of 100pA.  Using 10kOhm 0.1% resistors at the inputs should produce errors in the order of 1uV considering the input bias current mismatch. The design below shows a possible filter/protection circuit that could be adapted into your design.  A similar filter/protection circuit can be used at the inputs in your design.    This design could also be used with bipolar supplies by referring VREFN to AVSS.  The filter time constant across VREF and the ADC inputs of the device are also matched for good ratiometric cancellation.

     

  • 0 in reply to Franz Lamers
    TI__Guru** 113765 points

    Hi Franz,

    If the grounded TC is truly at the same potential as the analog ADC ground, then care must be taken with respect to preventing a ground loop.  If the 'grounded TC' is not connected to the analog ground, you should see no issues.  Just to be clear, when I say grounded TC I am referring to the tip of the TC directly connected to the sheath surrounding the TC tip.

    For example, I could have a grounded TC pushed into a piece of meat on a grill.  The grounded TC tip is electrically isolated from analog ground except for the connection to analog ground at the analog input.  This example has no ground loop as only one end of the TC is connected to analog ground. 

    Regarding the RTD, I said the high-side reference will not work because the RTD sensor (not reference resistor) is connected to ground which violates common mode.  You may want to read Luis' application note SBAA201 that discusses ratiometric RTD measurments. http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=SBAA201&fileType=pdf

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 40 points

    Thanks for the reply.

    I will use the circuit to measure RTDs (figure 10) on page 11 of the suggested doc, but connect the negative end of the RBIAS resistor to AVSS (-2.5V)

    I still have to use a +-2.5V power supply as I am also measuring +-Volts and +-mA also in a single ended configuration using the spare channels on the ADS1248.

    For the TC, I currently have it working perfectly with a TC simulator in a single ended configuration (VBIAS Off, swapping with the BCS to detect a broken TC).

    Will it be better to use the same input pins for the TC and RTD (the inputs to the PGA) and turn off the current sources? in this way I can save on terminals and still measure TCs and RTDs (customer will chose what sensor he wants to attach). The TC wont be single ended. Will this configuration work?

    Thanks in advance.

     

  • 0 in reply to Franz Lamers
    TI__Guru** 113765 points

    Hi Franz,

    Yes, you can use the same inputs for RTDs or TC.  Remember that the TC requires a cold junction measurement so that it is properly compensated.  You will need to properly create a cold junction at the connection point where the temperature at the connection can also be measured.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 40 points

    I modified the board and all is working. Yes, I am using a silicon temp sensor hooked up to one of the aux pins for the CJC.

    Thanks,

    Franz