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ADS1120 with earthed thermocouples

Stephen Hughes97
Prodigy 20 points
Other Parts Discussed in Thread: ADS1120, LM7705, TPS60403, ADS1118

I'm designing a system which monitors two type-J thermocouples. I'd made a prototype using the ADS1120 but hadn't realised the thermocouples used were earthed which in effect makes the negative input 0V and violates the input range of the PGA.

I've seen elsewhere the use of the LM7705 suggested to give a slightly negative value for AVSS, however according to the values on the datasheet this is outside spec - for unipolar operation it must be within 0.1V of DVSS and for bipolar it must be at least -2.3V.

So, my question is, is it actually possible to violate the specs in this way and still have accurate and reliable operation?

The alternative I'd seen was to use a 2.5V regulator to tap off the 3.3V supply then use a TPS60403 to generate -2.5V but the LM7705 solution would be a lot simpler - if it works.

  • 0
    TI__Guru** 113765 points

    Hi Stephen,

    Welcome to the forum!  If you bypass the PGA on the ADS1120, you can have up to a gain of 4 with the input range all the way to 100mV from the supply rails.  Using the LM7705 may work in some cases and modes of operation, but you will violate the common-mode restrictions given in the datasheet for (AVDD-AVSS)/4 when the PGA is enabled.  The best mode of operation is with symmetrical bipolar supplies of +/-2.5V for the analog supplies.  We have not characterized the usage with the LM7705 so it is difficult to say how good of success you will have without actually trying it.

    I realize you already made a prototype, but there is another option using the ADS1118 where you can connect two TCs and there is no common-mode restriction relative to ground.  Full-scale range for this part can go to +/-256mV which would be equivalent to the ADS1120 using a gain of 8.  I guess it really depends on the temperature range and resolution you are hoping to get.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 20 points

    Hi Bob,


      Thanks, you've highlighted what I missed. Using the LM7705 puts the input of the PGA within range but not the output.

      The ADS1118 doesn't give the range I need unfortunately - this is for a heater controller and it would only give around 13 codes per degree which I don't feel is enough to resolve temperature accurately enough to give a resolution of 0.1C.

      My previous design used the Microchip MCP3421 which gives 18 bit resolution and the same input range. This worked fine aside from the very low update rate. I'll stick with trying the modification to use split rails I think.


      Thanks again,

    Stephen