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LMP90100: LMP90100 with four RTDs

Manai Oussema
Prodigy 150 points
Part Number: LMP90100

Hello everyone,

I'm currently working on a project that involves the LMP90100 AFE. I've managed to make it work with the MCU and configure it. However, i wonder if anyone has tried this circuit with four RTDs(no matter for the type) connected at the same time, or anyone has an idea how to do it. Any help is welcome.

Thank you.

  • 0
    TI__Guru** 113765 points

    Hi Manai,

    First, you can connect 4 RTDs to the LMP90100, but just so we are clear there is only one ADC and the input channels go to a mux where only one input combination is read at a time.  There are not 4 simultaneously sampling converters, just the one converter.  So you will need to cycle through the mux to measure all the RTDs. 

    It does make some difference on which type of RTD you connect and how you intend to measure RTD.  For the LMP90100 there are 8 input channels that can be used.  The measurement itself will be differential so the maximum number of differential inputs is 4 allowing for the 4 RTDs.  The RTDs would be connected in parallel so that the each RTD is measured separately.  One end of the RTD would be connected in common across all four RTDs and that connection would also go to a bias resistor that is also used to establish a reference voltage.  This would appear similar to the connections in Figure 78 of the LMP90100 datasheet.  Note that for a 3-wire RTD you need 2 current sources in this configuration to cancel lead resistance from the conversion result.  The 2-wire and 4-wire would only need a single IB current source. 

    What becomes a little more challenging at this point is there are only 2 IB channels for excitation to the RTD/Reference.  Ideally you would want a separate IDAC for each RTD.  This could be accomplished by using an IB mux where the current can be routed 1 of 4 ways.  For 3-wire RTDs you would need a second mux to steer the current to compensate for the lead resistance.

    Just so that we are clear, this is not the only combination that can be used.  It is possible to voltage excite the RTDs, place RTDs in series combination, or use and external analog input mux instead of the current mux.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 150 points

    Thank you for your quick answer Bob,

    i wonder about connecting the RTDs in parallel, wouldn't that cause the current to be divided for each RTD ? and how can i measure them separately ?

     

  • 0 in reply to Manai Oussema
    TI__Guru** 113765 points

    Hi Manai,

    The reason you would need a 1 in and 4 out current mux would be to steer the current to the desired RTD for measurement.  This will prevent the current division and you can measure each RTD separately.  The RTD being measured is the one the current is be routed to at the time of the measurement.

    Best regards,

    Bob B