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how many RTDs measurement possible with ADS1248?

yavuz delican
Prodigy 140 points
Other Parts Discussed in Thread: ADS1248

We have decided to use two-wire RTD application as Fig.1. in application report of ADS1248 (as it looks like the first picture below) since our 1K RTDs are not far away (max 10cm line roughly equals 0.022ohm).But in this picture; it is only possible to read 4 RTDs.

But we have decided to use  as in the second picture  configuration since it is possible to read 5 RTDs and one extra signal to monitor.

It seems possible to read up to 7 RTDs as our configuration for the datasheet in Fig. 48 with 2 wire RTDs. But we want to make exactly sure about this. Also if this configuration is possible;what are the possible drawbacks if we compare to the attached first picture (4 RTD reading).

  • 0
    TI__Guru 55466 points

    Hello Yavuz,

    It is possible to measure 7 RTD's using the ADS1248 with the two wire configuration.  The main difference when comparing the two-wire versus the 4-wire configuration are the errors due to the series resistance on the lines connecting the RTD to the input of the ADS1248.  Provided that in your application the lines are kept short and the impedance of the RTD is relatively high (since you are using 1kOhm RTD); this error may not be very significant .  In the 2-wire configuration, since the IDAC excitation current goes through both lines connecting the RTD and the inputs of the ADS1248; any impedance in these lines will add to the over all resistance seen by the ADS1248 and cause an error in the measurement.  Since you are using a  relatively high impedance 1kOhm RTD; the 0.022 Ohm will cause a relatively small error

    Looking at the diagram below, on the 4-wire configuration, the IDAC excitation current flows through a separate line1 connection.  Since the inputs of the ADS1248 are high impedance (nA input bias current), the errors due to impedances on Line2 and Line3 do not have an effect on the accuracy of the measurement; this allows the user to place the RTD remotely without worriyng about the line impedances.

    In summary, provided that the RTD is in close proximity and the added line impedance is relatively small compared to the RTD being measured; the two wire could work well for you.  Please find attached a *.ppt file showing the ADS1248 connected to 7 RTD's on a 2-wire configuration, and 3 RTD's in the 4-wire configuration.

    Best Regards,

    Luis

    0412.Forum_RTD7.pptx

     

  • 0 in reply to Luis Chioye
    Prodigy 140 points

    Hello Luis;

    Thanks for the answer.

    Best regards.

    Yavuz Delican