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ADS1248: ADS1248 compatibility with ADS124S08

A GIR
Genius 3615 points
Part Number: ADS1248
Other Parts Discussed in Thread: ADS124S08, ADS1247

Hello

We develloped some year ago some application using ADS1248.

I have 3 questions:

1- Can I replace the ADS1248 by the ADS124S08, without change anything else? (Spi communication, Register access, hardware ..) Just keeping in mind the new one has two channel, then we don't use the 4 last channel..?

2- We devellop one application based on the ADS1248, but using only the AD0 ..AD3 based on the ADS1248 datasheet p76:

Figure 114. Ratiometric 3-Wire RTD Measurement System Featuring the ADS1247

With this, we have one channel of PT100 sensor.

Today, if we want add a channel of PT100, using the AD4..AD7, how can I adapt the schematic relatif to the REFP/REFN?:

- a) Can I have twice Ref filter from pin REFP0/REFN0, then duplicate all the schematic composed by the (CR_CM1, CR_CM2, CR_Diff, Rr1, Rr2)

- b) Keep only one Ref filter (CR_CM1, CR_CM2, CR_Diff, Rr1, Rr2) and have the lead 3 of 2 PT100 common?

3- New application note for the ADS124S08:

https://www.ti.com/lit/an/sbaa334/sbaa334.pdf

"Three-wire PT100 RTD measurement circuit with low-side reference and one IDAC current source".

The schematic is a little different on the "Lead 3" / AIN3.

- Is a specific for the new ADS124S08? Or twice schematic can be applied on both ADS1248 AND ADS124S08?...

- Which is the difference between the two solutions?

Thank

  • 0
    TI__Mastermind 31655 points

    Hello,


    The ADS1248 and ADS124S08 are not drop-in replacements for one another. Aside from the packages, the main concern is that the configuration register maps are different. Commands written for one of the devices will not access the same registers on the other.

    If you would like to add an additional PT100 channel and maintain the lead resistance cancellation features, you will not be able to use the same topology shown in Figure 114 in the ADS1248 datasheet. The main limitation is the number of IDAC current sources. You will need 2 current sources for each of the PT100 channels, and there are only 2 current sources available in the ADS1248 (as well as the ADS124S08). A solution for this is described down below in reference to the application note that you linked in your post.

    Since each of the RTDs will have its own supply, in order to have the ADC reference ratiometric to your supply you will also need to duplicate the REF filter, and attach lead 3 on each of your RTDs to separate reference inputs.

    The application note you referenced (www.ti.com/.../sbaa334.pdf) is not specific to the ADS124S08. It can also be implemented in the ADS1248. The main difference between this topology and your original design (from Figure 114 in the ADS1248 datasheet) is that this design uses only 1 current source, but in order to perform lead resistance cancellation there has to be a second reading across lead 3. So you trade 1 current source for a second measurement and ADC reading. In your case, if you would like to measure 2 RTDs there are only 2 current sources available so you can only use 1 per RTD, and this topology is a good way to do that.

    If you would like some more information about these different RTD measurement topologies, please see A Basic Guide to RTD Measurements. Section 2.3 goes over your original topology, and section 2.4 covers the single IDAC measurement circuit.

    Regards,

  • 0 in reply to Scott Cummins
    Genius 3615 points

    Hello,

    Thank for your complete back.

    Right, we use two current source, but to use a second way (with AN4 to AN7) we not realize a conversion in the same time, then we can switch all from the first way to the second way? (and then reverse)

    Currently, for only one way, I have the following scheme which seems ok for me:

    To start a conversion I do the follwoing:

    "Tag Begin:"

    Write MUX0.MUXSP = AIN1

    Write MUX0.MUXSN = AIN2

    Write MUX1.REFSELT = REF0

    Write SYS0.PGA = 8

    Write SYS0.DR = 5SPS

    Write IDAC1.IDIR1 = AIN0 & IDIR2 = AIN3

    Write IDAC0.IMAG = 1500µA

    Write MUX1.VREFCON = 01

    START pin is always set to +3.3V.

    I wait the DRDY\ Signal and then read the result.

    Then I restart on tag "Begin:"

    No If I have two way, the second on the AIN4 to AIN7, I can use the Ref1 for this second way, and then alternate the command above, and then the command bellow:

     

    Write MUX0.MUXSP = AIN5

    Write MUX0.MUXSN = AIN6

    Write MUX1.REFSELT = REF1

    Write SYS0.PGA = 8

    Write SYS0.DR = 5SPS

    Write IDAC1.IDIR1 = AIN4 & IDIR2 = AIN7

    Write IDAC0.IMAG = 1500µA

    Write MUX1.VREFCON = 01

    START pin is always set to +3.3V.

    I wait the DRDY\ Signal and then read the result.

    Then I restart on tag "Begin:"

    This seems Ok for you?

  • 0 in reply to A GIR
    TI__Mastermind 31655 points

    Hello,

    So my understanding of your last post is that it is not important to you to measure both RTDs at once, so your idea is to continue to use the dual current source method to cancel the lead resistance, and to MUX the current source outputs to different pins, switching back and forth between the two RTD circuits. Is that correct?

    As long as the delay for the settings change is acceptable, that seems like a possible solution. My only concern would be that the RTD may take a bit of time to settle once the excitation is applied. You may need to experiment to see how long it takes for the measurement to become stable once switching the excitation supply to your RTDs.


    Regards,

  • 0 in reply to Scott Cummins
    Genius 3615 points
    In fact, I never imagined that we could do simultaneous conversion on several channels!

    Anyway, we measure temperatures which evolve quite slowly.

    I need to check about the timming.
  • 0 in reply to A GIR
    TI__Mastermind 31655 points

    Just to clarify, the ADS1248 and ADS124S08 are not designed for simultaneous sampling. My main concern was just being able to convert as quickly as possible, but if time is not a large concern then the option of muxing the IDAC outputs to different pins for each of your RTDs is a good solution.

    Regards,

  • 0 in reply to Scott Cummins
    Genius 3615 points

    Thank for all these information