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ADS114S08: Is the ADS114S08 a replacement for the ADS1148? Which is better for both single-ended and differential measurements?

tim jager
Prodigy 120 points
Part Number: ADS114S08
Other Parts Discussed in Thread: ADS1148, ,

The ADS114S08 appears to be a new/improved version of the ADS1148. Should I consider the ADS114S08 first for new designs? Also, is this ADC preferred for a combination of both single-ended and differential measurements?  We will be reading 4 wire RTDs, a 4-20mA signal (by measuring voltage across a shunt resistor), and a 0-5V signal (scaled with a voltage divider to fit within the allowable input voltage range of the ADC). Which ADC is best for this application? 

Thanks, 

Tim

  • 0
    TI__Guru* 97585 points
    Tim,


    We think of the ADS114S08 as the next generation of the ADS1148. The ADS114S08 often has better specifications than its predecessor and more fault detection features.

    Additionally the ADS114S08 has a PGA that can be bypassed so that AVSS (or ground if it is so connected) is a valid input. This makes it easier for single-ended measurements. For the ADS1148, the PGA cannot be disabled and AVSS is not a valid input because it is outside the PGA input and output range.

    I would use the ADS114S08 for your application. Once you have a basic topology and design, feel free to post back if you need someone to review it.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 120 points
    Thanks Joseph. This is really helpful. Could you tell me the difference between the ADS114S08 and the ADS114S08B? Which should I choose?

    Thanks again,
    Tim
  • 0 in reply to tim jager
    TI__Guru* 97585 points

    Tim,

    The ADS114S08B is very similar to the ADS114S08, with some of the features removed. There are also a few differences in the specifications (from the MIN and MAX columns of the Electrical Characteristics).

    Here are the differences in a single graphic.

    What you decide to use will depend on what features you need and what specifications you need in your design.

    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 120 points

    Thanks Joseph. I think the ADS114S08 is best for our application. I really appreciate your quick responses.

    Do you know if the ADS114S08 can be configured to measure four (4) PT1000 RTDs without putting them all in series?  We have an application that requires measuring between 1 and 4 PT1000 RTDs, so we would prefer to not put them all in series with with the single reference resistor. In the case where only one is connected, this would break the series chain and no current would flow. Is there a way to use the MUX to switch the reference resistor between 4 different RTDs and still do a ratiometric measurement?

    Thanks, 

    Tim

  • 0 in reply to tim jager
    TI__Guru* 97585 points
    Tim,


    Connecting four RTDs without measuring them in series depends on what type of RTDs you are using. Are you measuring 2-, 3-, or 4-wire RTDs?

    You may also be able to use a multiplexer to help with additional channels, but you need to to consider the series resistance an current leakage.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 120 points
    Joseph,

    These are 2 wire RTDs. They are PT1000 (nominal resistance of 1000 Ohms) so the lead resistance is negligible.

    Is this doable with the ADS114S08 alone, or would we need to add an external MUX?

    thanks,
    Tim
  • 0 in reply to tim jager
    TI__Guru* 97585 points

    Tim,


    You should be able to connect four 2-wire RTDs to the ADS114S08 device. You shouldn't need the help of a multiplexer. I think something like this should work:

    All negative leads are connected and you select the IDAC to go from channel to channel.




    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 120 points
    Joseph, this is perfect. The key is connecting the negative leads of each RTD. You are the man!

    thanks,
    Tim