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Linux/ADS1147: ESD, Burst Protection 5kV

rainer scalick
Prodigy 75 points

Part Number: ADS1147
Other Parts Discussed in Thread: TPD2E1B06

Tool/software: Linux

Hi,

Right now I try to evaluate the ADS1147, but I cannot find a application for ESD, Burst or Surge.

I am specially interessted in the 3 Wire Ratiometric RTD Measurement System. 

  • 0
    TI__Guru** 100225 points
    Rainer,


    Normally, the ADS1147 is rated to ±2000kV HBM and ±750V CDM for ESD protection. Anything more than that will require additional external protection.

    That being said, I don't have too many many specific recommendations. In the past, we've recommended diodes to sink over-voltage event current to supply rails and used series input resistances to limit current going into pins. For protection diodes, the important characteristic is the leakage current. The leakage term from the diode becomes a gain error in an RTD measurement. Taking the case of 3-wire RTD measurement with a low side reference, imagine the leakage from a diode in a ratiometric measurement is 1uA and the excitation current is 1mA driving both input leads of a 3-wire RTD. The reference resistor sees 2mA minus the 1uA. This would add a gain error of about 0.5%. Ideally you want to keep the leakage terms low to reduce this error. I've seen circuits that use Schottky diodes such as a BAT54 type, but in many cases the max leakage is 2uA, which may be a bit high. A BAV199 will have lower leakage, which will likely be under 10nA. Also, something like the TPD21E1B06 might be useful protection because of the low leakage, but I've never used or tested them.

    After latching the inputs to the supply through a protection diode, Consider the series resistance. In general, series resistance from the input filtering can protect the analog inputs. I wouldn't go higher than 5kΩ for series resistance or it may disrupt the input sampling for the ADC, but this should limit any current going into the inputs. Additionally, you may protect the pin supplying the excitation current with a diode or another series resistance. Just remember that if you use a series resistance, that the total resistance that the IDAC current can drive will be limited. If the IDAC current output voltage gets to be to high, the current will drop as the current source runs out of compliance headroom.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 75 points

    Hi Joseph,

    thanks for your support. I have tried that already yesterday with series resistance but I would not go higher than 1000 Ohm. In Case of a PT100 you will become a failure of about 50% in the worst case! I have tested the circuit wit a SMAJ5A Diode from STMicro electronic with a leakage current of 1uA it is working pretty fine! I will try to test that this week and if you like i can share with you the results!

    Sounds good that you thought the same as I do ;)

    Rainer Scalick 

  • 0 in reply to rainer scalick
    TI__Guru** 100225 points
    Rainer,


    I'm glad that you are able to get the device working with good data. Hopefully, the ADS1147 will work for you.

    I would note that diode leakages will typically be worst at hot temperatures. If you are looking at data, the errors may be small at room temperature but may get worse at high temperature. I've tested RTD measurements with precision resistors with very low drift.

    One last point. I didn't understand your comment about using series filter resistance greater than 1kΩ and having a failure rate of about 50% in the worst case. Can you elaborate on that?


    Joseph Wu
  • 0 in reply to rainer scalick
    TI__Guru** 100225 points
    Rainer,


    Just to clarify, I'd misspelled the ESD device that I'd written in my last post. The device should be the TPD2E1B06 (again, I've never tested these).

    Also, the BAV199 may have a high forward voltage of 0.9V at 1mA, so it may still require some series impedance. If this is too high, I'd consider the BAS70 which has a forward voltage of 0.41 at 1mA, but with a slightly higher leakage current. This is certainly low, but may still require some series impedance.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 75 points
    I am sorry for the misunderstanding. I case of the two wire measurement Methode you will become an big error, if you put a series restistant on the pcb. The copper resistant is okay and you can adjust it during testing, but a resistant with a hugh R is bad.
  • 0 in reply to Joseph Wu
    Prodigy 75 points


    Red line = Voltage after serie resistant


    Green Line = Voltage after diode


    Blue Line = Current inside of the ADS1147


  • 0 in reply to rainer scalick
    TI__Guru** 100225 points

    Rainer,

    I'm not sure I understand. Did you have a schematic that you weren't able to include in your last post?

    For the two-wire RTD measurement, the lead resistance of the RTD will always add an error to the measurement. However, the pcb resistance is probably a very small error. Below is a standard two wire RTD measurement:

    The error from the leads will be:

    IIDAC * (RLEAD1+RLEAD2)

    The only other resistive errors will be the input series resistances (from the input filtering reacting with the analog input current - probably < 10nA). At the input, this may appear as an offset. Similarly, the reference input may have error with the reference input current reacting with the series filter resistances. Because this error involves the reference value, this will appear as a gain error in the measurement.

    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Guru** 100225 points
    Rainer,


    I should probably attach some numbers to the last post.

    The error for the RTD lead resistance is much larger than the error from the input current going through the filter resistors. First, look at the lead resistance error.

    If you have a 1mA IDAC current and 1Ω leads, then the error will be:

    1mA * (1Ω + 1Ω) = 2mV

    If you're using a PT100, this is a very large error, that you could significantly reduce by using a lower IDAC current and a PT1000 RTD. Using a 100uA current, you would get an error of 200uV

    Second, looking at the input current, check Table 4 of the ADS1147 datasheet. The input current is 0.5nA at DR=20SPS (getting larger at higher data rates and large input signal). For this case, lets assume the input voltage is small. If the input filter resistors are 5kΩ, then the error will likely be:

    0.5nA * (5kΩ + 5kΩ) = 10uV

    This 10uV error is very small compared to the lead resistance error of 2mV.

    Similarly you can look at the reference input. The input current is 30nA typical. For this you could remove the negative reference input filter (REFN0 input filter) because it is grounded and doesn't require a filter. This error would be:

    30nA * 5kΩ = 150uV

    This error will appear as a gain error. Again, lets assume that your IDAC current is 1mA and you use a 2kΩ reference. Here, your reference is designed to be 2V, but the reference is larger by 150uV. This means that the reference is 0.0075% larger than expected. That means all measurements will be 0.0075% smaller than expected and will be a gain error.

    Note that you'll need to use a reference resistor that is high accuracy and low drift. In more precise 3- and 4-wire measurements, the reference resistor accuracy will be a very large part of the error in the measurement.


    Joseph Wu