• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 46 subscribers
  • Views 718 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS1258 mux input leakage current

Nic Bard
Prodigy 30 points
Other Parts Discussed in Thread: ADS1258

I am using the ADS1258 to read an input signal which when open circuit, is terminated with a 499kohm resistor to 0V so that it does not float. The mux input leakage current is not specified for this device so I assumed a typical 10nA.

However, we are finding that at low temperature and room temperature this is fine, but at +70degC, it is very large. For assemblies up to now, it was less than 100nA but the last batch of parts showed up to 150nA which is failing our test limits.

Is there any data for this parameter?

Note that the MUX output is opamp buffered to the input of the ADC.

  • 0
    TI__Mastermind 44680 points

    Hi Nic,

    Welcome to the TI E2E Forums!

    I've been looking into this issue with Sally. Unfortunately, we do not have much characterization data on this. The closest thing I could find was the leakage current after exposing the inputs to a 2kV ESD pulse, which generally increases the input leakage current.

    Here is what I do know about the leakage current:

    • The average input leakage current after an ESD event was around 50nA, and the maximum was 500 nA.
    • The leakage current will increase with temperature (roughly doubling every 10 degC).
    • Leakage current will be highest when the input is shorted to ground.
      • It is better to terminate the inputs to AVDD, if you must terminate them. Biasing the inputs to mid-supply would be better still.
      • The best case is to leave the unused inputs floating. Placing a cap to ground can help mitigate EMI concerns.

    Best Regards,
    Chris

  • 0 in reply to Christopher Hall
    Prodigy 30 points

    Hi Chris,

    Thanks for your reply. The forum is indeed a better way to communicate!

    Unfortunately, in my application, the input must be terminated without affecting the signal and so that we can determine if it is open circuit. It is the open circuit threshold that we are trying to differentiate.

    For such a precision device, I am surprised that there is ESD leakage data but no normal operation data! Is there a similar device I can use the data from? Or am I going to have to characterise the device myself or make a guess?

    Regards,
    Nic

  • 0 in reply to Nic Bard
    TI__Mastermind 44680 points

    Hi Nic,

    I was under the impression that the inputs terminated to ground were unused, but it seems like you are actively using these channels. In that case, you may not be measuring "leakage" current. The delta-sigma modulator has a relatively low input impedance (65 kOhm differential) and could easily pull 200nA.

    • Do you use an input buffer between the MUXOUT and ADCIN pins? We recommend buffering with all delta-sigma ADC's, especially with high impedance inputs.

    • Would you be able to share a schematic with me? (You can email it directly to pa_deltasigma_apps@ti.com instead of posting it on the forum).

    For detecting an open circuit I would look into using the sensor bias current sources built into the ADS1258. I'm assuming the open-circuit occurs on a different input (not the input terminated by 500kOhm) and we could drive it high or low when open.

    Nic Bard said:
    For such a precision device, I am surprised that there is ESD leakage data but no normal operation data! Is there a similar device I can use the data from? Or am I going to have to characterise the device myself or make a guess?

    Let's determine if we are looking at leakage current. It may have been that it was measured, but I just don't have the data. Our characterization engineers are constantly increasing the amount of data they collect and have gotten better, over time, to organize and store this information for us application engineers.

    Best Regards,
    Chris