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ADS1120: Do the IDAC current sources have a limitation in capacitive load?

Dennis Eichmann
Guru 74080 points

Part Number: ADS1120

Hi everyone!

Is there a limitation of capacitance that the included current sources can handle? I do not find anything in the datasheet about this (or I'm just blind). Bob, if you read this - we once talked about this application as shown here:

This is indeed not a common application, but I needed it for rough temperature measurements and it works very well. I also added the two red capacitors and did a lot of tests and measurements and everything worked really good without any oscillations. I'm sourcing a current of 100µA out of AIN0 and also using this pin as REF+. Then I use AIN3 as REF- and I also sample AIN3 against AVSS. From the performance point of view this is surely not the best thing, but I do not need very precise results at all. The other channel is important for me (not shown here). Temperature is just a side info and can be wrong for a few degrees. But now I was wondering if there is a limitation of capacitance the current sources can drive since there is no information about the working principle of the current sources. Is there any known limitation? Honestly this is just for my personal information because there is not actual problem, but I would be interested to know.

Dennis

  • 0
    TI__Guru** 113765 points

    Hi Dennis,

    I am not aware of any limitation or stability issue by adding a large capacitive load to the IDACs.  Generally this would not be done due to the potential for large settling times.  I'll continue to do some investigation and update this thread if there is any limitation.

    I have built a similar circuit for RTD measurements using a high-side reference and saw no issues.  However the largest cap value I used was 100nF.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Guru 74080 points
    Hi Bob!

    Thanks for the reply - 100n is the value I used for both caps as well. As I said there is no problem at all - just wanted to know.

    Dennis
  • 0 in reply to Dennis Eichmann
    TI__Guru** 113765 points

    Hi Dennis,

    I talked to one of the designers and he didn't think that capacitance alone would be an issue, especially at the level of current output from these devices (thinking in the general range of 10uF to 47uF load).  However, if we think this through and enough inductance exists or is added to the path along with the large values of capacitance there could be the potential for an instability.  The likelihood of this being an issue is very small relative to the increased analog settling being unacceptable in an application.  Good question!

    Best regards,

    Bob B