• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 46 subscribers
  • Views 1143 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.

ADS1248, using burnout current sources

Vitali Lach
Expert 1130 points
Other Parts Discussed in Thread: ADS1248

Dear Sirs and Madams,

We are already using that ADS1248 in some projects for strain gauge measurement.

For some demonstration system I have to connect a sensor with a higher impedance up to 200k (single load measurement resistor, 10 Ohms, 10K, 200K). I am aware that ADS1248 is not designed for such very high impedance measurement, but it’s actually my task to make it possible using existing hardware which contains three ADC's in total.

 

My questions:

1.Is that basically possible to use burnout current source for that measurement? I’m well aware that I don’t have to expect a proper measurement precision. (I also able to use an external 2,048V reference voltage.)

 

2.The idea is to use a full bridge configuration (please see attached picture) or would you recommend  4 wires ratiometric measurement in that case?

 

Many thanks in advance.

 

Best Regards

  • 0
    TI__Guru* 97415 points
    Vitali,


    No, it is not possible to use the burnout current sources to drive the bridge measurement. There are a several reasons for that. First, the burnout sources are only active on the inputs being measured. In the example that you've drawn, the current sources would only be forcing current on AINP and AINN. Second, the current is forced into AINP and pulled from AINN and they may not match in magnitude. Lastly, there's a large variance in the current source magnitude. I would guess that this is somewhere near 30% at 0.5uA, and about 20% near 10uA.

    If you want to do bridge excitation, the only integrated solution would be using the excitation current DAC (IDAC). Unfortunately, the current only goes as low as 50uA. If you need a lower current value, I would use some other device for a current source.


    Joseph Wu
  • 0
    TI__Guru* 97415 points
    Vitali,


    And yes, you would generally use a ratiometric measurement for this application. I would note that since the resistors are very large, you need to watch how large the reference voltage gets. Even a modest amount of current could make the reference voltage large. Note that the maximum reference voltage is limited to (AVDD-AVSS)-1. At a supply of 0-5V, this limits the reference to 4V.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Expert 1130 points
    Hello Joseph and thank you very much.
    This help us a lot.

    Best Regards.