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ADS1248: Basic question using this part.

geunwoo park
Prodigy 30 points

Part Number: ADS1248
Other Parts Discussed in Thread: INA190

I have questions about ADC with Amp (ADS1248).

I'd like to use the ADS1248 for measuring current consumption on 6V power line at test set.

Q1) Is it possible to measure less than 10uA Unit?

Q2) If I measure about current less than 10uA, What is the shunt resistor value for measuring current between AINP with AINN?

Q3) they use External Voltage REF (REF5020ID) At ASD1248 EVKit schematic, but ADS1248 has internal Voltage REF.

I want to just use internal Vol REF. Is it possible to use only this? If I do this, Is there any problems predicted.

  • 0
    TI__Guru* 99465 points
    geunwoo,


    It would help for you to draw a diagram of how you want to make a measurement. There are two basic problems that I can see using the ADS1248, without even knowing what you have planned about your circuit. First, if your supply is 6V, you will need an LDO to lower the voltage for the supply of the ADS1248. The absolute maximum AVDD is 5.5V. After that, a shunt measurement will be at one of the supplies. This means that the shunt resistor should be near 0V or 6V. Even if the supply were at 5V, input range of the PGA is limited in range. There is an equation for the PGA common-mode input range that is listed as equation 3 in the datasheet. The input is limited to be within AVSS+0.1 to AVDD-0.1 and this is even more limited with larger inputs and more gain.

    If you want to make some sort of current shunt measurement, I think you'll need an additional amplifier to give gain and then shift the voltage measurement to the middle of the ADC's PGA range. I think the INA190 might work for you if the noise isn't too high

    Regardless, I'll answer your questions. I'm making a few assumptions as to how you will end up making the measurement, however, it should help you get started.

    1. and 2. Yes, you should be able to measure 10uA as the smallest measurement. However, it does require some thought as to what you use for your current shunt resistor. I would start with the largest current that you expect to measure and that will help determine what your largest full scale range should be. After that, the limiting factor in making a 10uA reading will be how much noise there is in the ADC measurement. In the ADS1248 datasheet, Tables 1-6 show the noise performance of the ADC. You should be able to calculate what your full scale range and determine what noise level you expect from the given tables.

    3. Using the internal reference vs the external reference will depend on what noise you might tolerate in your design. There's a link below on the reference noise impact to the measurement:

    e2e.ti.com/.../the-impact-of-voltage-reference-noise-on-delta-sigma-adc-resolution

    Often the external reference will have slightly better noise performance. However, you'll also need to consider the reference value and initial accuracy as well. They will have an impact on the range of measurement and the gain error.


    Joseph Wu