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INA240: INA240A1 to ADS1115 for measuring +-50mV current shunt

ferdster
Expert 1125 points
Part Number: INA240
Other Parts Discussed in Thread: ADS1115

I want to build something similar to:

TI Designs: TIDA-03040
Automotive Shunt-Based ±500-A Precision Current Sensing Reference Design
www.ti.com/.../tiducj6a.pdf

But instead of the PGA400, I want to just use a 16-bit ADC (ADS1115).

Our board has a regulated 3.8V supply.

Plan is to use a voltage divider to divide 3.8V by 2 for the reference voltage. Does this need to be buffered or is a simple resistive voltage divider OK?

The output of the INA240A1 would then be 0.9-2.9V. Followed by a 1/4 voltage divider gives me 0.225-0.725V, which is perfect for the ADS1115 +-0.256V differential input.

Does the reference voltage need to be super precise? I mean let's say it drops from 3.8V to 3.4V, then the input to the ADS1115 would become 0.175-0.675V. But since the ADS1115 is measuring differentially, wont the result still be the same?

Another scenario is that my board could be powered by a 3.6V battery. And if I use it divided by 2 for the reference voltage it would vary as the battery discharges. This goes back to my question above on if it matters if the reference voltage is not stable when going into a differential ADC?

Thanks for the help.

  • 0
    TI__Mastermind 21760 points

    Hello ferdster,

    Thank you for considering to use Texas Instruments in your design. Swapping the PGA400 with the ADS1115 should not be an issue.  As for using a voltage divider on the REF, we usually recommend a buffer or hooking the output and REF pin directly to a differential ADC like what you are proposing (click here if you want more info on this topic).   Since your using our proposed ADC method, I think your implementation should be fine and the measurements should be approximately the same as long as the differential voltages are the same.

  • 0 in reply to Patrick Simmons
    Expert 1125 points
    Taking into consideration that I might be on battery power, what resistor values would you suggest for the 1/2 voltage divider?

    Thanks.
  • 0 in reply to ferdster
    TI__Mastermind 21760 points

    Hello Ferdster,

    As you are wanting to save on power, you can use resistors in the 100 kohm range or higher.  However, keep in mind that larger resistors will produce more noise on your Ref pins, which then may require you to put a filtering cap on your ref pin.  This will then slow-down your startup and response time.   Based on the RC time constant, you can determine what is acceptable for your system if response time is an issue.