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TIDA-010015: Current sensing accuracy question using INA181 in TIDA-010015

Part Number: TIDA-010015
Other Parts Discussed in Thread: INA181,

Hi,

On the third page of the TIDA-010015 schematic I see a 10Ohm resistor in series with a 0.5mOhm sense resistor. There is another 10Ohm on the IN- input. The problem is that the best 10Ohm resistor has 0.5% error, which is 50mOhm or 100 times above the sense resistor value. Wouldn't that cause sensing issues? The INA181 datasheet shows there is no need for input resistors on IN+ and IN- so can they be omitted? 

Please help me answer these questions:

  • Will resistors 66 and 78 on TIDA-010015 schematic cause sensing errors?
  • What are these resistors protecting INA181 against?
  • Can these resistors be omitted?

Thank you for your help,

Arthur

  • Arthur, 

    Thanks for the question!

    The schematic is not laid out the best, but you can see that the connection of the shunt is actually between the nodes "RSense" and "SGND," which are the netnames on the bottom end of each of R66 and R78. R66 is therefore not in series with the shunt resistor, but rather in the sense line connecting to the shunt, so the schematic could be redrawn similar to this:

    Where R66 and R78 are the "Rf" resistors seen above. 

    That said, these resistors do inject a small amount of error into the measurement, but not due to skew of the sense resistor, but rather by slightly altering the gain resistors internal to the INA181. This is discussed in depth in section 9.1.3 of the INA181 datasheet, and provides an equation for the additional error. Keep in mind though that the internal resistors of the INA181 are trimmed to each other rather than an absolute value, so they themselves can vary by up to 20%. Provided these resistors are kept to a small value (<10Ωs), this addition is minimal, even with this variation. 

    Lastly, it doesn't look like these were intended as use for filter resistors, as the designer did not include the differential capacitor used to set the cutoff frequency of the input filter (unless it is elsewhere in the schematic). One thing I'll point out is this. The problem here is the low side measurement is close to GND. Internal to the device there are ESDs to GND, and during any transition 0.3V below GND, the diodes will start to conduct. These resistors could be there to limit the current into the device in this type of event, although 10Ω's will not provide much protection.

  • Thank you for a quick answer!

    It makes more sense to me now. According to the equation in the INA181 datasheet, the error from the 10Ohm resistors there should be no more than 1.2%, which I think is not a problem so I'll keep the resistors.