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INA219: Max wire length between shunt and Vin+/Vin- ?

Mats Ekberg
Prodigy 10 points
Part Number: INA219
Other Parts Discussed in Thread: INA237, INA238, INA228

Max wire length between shunt and Vin+/Vin- ?

I have a design in a RV and the wire length from the battery shunt to the INA219 is about 1.5 metres, is that okay or does it affect resolution etc?

  • +1
    TI__Mastermind 34220 points

    Hi Mats,

    The distance shouldn’t be a problem. However it does mean more opportunities for external interference to couple into the signal path, which may impact system performance or in the worst scenario damage the device.

    Some common best practices should help minimize these possible negative consequences. Examples I can think of - diff/twisted pair, common mode filtering and TVS clamps close to the device for EOS protection.

    Regards, Guang

  • 0 in reply to Guang Zhou
    Prodigy 10 points

    The resistance of the looong wires will of course affect the offset voltage on Vin+/Vin- ??

  • 0 in reply to Mats Ekberg
    TI__Mastermind 34220 points

    Hi Mats,

    I hope the wire resistance is small, in the single digits? If so its effect can be ignored. For example the average resistance of a piece of 10 feet, #28 wire is 0.7Ohm. This is the reason why I didn’t mention it.

    Here is a relevant paragraph from datasheet about the effect of small resistance:

    Regards, Guang

  • 0
    Guru 140200 points

    Hi Mats,

    the INAs are designed to have the shunt mounted close to the input pins. Increasing the distance has several unwanted side effects: The farer the shunt is moved away from the input pins, the less balanced the source impedances are seen from the two inputs. I mean especially the complex impedances at higher frequencies here. This will have a negative impact on the common mode rejection. As a countermeasure, route the two input wires from the shunt to the input pins of INA as twisted pair. This will improve the situation to some degree but you are still far away from an ideal situation.

    Also, moving the shunt away from the input pins of INA drastically increases the loop area formed by the input wires. By this, magnetic fields can induce unwanted EMF between the input pins of INA appearing now as differential signal which the common mode rejetion of INA can no longer reject. Routing the two input wires as twisted pair again is a countermeasure. This will improve the situation to some degree but you are still far away from an ideal situation.

    Kai

  • 0 in reply to Guang Zhou
    Prodigy 10 points

    With 10 ohms I got 46 in the shunt voltage register and with 0 ohms I got 4. This gives 0.42mV offset which in turn indicates 0.56 amps of current with my 0.075mV/100A shunt. That is a bit high in my application; a RV battery supervisor, 

  • 0 in reply to Mats Ekberg
    TI__Mastermind 34220 points

    Hi Mats,

    It is great that you’re checking it out. Although the first result might not be as good as you would like, there are probably ways to improve.

    If the readings jump around due to noise, you could set PGA=1; use ADC setting of longer conversion time and greater number of averaging. Once the offset is repeatable and consistent, maybe it could be subtracted from the calculation.

    If none of these helps, you may want to take a look at INA238, INA237 or INA228. This family of new, high performance devices are less sensitive to external resistances.

    Regards, Guang