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INA219 Shunt resistor

Aska
Intellectual 740 points
Other Parts Discussed in Thread: INA219, INA282

Hi

 if measured under the conditions, how to select the appropriate shunt resistor? Thanks.

Voltage:17V~4V

Current:0.07A~1.5A

Shunt A/D Configuration:12bit

  • 0
    TI__Guru 62035 points

    Hi Huang,

    The INA219 is a bit unique compared to other current shunt monitor devices because it has a configurable PGA.  This enables the user to change the gain during the span of the measurement.  This allows for the use larger shunt resistor values which help with accuracy issues at the low level inputs without saturating the ADC at the higher level inputs. 

    So to determine the max allowable shunt resistor we will look at the maximum input current which is 1.5A.  We will use a PGA gain of /8 at the highest input current levels which allows for a maximum input voltage of 320mV.  The maximum shunt resistor value we can use to achieve a full-scale input voltage is :  0.32V / 1.5A = 0.213 Ohms. 

    With a 0.213Ohm shunt resistor let's examine the system at the minimum current input of 0.07A.  The minimum current will produce an input voltage of:  0.07A * 0.213Ohms = 0.0149V.  If we kept the PGA set to /8 then we would only be using 0.0149V / 0.32V = 4.66% of our available ADC range.  So at the low current levels we'll increase the PGA to /1 which allows for a full-scale input of 40mV.  Therefore we'll be using 0.0149 / 0.040 = 37.25% of our conversion range.  A marked improvement. 

    The dominant error in this system will be the initial offset voltage and the highest error will occur at minimum load current.  At a PGA gain of /1V/V the input offset voltage is max 100uV.  Performing a quick calculation shows that the error at the minimum current will be:  Vos / Vshunt = (100uV / 0.0149V) * 100% = 0.67% resulting in a system with 99.33% accuracy.

    In your final system you will control the gain of the INA219 such that the PGA is always kept at the lowest division as possible to ensure the highest accuracy and when the input current begins to saturate the ADC input you will switch to the next lowest PGA gain setting until you are able to resolve the signal again.

    So in summary choose the maximum shunt resistor value to achieve the highest accuracy at the low current input levels.  The maximum allowable shunt resistor will be determined by the maximum input current and the maximum allowable input voltage.

    Also to learn more about general current sensing systems please read through the following series:

    Current Sensing Articles:
    Overview:
    http://www.eetimes.com/design/industrial-control/4236026/A-Current-Sensing-Tutorial-Part-1--Fundamentals

    Devices:
    http://www.eetimes.com/design/industrial-control/4236380/A-Current-Sensing-Tutorial-Part-II--Devices

    Accuracy:
    http://www.eetimes.com/design/industrial-control/4237268/A-Current-Sensing-Tutorial-Part-III--Accuracy

    Layout and Troubleshooting:
    http://www.eetimes.com/design/industrial-control/4237683/A-Current-Sensing-Tutorial-Part-IV--Layout-and-Troubleshooting-Guidelines

    Best Regards,
    Collin Wells
    Precision Linear Applications

  • 0 in reply to Collin Wells
    Intellectual 740 points

    Hi Collin,

                Thank you for your detailed instructions.
                I still have questions, please help, thank you.

    1.Why Datasheet page 17 and the page 23 will be 15Bit the calculation? The ADC maximum is 15bit?

    2.If I want to measure the over voltage of 32V if 40V can how design?Divider resistor?


  • 0 in reply to Aska
    TI__Guru 62035 points

    Hello,


    Unfortunately I'm not sure I understand your first question.  The INA219 can have up to 15 bits of data with 1 bit of sign equally a full 16-bit word.

    The maximum common-mode input voltage of the INA219 is 26V, you may not exceed this value without damaging the INA219.  The voltage measurement in the INA219 is used to measure the bus voltage that the shunt resistor is located on.  It is not possible to externally divide down the shunt and bus voltage without negatively affecting your current measurement.  If you need to measure the current on a bus with a common-mode voltage greater than 26V you will need to use a different current-shunt monitor such as the INA282 which has a wider common-mode input voltage range.

    Regards,
    Collin Wells
    Precision Linear Applications

  • 0 in reply to Collin Wells
    Intellectual 740 points

    Hi Collin,

                        Does the INA219 have the Layout guide? thanks.

  • 0 in reply to Aska
    TI__Guru 62035 points

    Hello Huang,

    The INA219 does not have a specific layout guide.  However, you can learn about common current sensing layout pitfalls and how to avoid them in the following article:

    Layout and Troubleshooting:
    http://www.eetimes.com/design/industrial-control/4237683/A-Current-Sensing-Tutorial-Part-IV--Layout-and-Troubleshooting-Guidelines

    The main concerns are to place the shunt resistor as close as possible to the INA, keep the input traces short and symmetrical, and ensure you've applied a Kelvin (4-wire) connection to the shunt resistor. 

    Regards,
    Collin Wells
    Precision Linear Applications