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INA219 Design Considerations

Mark Meisner
Prodigy 120 points
Other Parts Discussed in Thread: INA219

I am considering using the INA219 in a design.  Here is a brief overview of this particular part of the circuit:

Bus Voltage: 6.7V

Load: 50ohms

Max expected current : 134mA

Min expected current: 0 mA

We need to digitally control the power delivered to the load.  Therefore, a MOFET will be put between the load and ground.  It will be switched on/off via MCU PWM.  Modifying the duty cycle will change the average power.  We have verified that this method works, however we need a way to monitor power being delivered to the load.

The PWM signal has to be above about 20kHz to get out of the audible frequency range.  I am aware I cannot use an RC filter before the INA219 because the series resistance will produce errors in the results.  

1) Can I use the INA219 for this application?  I do not need instantaneous power.  Since the ADC is a delta-sigma converter, can I just oversample the signal and the device will produce an average? Or is there a limiting factor I am overlooking?

2) How would I calculate a shunt value for this? For more accurate results, would I have to manually change the PGA gain for what I would expect the power to be?

Thanks,

Mark

  • 0
    TI__Genius 12335 points

    Mark,

    Zero-drift amplifier typologies can have issues with large steps/transients in the input common mode voltage, which is exactly what a PWM waveform would produce. Therefore, I'm not sure that directly sampling a PWM current waveform with the INA219 would provide very accurate results. As you said, very large resistances in an input low-pass filter will introduce inaccuracies in the measured current, but it may be possible to use an input LC filter (rather than an RC filter) to low-pass filter the PWM before it is read by the INA219. Another possibility is increasing the PWM frequency which would allow a higher corner frequency on the input low-pass filter, improving accuracy. 

  • 0 in reply to John Caldwell
    Prodigy 120 points

    Sorry for the late reply, I was on vacation this past week.

    When you say the "input low-pass filter", are you referring to the external filter I would add, or the internal low-pass filter within the ADC?  If I added the maximum external recommended 10ohm and 1uF components, the frequency cutoff would be 16kHz.  For the LC filter I could use, 10uH and 10uF, producing the same cutoff frequency of 16kHz; however those components are more expensive.  This leads me to believe that I should try the RC filter and increase my PWM frequency as much as possible.  

    Any thoughts on my second question above?