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INA220 / measure the AC current

Ryuji
Guru 20090 points
Other Parts Discussed in Thread: INA220, ADS131E04, ADC101C021

Hello,
 
Our customer use INA220 for measurement the AC current of +/- 10mA.
I beieve that , we can use INA220 for measurement the AC current.
But please kindly let me know as following.
 
(1)
Please see the attached file as schematics idea.
If there are any concern please let me know.
 
(2)
If there are any recommend device without INA220, please let me know.
 
Best regards,
Ryuji Asaka
 
 
  • 0
    TI__Genius 14825 points

    Ryuji Asaka,

    Thank you for your interest in the INA220.  I have a few questions about this application before I can decide if the INA220 is a good fit for it.

    1. What frequency and amplitude is the AC waveform?

    2. Is the +/- 10mA current riding on a larger current or would the shunt voltage be centered around 0V?

    3. What is the Common Mode Voltage across the current shunt ("Supply" in the schematic you provided)?

    4. The INA220 can convert at 9, 10, 11, and 12 bit resolutions at the cost of slower sampling rates.  What is the target resolution of the application?

    5. Is the goal to get an RMS current by taking sequential readings and calculating it, or simply to get a peak-to-peak current by constantly evaluating the measurement results?

    6. What is the smallest current the customer expects to measure?

    Thank you; I hope to help you with your application soon!

    Jason Bridgmon

  • 0 in reply to Jason Bridgmon
    Guru 20090 points
    Hello Jason san,
     
    Thank you for the reply.
    Please see the following as reply to your questions.
     
    1. What frequency and amplitude is the AC waveform?
    >>about 9kHz ~10kHz, amplitude is from -43mV to +43mV.
     
    2. Is the +/- 10mA current riding on a larger current or would the shunt voltage be centered around 0V?
    >>Yes. Centerd around 0V(i.e. 0A)
     
    3. What is the Common Mode Voltage across the current shunt ("Supply" in the schematic you provided)?
    >> Supply voltage is from -43mV to +43mV. (i.e. 4.3ohm * +/-10mA)
    >>  Load tied to ground.
     
    4. The INA220 can convert at 9, 10, 11, and 12 bit resolutions at the cost of slower sampling rates.  What is the target resolution of the application?
    >>9bit resolution is no problem.
     
    5. Is the goal to get an RMS current by taking sequential readings and calculating it, or simply to get a peak-to-peak current by constantly evaluating the measurement results?
    >>Goal is to get a Peak-to-Peak Current.
     
    6. What is the smallest current the customer expects to measure?
    >>Smallest Current is 0mA.
    >>They would like to measure the current range  -10mA to +10mA.
     
    Best Regards,
    Ryuji Asaka
  • 0 in reply to Ryuji
    TI__Genius 14825 points

    Ryuji Asaka,

    Thank you for your answers.  I have some suggestions and follow-up questions.

    1. What frequency and amplitude is the AC waveform?
    >>about 9kHz ~10kHz, amplitude is from -43mV to +43mV.
    Jason> With a ±43mV swing, you'll need to use PGA ÷2 to get the ±80mV full scale range.  The INA220 in 9-bit mode can sample at 93uS per sample, or rather, 10.752kHz.  Sampling a 10kHz wave at 10.752kHz will likely cause aliasing and make it very difficult to get a good idea of the peak-peak value of the waveform.
    2. Is the +/- 10mA current riding on a larger current or would the shunt voltage be centered around 0V?
    >>Yes. Centerd around 0V(i.e. 0A)
    Jason> Is this 0V the neutral line of an large AC power source?  It may require some isolation if it is.  If you're just trying to measure the current literally from a -43mV to +43V source to a load, then you will likely not need this isolation.
    6. What is the smallest current the customer expects to measure?
    >>Smallest Current is 0mA.
    >>They would like to measure the current range  -10mA to +10mA.
    Jason> It will likely be that when the current is close to 0mA, the bias currents of the device may be larger than the signal. This means the customer will lose some accuracy at times when the current is very small (close to 0mA).
    I'm thinking that a good Instrumentation Amplifier with a good ADC may be a good fit for your application.  I'll run it by those apps engineers and see what they have to say about it!
    Thank you,
    Jason Bridgmon
  • 0 in reply to Jason Bridgmon
    TI__Genius 14825 points

    Ryuji Asaka,

    After discussing the application a bit with a few other engineers, one suggestion for a different part that may be more suited to this application is the ADS131E04.  This device can sample at 32kHz (more than the 20kHz needed for decent sampling of the input waveform) and has a built in PGA.  With the PGA set to 12V/V, the number of bits of resolution is 11.3.  The input impedance is 200MOhms, so 4.3ohms on it shouldn't be much of an issue.


    This solution does have a few major differences, it is a serial interface and not an I2C interface, and it has substantially more pins and multiple power supplies.  The customer may not be willing to accept this and possibly the price could be an issue as well, with the INA220 at about $1 US, and the ADS131E04 at $3.95 US.


    Please let me know how else I can assist!

    Jason Bridgmon

  • 0 in reply to Jason Bridgmon
    Guru 20090 points
    Hello Jason san,
     
    Thank you for the reply.
    We would like to propose Current Shunt Monitor and ADC  separately due to cost issue.
    I will propose INA199A1(GBW=80kHz, gain=50, Bi-directional, Low cost) + ADC101C021(10bit, 188.9 kSPS, Registered device in the customer).
     
    Are there any concern in this proposal ?
    And if you have any recommend solution of separate Current Shunt Monitor and ADC, please let me know.
     
    -Maximum Sense Current: +/-10mA
    -Shunt Resistor: 4.3ohms
    -Singal Frequency: 10kHz
     
    Best Regards,
    Ryuji Asaka
     
     
     
     
  • 0 in reply to Ryuji
    TI__Genius 14825 points

    Ryuji Asaka,

    This INA199A1 and ADC101C021 solution seems reasonable.  I read up on the ADC101C021 and I see that it requires a stable Va because the power supply is also the reference to the device.  In addition to that, the I2C bus is supposed to be pulled up to the same potential as Va, so the customer should take this into consideration when designing their circuit.  See page 31 of the ADC101C021 for more explanation and application reference circuits.

    The INA199A1 appears to be gain stable out to 10kHz and begins to drop off after that for a 15mV sine wave, which should satisfy your customer's requirements.  Refer to figure 3, page 5 of the data sheet.  At the bottom of page 9, the data sheet says that the INA199A1 operating on a 3.3V supply could easily handle a 60mV full scale shunt drop with only 150uV of offset.  I believe this is more than adequate.

    Jason Bridgmon