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INA228: Question about INA228

Part Number: INA228
Other Parts Discussed in Thread: INA229

Hi, support team

My customer has the questions as follow:

I am thinking to use INA228 for VBUS voltage (max. 52V) and temperature measurement. Differential shunt voltage is not used.

1. Any other part numbers which may be more suitable for the above purpose?

2. For VBUS data accuracy, the datasheet lists the following but seems no specifications on reference and ADC accuracy. Are the specifications on reference and ADC accuracy available?

Vos_bus VBUS offset voltage
dVos/dT VBUS offset voltage drift
GBERR VBUS voltage gain error
GB_DRFT VBUS voltage gain error drift

3. How can I assess the accuracy of VBUS data? e.g. if VBUS is 52V, what is the error in the data from INA228?


Thanks so much.

Best regards,
Yuki

  • Hello Yuki,

    Thank you for using the TI forum, here are my responses to your questions:

    1. The INA228 (and INA229, SPI version) are the best devices we have for this application. 
    2. The accuracy specs for VBUS in the datasheet is the total VBUS accuracy and includes all relevant internal reference and ADC parameters.  This is why those are not also specified. 
    3. The accuracy can also depend largely on your settings, because if this, you may want to use the effective number of bits (ENOB) table.  This can help you see your noise free resolution based on your measurement settings. We also have a variety of training videos that show you how to look at different error sources in measurement.  Those can be found here: https://training.ti.com/ti-precision-labs-current-sense-amplifiers-introduction-different-error-sources. (This is the first video is section 3, but the whole section is about error sources). 
  • Thanks for your kindly reply.

    1. Table 8-2 lists ENOB, but seems only for shunt voltage, as the header says,
    NOISE-FREE ENOB (±163.84-mV) (ADCRANGE = 0)
    NOISE-FREE ENOB (±40.96-mV) (ADCRANGE = 1)

    Is there ENOB for VBUS?

    2. I am thinking to put a RC low-pass filter (R=15Kohms, C=1uF) on VBUS input, any comment?

    3. Understand VBUS voltage gain error drift is ±20 ppm/°C. Is this error constant or random at a temperature?

    Thanks.

  • Hello Yuki,

    Here are my responses to your questions:

    1. Sorry, I should have been a little more clear about this. The table in the datasheet is specifically for the shunt voltage, but the bus voltage measurement is done using the same ADC and has the same conversion and averaging settings.  Because of this, the ENOB for VBUS should be fairly similar to the ±163.84-mV column of the ENOB table, though it is not specified directly in the datasheet.
      1. I have reached out to the validation team to see if they can confirm if this ENOB table should be more directly comparable with VBUS, and will get back to you once I hear from them.
    2. This is probably fine, but just make sure to take the input resistance of VBUS into account for your calculations (Zvbus in datasheet, 1MΩ typical).
    3. Figure 6-13 of the datasheet shows the typical characteristics of how the bus gain error can change over temperature. I will paste that here for convenience: 

  • Thanks, looking forward to the confirmation on ENOB for VBUS.

    In that case, a RC filter before VBUS is not a good idea, since it reduces accuracy. It seems INA228 does not have any anti-aliasing filter before sampling, is the noise frequency above 1/2 Nyquist frequency a concern here?

  • Hello Yuki,

    The INA228 has a dynamic filter for the ADC based on the conversion rate to help reduce noise, see section 7.3.4.1 of the datasheet for more details about that. 

    Due to the holiday weekend, I haven't heard back yet for the confirmation, and it's likely I won't hear back until Tuesday or Wednesday...  sorry for that delay. 

  • Thanks.

    The filter mentioned in 7.3.4.1 is a low-pass digital filter, after the ADC.

    It seems no built-in analog anti-aliasing filter before the ADC, right?  The noise frequency above 1/2 Nyquist frequency may be a concern.

  • Correct, there is no anti aliasing filter before the ADC, since this is primarily designed for a DC application.  

    Also, I heard back from the validation team, and the ENOB has not been tested for VBUS, so is hard to say how similar it should be to the sense voltage ENOB.

  • Thanks.

    1. Understand INA228 ADC conversion time TCT is from 50 μs to 4.12 ms. Usually the delta-sigma ADC’s modulator (over)samples the input at the frequency, fMOD. So INA228 ADC fMOD = 1 / TCT?

    2. INA228 datasheet 8.1.4 says: "The internal ADC has good inherent noise rejection; however, the transients that occur at or very close to the sampling rate harmonics can cause problems. Because these signals are at 1 MHz and higher, they can be managed by incorporating filtering at the input of the device."

    Does this imply that fMOD = 1 MHz?

  • Hello,

    The internal oscillator is spec'd in the datasheet:

    So, yes, it is 1 MHz.