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AMC3302-Q1: High Side BMS Shunt Current Measurement

Lukas Geiger
Prodigy 10 points
Part Number: AMC3302-Q1
Other Parts Discussed in Thread: ADS131B02-Q1, ADS131B04-Q1, ADS131B26-Q1

We are trying to design an isolated current measurement design based on a shunt measurement principle for high battery charge and discharge currents. The design should feature digital signal transmission to a master MCU, preferably via CAN. The AMC3302-Q1 could be a starting point for the design, but i wondered wether there is a more comprehensive solution to this problem.

With our power loss requirements (see below) the input voltage range of the AMC3302-Q1 is not favorable. We would not be using the full range when using a small shunt resistance (25µR to 35µR), therefore sacrificing resolution.

We would like to meet the following specifications for our measurement:

 

Parameter

Value

Unit

Nominal Measurement Range

±500

A

Power Loss (Shunt)

<9

W

Total Accuracy

±0.4

%rdg

Offset

75

mA

Linearity

0.01

% of range

Resolution

27

mA

Additionally we want to integrate various temperature measurements. Specifications on this still need to be worked out, but it may be relevant to an answer for the current sensing architecture.

  • +1
    TI__Mastermind 47625 points

    Hi Lukas,

    27mA of resolution for a full-scale range of 1000A is very high accuracy requirement, 27ppm of error.

    Are the values listed typical or maximum allowable errors? 

    What is your sampling rate requirement? 

    We may need to explore considering a higher accuracy delta sigma or SAR ADC then using a digital isolator to communicate across the barrier. 

    https://www.ti.com/data-converters/adc-circuit/precision-adcs/products.html#p84=24;32&sort=p1130;desc

    https://www.ti.com/isolation/digital-isolators/products.html

  • 0 in reply to Alexander Smith
    Prodigy 10 points

    Hi Alexander,

    thanks for your quick reply.

    27mA is typical, all other listed values are maximum.

    Sampling rate is quite low (minimum 50 S/sec), as this measurement is not needed for any regulation task, only for monitoring/functionaly safety.

    I will explore your suggestion, and maybe get back to you, if anything comes up.

  • +1 in reply to Lukas Geiger
    TI__Mastermind 20845 points

    Hi Lukas,

    thanks a lot for reaching out with your request. Alex just made me aware of your inquiry.
    I have a lot of focus on current shunt sensing in BMS. Your requirements seem pretty typical for this application.

    We usually solve this problem by using a high resolution, high accuracy, 24-bit delta sigma ADC. The ADS131B02-Q1 and ADS131B04-Q1 are great options for it. Unfortunately those ADC do not integrate the data isolation and isolated DCDC-converter. Means you would need to add them externally. But TI has many different options for the digital isolator and isolated DCDC-converter.

    Following some rough estimations using the ADS131B04-Q1 with a 35uOhm shunt:

    • Max voltage across shunt: ±500A x 35uOhm = ±17.5mV
    • FS range of ADS131B0x-Q1 @ Gain = 32: ±37.5mV
    • Input-referred noise of ADS131B0x-Q1 @ Gain = 32, Data Rate = 250SPS: 0.42uV_rms
    • Resolution in mA: 0.42uVrms/35uOhm = 12mA_rms
    • Offset @ 25°C, Global chop mode enabled: 2uV (max)
    • Offset in mA: 2uV/35uOhm = 57mA
    • Gain error needs to be calibrated at room temperature at least. Only the gain and VREF drift are important then.
      Gain error due to temperature drift (e.g. Ta = -20°C to 60°C): 25ppm/°C x (25°C - (-45°C)) = 1125ppm = 0.11%

    Overall it looks like the device would meet the requirements pretty well.

    You can leverage some of the other available ADC channels in ADS131B0x-Q1 to measure battery voltage and temperature.

    We also provide some basic functional safety documentation (Base FIT rate, FMD, Pin FMA) on the product folder for those ADCs.

    Regards,
    Joachim Wuerker

  • 0 in reply to Joachim Wuerker
    Prodigy 10 points

    Thank you Joachim,

    i think we're gonna sacrifice some resolution and choose Gain 8 to extend measurement range. This was very helpful!

    Regards

    Lukas Geiger

  • 0 in reply to Lukas Geiger
    TI__Mastermind 20845 points

    Hello Lukas,

    gain = 8 is actually what is commonly used, at least in automotive BMS to also detect overcurrents of multiple kA.
    If you look at the noise table in the ADS131B04-Q1 datasheet you will actually realize that the input-referred noise doesn't really degrade much between gain = 8 and 32.

    Regards,
    Joachim Wuerker

  • 0 in reply to Joachim Wuerker
    TI__Mastermind 20845 points

    Hello Lukas,

    I couldn't mention it two weeks ago, but I can now finally tell you that we put our new automotive BMS analog front-end for current shunt, voltage and temperature measurements in preview on ti.com. The device is called ADS131B26-Q1 and is developed according to our internal development process for functional safe hardware.

    Product folder: https://www.ti.com/product/ADS131B26-Q1

    Regards,
    Joachim Wuerker

  • 0 in reply to Joachim Wuerker
    Prodigy 10 points

    Hi Joachim,

    thanks. If this isn't good timing Smiley

    Kind regards,

    Lukas Geiger