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Using an MSP430F6 to measure strain gauges?

Craig Easdale
Expert 2170 points
Other Parts Discussed in Thread: MSP430F6779A, MSP-DSPLIB, MSP430F67641, MSP430I2041, MSP430F6736A, MSP430F6726A, MSP430F6736, EVM430-F6736

I have a project where I need to measure strain gauges on 3-axes in a half-bridge configuration. I notice that most projects like this us a an HX711 load cell amplifier (or similar). Usually any device for this purpose has an amplifier stage (programmable gain of 32-128) and high resolution differential ADC (24-bit) with 50/60Hz noise suppression (HX711 datasheet).

I had been wondering if any microcontroller with an on-board ADC could handle this task on it's own, and my search led me to look at the MSP430's. I noticed the MSP430F6 could potentially provide since it appears to have 3-6 differential sigma-delta ADCs with a programmable gain amplifier.

Would this be an appropriate part for measuring strain gauges? Are there any other parts that might be suitable?

Essentially my requirements are:

  • on-board PGA (gain 32-128)
  • ob-board ADC (24-bit, differential x3)
  • low-power microcontroller (<1uA sleep current)
  • ideally some 50/60Hz noise suppression on the Analog Front End
  • Smaller package? (the 80-pin LQFP is overkill)

Also, is there a cost-effective evaluation kit for the MSP430F6, like the launchpad?

  • 0
    TI__Mastermind 49692 points

    Hello Craig,

    Thank you for your detailed post. I'd recommend the MSP430F67xxA family, which includes the F677xA, F676xA, and F674xA devices (you can pick depending on number of peripherals, Flash memory, RAM memory, SD24 channels, etc., required). Take a look at Table 3-1 in the datasheet for a helpful comparison between this family of devices. While primarily targeting e-meter applications, these parts would be appropriate for measuring strain gauges as well.

    Craig E said:

    Essentially my requirements are:

    • on-board PGA (gain 32-128)
    • ob-board ADC (24-bit, differential x3)
    • low-power microcontroller (<1uA sleep current)
    • ideally some 50/60Hz noise suppression on the Analog Front End
    • Small package (the 80-pin LQFP is probably overkill)

    For example, I'd start with the MSP430F6779A, since it meets most of your requirements above.

    F6779A Capabilities:

    • On-board PGA
      • Gain: 1 - 128
    • On-board SD24 ADC (24-bit)
      • F677xA: 7 differential channels
      • F676xA: 6 differential channels
      • F674xA: 4 differential channels
    • Low power sleep current
      • Standby Mode (LPM3): 2.1uA @ 3V supply voltage
      • RTC Mode (LPM3.5): 0.34uA @ 3V supply voltage
    • Noise suppression
      • Filtering can be done using passives externally to reduce 50/60Hz noise
      • Our Digital Signal Processing (DSP) Library (MSP-DSPLIB) could perform FFT's to remove noise
    • Package size
      • Unfortunately, the smallest package size for these devices seems to be 100-pin LQFP package
    • Alternative Devices
      • MSP430F67641 which is offered in a 80-pin LQFP package and features several SD24 differential channels
      • MSP430I2041 which also features 4 SD24 differential channels but has fewer PGA gain settings and worse sleep current performance

    Craig E said:
    Also, is there a cost-effective evaluation kit for the MSP430F6, like the launchpad?

    Unfortunately, there isn't really a cost-effective EVM for these devices. However, you could purchase a target socket board that's cheaper but allows you to develop some code and access the peripherals. Here are some tools for the MSP430F6779A for your reference.

    Hopefully this helps!

    Regards,

    James

    MSP Customer Applications

  • 0 in reply to James Evans
    Expert 2170 points

    Thanks for your reply James.

    I've done a bit more research into the MSP430F67xx parts and I've become fairly confused. 

    Looking at the MSP430F6726A and the MSP430F6736A, I think there may be a typo in the description/data sheet.

    The F6726A is listed as having 3Ch SD ADCs on the product page (http://www.ti.com/product/msp430f6726a, seemingly the only F672xx part to have this. However the datasheet has a table which suggests that ALL the MSP430F672xA parts only have 2ch SD ADCs: http://imgur.com/a/hyL98

    Obviously I need the three ADCs, and I think the MSP430F6726A would meet my criteria so long as it has these.

    And a quick follow up question about the SD24_B ADC... I can't find the maximum rating in the datasheet... Can I provide the microcontroller VREF pin with the same 3.3V I'm using to excite the strain gauges?

  • 0 in reply to Craig Easdale
    TI__Mastermind 49692 points
    Hello Craig,

    I'm sorry for the confusion, and I see what you're saying. It does appear that the product page for the MSP430F6726A has a typo - it should say 2 SD24 channels instead of 3. I'll inform our Marketing team that this should be fixed. In case you see something like this in the future, the datasheet will always have precedence over the product folder, and Table 3-1 in the datasheet confirms that the MSP430F6726A only has 2 SD24 channels.

    Since you do need 3 channels, I'd recommend the MSP430F6736A. We do offer the EVM430-F6736, which is a single-phase e-meter but could be used for your development. However, the target socket board that I mentioned earlier is another great development platform. While the MSP430F6736 is functionally equivalent, the MSP430F6736A is recommended for new designs.

    Regarding VREF, the range for the external reference voltage (VREF) is from 1.0V (min) to 1.5V (max) and can be found in Table 5-42 in the datasheet. Keep in mind that you could also use the internal reference module which can be configured to 1.5V, 2.0V, and 2.5V.

    Hopefully this clears up any confusion.

    Regards,

    James

    MSP Customer Applications

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