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MSP430 for Medical Implantable Devices

Efrain Gaxiola-Sosa
Intellectual 385 points
Other Parts Discussed in Thread: MSP430FG4617, MSP430FG4618

Hi everyone,

Currently I am working with the MSP430fg4617 in a medical implantable device and I have some questions regarding to use the MSP430 in this kind of application.

 

1.- I have observed some Medical Implantable Designs using MSP430 microcontrollers on the IEEE Journal database but I am not sure if it is suitable to use them. Does anybody has a recommendation or experience working with any MSP430 in medical implantable devices?

2.- What is the value of R in the schematic that appear in figure 22.5 in the MSP430x4xx User's Guide (SLAU056G)?

3. Using the internal OA's, I may need a gain higher than 15 (the maximum programmable gain) and I am trying to use this stage as antialiasing filter as well. I think that I can I get more gain using external resistors and capacitors but, is it suitable? Are those OA's designed to have that kind of performance?

4. I need as less external components as possible so depending on the value of the R (from question 2) I will try to design an antialiasing filter combining this R with an external cap, is it suitable?

5. From figure 22.1 of the same document mentioned in 2,P430x4xx User's Guide (SLAU056G),  it seems that one can use the three op amps independently with different configurations and different gains. If I use OA0 and OA1 in non inverting configuration and OA2 as differential amplifier it looks like an instrumentation amplifier. Is it possible to design an 3 op amp Instrumentation amplifier using the internal OA's of the microcontroller?

6. Does anybody knows about some kind of SPICE macromodel for the internal OA of the microcontroller? I would like to use PSPICE to do some simulations and see if I can reach my specs using them.

Please don't doubt to give me your opinon. Any comment will be very welcome.

 

  • 0
    TI__Mastermind 23685 points

    Efrain,

    The value of R is not spec'd in the datasheet, but it should be of no consequence, since the resistor ladder will produce a ratio used to determine the programmable gain. If you need a larger gain than what is provided, please note you can cascade the internal OA internally, so if you are using a device with multiple internal Op-Amps this can be done with no external connections. Alternatively you can cascade using an external op-amp circuit of your choosing.

    If you want to see what internal cascading options are available, see page 22 of the datasheet: http://focus.ti.com/lit/ds/symlink/msp430fg4619.pdf Similar tables are present in other MSP430 datasheets.

    I realize this does not address all of your questions, but I hope this helps. Perhaps someone can comment on the remaining questions.

  • 0
    TI__Genius 16165 points

    It really depends on what kind of signal Vp-pk from your subject.

    For example, rat's brain-waves are like 50 uVp-pk and the noise floor is approximately 20uVp-pk. You will need a pretty high gain (20,000x) just to have a good resolution (1Vp-pk) so that the ADC can sample it. Also, another important thing to keep in mind is the noise being introduced by the op-amp when  you are amplifying such a small signal. Another important consideration when choosing such amplifiers is that it must have a really high input impedance.

    I would highly suggest doing some experimentation with a signal generator that you can attenuate the signal down to the voltage levels of your subject that you are trying to sample. Even then, it may not simulate real-life subject as noise and impedance can be an issue.

    With that in mind, the MSP430 may be ok for signals that are relatively large enough input signals to be amplified. But, if it is a really small signal, either a custom ASIC op-amp or some ultra-low-power TI amplifiers that you can use. Try taking a look at this part OPA2337.

    Efrain Gaxiola said:
    6. Does anybody knows about some kind of SPICE macromodel for the internal OA of the microcontroller? I would like to use PSPICE to do some simulations and see if I can reach my specs using them.

    We don't have an official spice model for it. These op-amps were designed for general purpose amplification only.

    Hope it helps!

    Regards,

    William

  • 0 in reply to Brandon08
    Intellectual 385 points

    Thank you for your answer Brandon. I asked for the value of R because I wanted to add a capacitor in pararell with the resistor between the OA inverting input and the OA output. Doing so I will have a 1st order active RC filter that may work as antialiasing filter. If I have the value of R, depending on my selection of the PGA I will have a value for the final R. With this value and my 3dB freq I can compute the value of C. With R and C I can get the antialiasing filter needed for the operation of the ADC12. I remember that the ADC12 of the MSP430FG4618 does not has an included antialiasing filter. What can you recommend me about using the ADC12 in cascade with an 3OA differential amplifier?

  • 0 in reply to William Goh
    Intellectual 385 points

    Hi William and thank you for your answer. The device will monitor a blood pressure signal from a rat. The signal come from a sensor so it is in the level of mV. Regarding to the noise, I am trying to use the three op amp differential amplifier configuration because of the extra CMRR that the input buffers add. I don't know if I can use the 3 OA's as an instrumentation amplifier because normally the resistor that controls the gain is connected between the two inverting inputs of the input amplifiers. For the MSP430FG4618 it is only possible to connect the resistor from the inverting input to ground so I am not sure about use them as Inst Amp. I will experiment using the two input amplifiers in non inverting configuration and see how big is the maximum gain that I can get. I don't want to add an external Ins Amp because of the space and power restrictions.

    Finally, regarding to the PSPICE Model, I realize that the specifications of the internal OA are very similar to the ones for the 741 do you think that it is suitable use the 741 model for simulation purposes'?

     

    Regards,

     

    Efrain.

  • 0 in reply to Efrain Gaxiola-Sosa
    TI__Genius 16165 points

    Efrain,

    Yes you can use a 741 as your PSPICE model. We won't have an official PSPICE model for our internal OA. But, they should be almost similar.

    Regards,

    William G

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