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Pulse Oximeter Using MSP430 Without Internal OPA

Xiaohai Li
Prodigy 30 points

Other Parts Discussed in Thread: MSP430FG437, MSP430F2272, TLV274

I want to develop a Pulse Oximeter with MSP430 and I have read SLAA274B which had used a MSP430FG437 with OPAs inside. 

But I need to finish this job using a MCU without Internal OPAs, like MSP430F2272. How can I change the circuit to get a similar result. Is there any examples or Application Notes?

  • 0
    Guru 227245 points

    Xiaohai Li said:
    I need to finish this job using a MCU without Internal OPAs, like MSP430F2272. How can I change the circuit to get a similar result.

    What about addign an external OPA when there is no internal one? Sounds liek an obvious (and probably the only) answer.

  • 0 in reply to Jens-Michael Gross
    Genius 3770 points

    MSP430F2272 actually has two internal op-amps; did you mean to mention another MSP430?

    It may be possible to rig the Comparator_A/B inside many MSP430s to perform like a very crude op-amp; however you'd need an external bias network to trick the comparator to operate in the linear region, and you'd also need to connect CAOUT to an ADC channel externally. Also there's only one comparator, whereas the app note requires two op-amps.

    So you should probably follow JMG's advice, unless you really don't want an external op-amp.

    Tony

  • 0 in reply to TonyKao
    Guru 58965 points

    It is even possible to make that meter without any MSP430. It is also possible to use any MSP430 in such a meter.

    But why?

  • 0 in reply to Jens-Michael Gross
    Prodigy 30 points

    Jens-Michael Gross said:
    What about addign an external OPA when there is no internal one? Sounds liek an obvious (and probably the only) answer.

    I’m sorry that I didn’t make myself clear. Of course external OPAs are needed. And my question is that should I just design the external OPA circuits exactly like the structure of the internal OPA module seen from the datasheet? Can I use a TLV274 to replace the internal ones? Does any OPA from TI perform the similar characteristics?

  • 0 in reply to TonyKao
    Prodigy 30 points

    TonyKao said:

    MSP430F2272 actually has two internal op-amps; did you mean to mention another MSP430?

    It may be possible to rig the Comparator_A/B inside many MSP430s to perform like a very crude op-amp; however you'd need an external bias network to trick the comparator to operate in the linear region, and you'd also need to connect CAOUT to an ADC channel externally. Also there's only one comparator, whereas the app note requires two op-amps.

    So you should probably follow JMG's advice, unless you really don't want an external op-amp.

    Tony

    I have checked the datasheet again and it says the operational amplifier module is “MSP430F22x4 only”. I don’t know whether my understanding is correct.

  • 0 in reply to Xiaohai Li
    Genius 3770 points

    You are right of course, I over looked the tiny print on the functional diagram stating exactly that. Should get my eyes checked etc...

    Tony

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