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Connection of a microcontroller pin to the amplifier

Alexander Anufriev
Prodigy 120 points

Hello everybody,

I would be grateful for some advices concerning the following point:

is it a good solution to amplify DAC signal from MSP430 by the OP amplifier till within 10 V.  I doubt about connecting the uC pin to the amplifier. Could anybody give me some suggestions?

Thanks in advance.

Alex

  • 0
    Guru 227245 points

    Where's the problem? Which OP amplifier? The internal one of hte MSP? Then it won't work. The maximum OP output is the MSPs VCC. With an external OpAmp there shouldn't be a problem.

  • 0 in reply to Jens-Michael Gross
    Prodigy 120 points

    Hello Jens-Michael,

    thanks for the answere. The OP is external with 24 V power supply. If I connect uC DAC to this amplifier, should I somehow protect this pin?

     

  • 0 in reply to Alexander Anufriev
    Guru 227245 points

    Alexander Anufriev said:
    The OP is external with 24 V power supply

    Symmetrically used (+-12V) or GND based (GND/24V)?
    However, it should be mostly unimportant. If the OpAmps inputs accept full input range, the onlöy important thing is that the input voltage on the two input pins is not above or below the supply. The OpAmp amplification is based on the current that flows between the two input pins.

    The main problem is that you get only one voltage form the MSP. And voltages are relative, so the MSP output voltage is relative to MSP GND. But for the OpAmp, the input voltage needs to be relative to something too. So you must have a connection between the MSPs GND and teh OpAmps 'reference base'. Here it is important whether the OpAmp is running symmetrically or not, and whether it has Rail-to-Rail inputs (or at least accepts an input down to the negative rail).

    The best setup would be to make a 1:1 voltage divider on the 24V supply (so in teh middle it is on 12V level) and connect this middle point to the MSP GND. (and do not connect the 24V GND and the MSP GND).
    Then the MSPs output voltage will be (for the OpAmp) relative to 12V, which is far from the upper and lower supply voltage. This is called a virtual GND and lets the OpAmp work as if it were operated by +-12V instead of 24V. This virtual GND does not need to be in teh middle of the supply. Any resistor pair that splits the supply vootage is good. So you can have a virtual GND that splits the supply to -1/+23V.
    The important point is that the input voltage is not near the negative supply, as OpAmps are usually really bad when the inputs come near the supply rails (some won't work in this region at all).

    In any case, the input circuit of the OpAmp should be protection enough for the MSP pin. The dangerous thing is not the voltage, but the current that flows, caused by the voltage. And OpAmp inputs have a very low input current / high input impedance, as long as the input voltage is not above or below the rails.

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