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MSP430FR2355: EXP430FR2355 - Strange behavior at some opamp gains

Haroldo Amaral
Expert 1105 points
Part Number: MSP430FR2355

Hi,

I received one launchpad exp430fr2355 and did some tests based on the out of box demo.

I connected an Analog Discovery on pin P3.5 to measure the signal of DAC and pin P1.5 to measure the signal at the opamp output, but I noted some strange behavior.

Using the non-inverting configuration of opamp the gain 2x and 3x show a huge noise when connected to oscilloscope tip configured to 1x mode.

But if I activate the attenuator (10x mode), everything works well.

The same thing happens in the inverting configuration, but with the gain 1x and 2x.

Note that from on other gains it works very well.

Does somebody have an idea what's happening?

Thanks.

  • 0
    TI__Expert 6635 points
    Hey Haroldo,

    I am going to have to look further into this. I will update you with information as soon as possible.

    Thanks,

    Mitch
  • 0
    Expert 2740 points

    This looks like a simple case of capacitive overload.

    Your 1x probing load is probably in the 200 pF range. This is enough to start an oscillation on the opamp output.

    To verify this connect a 2k resistor between the signal source and your 1x probe. 

    Peter

    PS The maximum recommended capacitive load may be buried somewhere in the device documentation.

  • 0 in reply to Peter Dvorak
    Expert 1105 points
    Hi, you're right.

    I simulated the probe loading putting a 1M resistor in parallel with a 220p capacitor and connect to opamp output (pin P1.5). Bingo, the oscillation appeared. Putting a 2k resistor between the tip the oscillation vanishes.

    It's the first time that I see this behavior in practice, very interesting.

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