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INA333 Resistive Load Required?

matt wilson
Prodigy 130 points
Other Parts Discussed in Thread: INA333, MSP430F5528

I'm using an INA333 to drive a MSP ADC12. It appears that the INA333 is very unstable unless I add a 10k to ground at the output. The datasheet specs are all shown with that condition, but the datasheet doesn't say anything about resistive loading as a requirement. This is a coin cell circuit, and the part was chosen for the low quiescent current. If there is a mandatory 10k resistive load on the output, that changes the math dramatically on whether this is the appropriate part.

This application is a 3.0V unipolar supply. The inputs are biased to mid-supply. The ref pin is tied to ground. The circuit has a gain of about 200x and the input has a slight offset to drive the output to 0.200V at null which keeps us off the negative rail. I'm driving the input of an MSP430 ADC12.

Does anyone have any experience with this? Can someone from TI explain this? I'm considering just scrapping these parts and swapping in another part.

  • 0
    Genius 5430 points

    1266301;

    An INA333 should not require a load resistor. Make sure you are within the required common-mode input voltage; single-supply operation puts limitations on CMV and output voltage swing (see the data sheet).

    BTW, the most common method of offsetting the output to keep it away from the rail is to apply the offset to the REF pin, not the input.

  • 0
    TI__Mastermind 25825 points

    Hello 1266301,

    Can you please post your schematic and scope shots of the instability?  Please include the MSP430 part number.

  • 0 in reply to Pete%20Semig
    Prodigy 130 points

    This circuit is feeding the input of the ADC12 of an MSP430F5528.

    This is a really bad one. The oscillation frequency is ~80kHz.

    This one is better, but you can see that the when the ADC S&H fires, it causes the INA to begin to oscillate. I can get this by reducing capacitance on the INA output. The severity of the oscillation seems to be related to capacitance. (despite a relatively healthy series resistance protecting the INA.) The other thing that helps is a resistor to ground at the output of the INA. Unfortunately, I don't have a capture of that handy. I'll try to post if I can get it.

    Just for reference, this is what happens if I use a POT to generate the voltage instead of the INA333.

  • 0 in reply to matt wilson
    Prodigy 130 points

    My images don't seem to be coming through. I'll try again.

     

  • 0 in reply to matt wilson
    Genius 5430 points

    1266301;

    If the INA333 output is clean when your converter is removed, I'd guess that you need to buffer the output of the INA333 before connecting it to the converter. Micropower amplifiers do not have wide bandwidth or very low output impedance.