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LMP2012: Output saturation of one amplifier affects the output of the other amplifier output

Michael Ross
Prodigy 15 points
Part Number: LMP2012

We use one amplifier in the LMP2012 for signal conditioning and the other is used to drive an ADC. If the amplifier driving the signal conditioning drives a heavy current load, there is a corresponding small offset on the other amplifier driving the ADC. The change in voltage output of the ADC amp is small but detectable due to the resolution of the ADC.  Is this behavior expected due to shared circuitry internal to the package or perhaps temperature effects of the additional power in the saturated amplifier? 

  • 0
    TI__Mastermind 18847 points

    Michael,

    There are many different possibilities for this issue.  Having more detail on how large the current load is, and the detailed schematic with the two amplifiers would help me to give a more precise answer.  Here are a few possibilities.

    1. It is possible that the self heating of the device could cause an offset shift.  However, in this case, the op amp have very limited output current capability.  I don't see a short circuit limit in the data sheet, so for an example calculation I will just assume 10mA @ 2.5V (internal voltage across output transistor sourcing current).  This would dissipate (2.5V x 10mA = 25mW).  For this power the self-heating would be (119C/W x 25mW = 3C).  A 3C temperature rise would cause offset to shift by )0.015uV/C x 3C = 0.056uV).  Maybe I am missing something, but based on the limited output current capability I wouldn't expect the offset to change much.
    2. Some circuitry on the dual amplifier will be shared.  However that is generally bias circuitry and I wouldn't expect one amplifier to dramatically impact the other.  The exception to this is power dissipation.  However, as I mentioned in item 1, I doubt that you are dissipating enough power to raise the die temperature much.  Bruce Trump did a nice article on dual amplifiers that may be interesting to you:  Matchy Matchy—how alike are dual op amps? 
    3. Are you sure that the offset is in the ADC drive and not the high-power amp?  Since I don't have the schematic, I'm not sure how the amplifiers are connected together.  The higher current amplifier can have output swing limitations due to the heavy load (see Op Amp Input and Output Swing Limitations ).  The output swing limitation could look like an offset.  Is it possible the "offset" effect is not in the ADC amp but from the previous stage?
    4. You mention "small offset".  The LMP2012 has a max offset of 25uV at room temperature and 65uV across temperature.  If it is self heating, I would not expect more than 65uV of offset.  I suspect that your offset is probably much larger than that.  If you are getting offsets substantially higher than 65uV, I think you are seeing another effect (likely output swing or common mode limitation as mentioned in item 3).

    I hope that helps.  Best regards, Art

  • 0 in reply to Art Kay
    Prodigy 21 points

    Hi Art,

    Thank you for the reply!

    We shouldn’t have any heating from output currents; they’re very low.  The output currents are very low.  But I can’t say whether there’d be any heating due to internal currents during saturation.

    We can’t share the circuit, but the image below gives a situation with the same results.  When one op amp is saturated high, the output from the ADC implies the other op amp (U2 below) has an offset of +110 to +120 mV from the value we expect.  When the op amp is not saturated, the ADC output is just as we expect.  The circuits with U1 and U2 are completely separate.  Also, other inputs on the same ADC that don’t use op amp U2 give the expected answer whether U1 is saturated or not.  Furthermore, the +110 to +120 mV offset is about constant regardless of sense voltage value at the input to U2 – so it’s an offset error, not a gain error.

    Example circuit with a dual op amp.

    Thanks,

    Mike

  • 0 in reply to Michael Duckwitz
    TI__Mastermind 18847 points

    Mike,

    1. Based on the simulation schematic, I think the two amplifiers to not have any connections between them.  I understand that the amplifiers are both part of a single package dual amplifier, but the two amplifiers do not share input or output connections.  Correct?
    2. Assuming item 1 is correct, I would not expect saturation or other issues with one amplifier to have much impact on the other.  The offset on U2 is very large (120mV).  The nominal offset of the device is 25uV, so when the offset shift by more than 1000 this is highly abnormal.
    3. Since you see the offset shift on U2 when U1 is saturated, and see normal operation on U2 when U1 is not saturated I think you may have found an unexpected relationship between the two halves of the dual.  
      1. Can you change the amplifier model?  
      2. Even if you cannot change your design, I think you should try a different dual amplifier under the same conditions and see if the problem persists.
    4. Dual amplifiers generally contain some shared circuitry.  Nevertheless, they are designed to act independent of each other.  Saturating one amplifier should not impact the other.  If one amplifier draws high current (due to saturation), that can cause self-heating, but this shouldn't create the major shift that you see.  The vast majority of op amps have an output current short circuit limit which will limit the current to a safe level.  I don't know if there is something unusual about the LMP device or if there is some issue we haven't identified in your design.  Trying a different amplifier model should help clarify that.  I can help choose a model.  Let me know your package type and key specifications.

    Best regards,  Art

  • 0 in reply to Art Kay
    Prodigy 21 points

    Hi Art,

    You are correct with #1 above.  There are no circuit connections except that they both use the same ground return.

    Using a different op amp to see whether we get the same thing is a good idea.  If we see the same thing, there must be something external to the IC that I'm not seeing.  If the problem goes away with a different op amp, then we'd suspect something internal to the LMP2012.  I'm not sure I'll get that opportunity, though.

    Thank you for all of you help, Art!

    Mike

  • +1 in reply to Michael Duckwitz
    TI__Mastermind 18847 points

    Mike,

    If the problem goes away with another amplifier, I can take a look at LMP2012 and try to reduce your issue.  I suspect that the problem will remain, and we will have to look more closely at the configuration.  However, there is always a chance that it is device related so this is a quick way to check.  I look forward to hearing about your experimental results.

    Best regards, Art