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LMH6722: LMH6722

Abdelmoumin ALLIOUA
Prodigy 15 points
Part Number: LMH6722

Hello,

We are using the LMH6722 for an automotive active filtering application, and during test measurements, we have encountered some distortions, which resemble crossover distortion, at certain operating points.

This issue also appeared in simulations with the SPICE model, so we are wondering what could be causing it and how we might counteract it. Attached is the measurement of the op-amp output for a 1 MHz sinusoidal signal with an 80mV peak amplitude. The blue curve represents the signal at the output after the injection circuit (RC circuit), and the purple curve represents the output of the op-amp itself. Sensing is also done with an RC circuit. The full active topology is a voltage-sensing current injection topology, with a feedback mode.

We would appreciate any information that can help in resolving this issue.

Kind regards

,

  • +1
    TI__Genius 11000 points

    Hi Abdelmoumin,

    This is an interesting result you are seeing. I was curious if you could send the circuit file you are using that shows the result you are seeing in the working circuit. Just for a quick debug step does the signal get better if you increase the load resistance value (load the device less), if the design allows. The output load could be contributing to the distortion, but it might be something else that is also affecting the device. Another quick test would be to maybe add an offset to the input, so the output doesn't cross mid-supply and see if the distortion improves. 

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 15 points

    Hello Ignacio,


    Thank for your reply, the circuit itself is composed from a sensiing stage (RC circuit) to sense the voltage, and an injection stage (RC) to inject a current, both sensiing and injection circuit are connected from their capacitor to form a feedback loop, as a classic VSCI FB active filter should operate, well this issue appears at some operation point only which we couldnt find a proper explanation to it , however i tried what you suggested in simulation and it improved the results somehow  when increasing the injection resistor value, also when adding a voltage offset at the non inverting input it improved the results, attached are the figures of the tests, however im still wondering of the reason why this is happening, is it related to the output current that the opamp can draw ? or to the high gain im using (i tried to reduce the gain, the distortion was also there), looking forward to hear from you .

    kind regards 

  • 0 in reply to Abdelmoumin ALLIOUA
    TI__Genius 11000 points

    Hi Abdelmoumin,

    I am glad you are seeing a difference in your results; it definitely sheds some light on what you are experiencing. It seems like your initial idea of crossover distortion is correct. Output crossover distortion appears when the amplifier's output is crossing over mid-supply and either none or both output stage transistors are turned on. This results in a discontinuity at this very small operating region near mid-supply. A device with a small output amplitude and high output current is affected the most. This is because at low output amplitudes, the device is constantly hovering in this small operating region, and with any device the more current you are forcing the transistors to supply, performance will always degrade leading to more distortion in the signal. This is why having an offset at the output helped as you stayed completely out of the mid-supply crossover region.

    Best regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 15 points

    Hi Ignacio, 


    Thanks for your reply, when you mentioned (A device with a small output amplitude and high output current is affected the most. This is because at low output amplitudes, the device is constantly hovering in this small operating region, and with any device the more current you are forcing the transistors to supply, performance will always degrade leading to more distortion in the signal. This is why having an offset at the output helped as you stayed completely out of the mid-supply crossover region.)

    i belive you meant at small input amplitudes is that correct ? because this issue happened most of the time when the input amplitude is small 

    thanks again 

    kind regards,

  • 0 in reply to Abdelmoumin ALLIOUA
    TI__Genius 11000 points

    Hi Abdelmoumin,

    Output crossover distortion occurs at the mid-supply crossing point, so a small output amplitude signal will be more distorted as it is close to this region.  In a low gain a small input signal will translate to a small output amplitude signal, but the effect is from a small output amplitude signal. The goal is then to ensure the output signal does not crossover the mid-supply region. It will limit your output headroom as you are now biasing the output to be above the mid-supply point at all points but if your signal is not very large it could work. You can create a voltage divider at the non-inverting signal to create this bias at the output the small amplitude signal can ride on.

    Best Regards,

    Ignacio