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OPA2830: Zero-crossing distortion for high frequency sinusoidal signals

Samuel Lourenco
Intellectual 390 points
Part Number: OPA2830
Other Parts Discussed in Thread: LMH6612

Hi,

I've built a function generator that has the following output stage:

The OPA2830 works without a problem when amplifying sinusoidal signals up to 2MHz. Above that, if the output is terminated with 50 ohms, it starts to show zero-crossing distortion. This distortion worsens with frequency. However, it can be alleviated, and completely eliminated if the signal amplitude is decreased. This shows me that the distortion seems to be slew rate dependent. What confuses me, is that the OPA2830is being used within the specification.

This issue is only caused by the output stage. If the output is left unterminated, or unloaded, it can go well above 4MHz without showing any kind of distortion. What is happening? You can see that there is no distortion at 8MHz, with the amplitude set to 7Vpp and without load.

This is indeed strange.

  • 0
    TI__Expert 8000 points
    Hi,

    This is indeed a strange behavior, and I do not see any issues with your schematic. Just to rule out a few things, have you observed this behavior on more than one part? Is there any type of DC offset present on the signal? When measuring with a 50-ohm load, is the load terminated to the same ground potential as the amplifier's ground reference?

    Regards,
    Jacob
  • 0 in reply to Jacob Freet
    Intellectual 390 points

    Hi Jacob,

    To better answer your question, let me do this in parts.

    "Just to rule out a few things, have you observed this behavior on more than one part?"

    Yes. On another similar board. I see that this distortion only happens on the last stage.

    "Is there any type of DC offset present on the signal?"

    I measure a small offset of minus 16mV at the output. That is because the input signals are not exactly complementary, but the inverted one is a bit higher. Both input signals have an offset that is ideally nulled by the subtracting stage.

    "When measuring with a 50-ohm load, is the load terminated to the same ground potential as the amplifier's ground reference?"

    Yes, right on the connector ground, that is connected to the analog ground. I'm using a coaxial cable between the generator and the terminated scope. Changed to a shorter cable and saw no difference. Also changed the terminating resistors to no avail.

    Kind regards, Samuel Lourenço

  • 0 in reply to Samuel Lourenco
    TI__Expert 8000 points
    Hi Samuel,

    One further question. When you say that "distortion only happens on the last stage" are you driving just the last stage in that case with a function generator and observing the output?

    Regards,
  • 0 in reply to Jacob Freet
    Intellectual 390 points

    Hi,

    The schematic depicted is the output stage of a function generator that I've designed. What happens is that I only observe that distortion at the output and with the 50 Ohm load. The input signal to the last opamp is clean, even if the output is distorted. So, distortion is added by the second half of the OPA2830.

    I just hope I don't have a batch of fake chips. I got them from Mouser. It is the one on the bottom right corner. The bottom line says "56R4S".

    Kind regards, Samuel Lourenço

  • 0 in reply to Samuel Lourenco
    TI__Expert 8000 points
    Hi Samuel,

    I looked up the lot code on the device and it looks like you have legitimate devices. I am not sure at this point what could be causing you to have this issue.

    Are you able to put a different device on this board and notice a clean signal from the output? Or do you have another fixture where you could measure the bad device? I am just trying to rule out any board issues at this point, although everything looks like it should be good based on the information you've provided.
  • 0 in reply to Jacob Freet
    Intellectual 390 points

    Hi Jacob,

    I have ordered fixtures to test the OPA2830 and see how it fares in comparison with the LMH6612, and vice-versa. I'll be using both opamps as voltage followers (because in previous tests I ruled out any feedback network effect) and see how it goes. The output will have a 49.9 Ohm resistor in series. No TVS diode will be used (I ruled out that too).

    In the end, I might have to launch a new board revision, where the OPA2830 is replaced by the LMH6612, if all goes well.

    Once I get the boards, I'll post the results here.

    Kind regards, Samuel Lourenço

  • 0 in reply to Samuel Lourenco
    TI__Expert 8000 points
    Hi Samuel,

    Thank you, please do let me know what your test results show. If you do continue to observe this strange behavior on the new boards then it will probably be best to send them back to us as a return for analysis.

    Regards,
  • 0 in reply to Jacob Freet
    Intellectual 390 points
    Hi Jacob,

    It is strange that you ask that. I'll see what I can do.

    Kind regards, Samuel Lourenço
  • 0 in reply to Samuel Lourenco
    Intellectual 390 points

    Hi Jacob,

    I've replaced the problematic OPA2830 at the output by a LMH6612. It basically solved the problem. Now, I don't see any distortion when the output is terminated.

    I still have to make those three breakout boards, two with OPA2830 ICs from different batches, and a third one with a LMH6612. I'll post the results.

    Kind regards, Samuel Lourenço

  • 0 in reply to Samuel Lourenco
    Intellectual 390 points

    Hi,

    The boards finally arrived, and I've assembled them right away. Two of them employed the OPA2830 (chips from different batches), and one employed the LMH6612.

    I've tested all of them under the same conditions: +/-5.5V supply voltage, 50 ohms termination. The results are very telling. The OPA2830 distorts the 8MHz and 16MHz signals passing through, when the respective board is terminated:

    Both boards assembled with the OPA2830 present the same distortion. Conversely, the LMH6612 behaves very well under the same conditions:

    This tells me that the OPA2830 indeed has an issue when driving high current loads. Notice that, in all cases, the load is 100 ohms (the external resistor in series with the output of the opamp is a 49.9 ohm resistor. Both opamps inside each chip are configured as voltage followers.

    Replacing the OPA2830 by the LMH6612 solved my issue, and hence I marked the corresponding post as a possible solution. However, the issue with the OPA2830 still persists.

    Kind regards, Samuel Lourenço