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THD of THS4271 is bad in inverting configuration with gain=1

Arvind Shanker
Prodigy 20 points
Other Parts Discussed in Thread: THS4271, THS4271EVM-UG

Hi,

I am evaluating the THD performance of THS4271. In the non-inverting configuration with gain=1, THD comes to around 82dB, but in the inverting configuration with gain=1, THD is about -71dB with RL=150 ohms and about -79dB with RL=500 ohms. In the datasheet, THD information is not given for inverting configuration, so I want to know if this result is expected?

Please find the schematic below:

Regards,

Arvind.S

  • 0
    TI__Genius 13335 points

    Hello Arvind,

    Could you please reupload your schematic? It appears as though there was a problem when you posted.

    Also, what frequency and amplitude is the input signal that you are testing?

    Keep in mind that when operating the THS4271 in a unity gain configuration, the value of Rf will have an impact on the stability as shown on figure 93 of the THS4271 datasheet. One quick check would be to take a frequency response of each to see if one configuration produces more instability than the other.

    Also note that it is expected that the distortion will get better with a larger load. This result can be seen in figures 6 - 13 of the THS4271 datasheet.

    Other than that, there is no stead fast rule of which input will provide the best distortion other than empirical testing, although a difference of 11dB is quite large. Have you tested this on a different part?

    Regards,
    Luke

  • 0 in reply to Luke LaPointe
    Prodigy 20 points

    Hi Luke,

    Thanks for the reply.
    The input frequency is 10MHz pure sine wave and the amplitude is 2 Vp-p.
    As you had suggested,the frequency response till 100MHz was checked and no instability was noticed in both the configurations. The THD was also checked in a different part and the results were same.
    The schematic has been uploaded again.

    Regards,
    Arvind.S

  • 0 in reply to Arvind Shanker
    TI__Genius 13335 points

    Arvind,

    Keep in mind that using an inverting configuration with a gain of -1 will correspond to a noise gain of 2 or 6dB. Therefore, the output referred THD will inherently be 6dB higher than the noninverting case where the closed-loop gain and noise gain are equal to 1.

    Additionally, this device has a very sensitive inverting input node and the layout will have a large impact on the stability of the circuit. There was an separate EVM made just for the unity-gain configuration (THS4271EVM-UG) that optimized the feedback path to the inverting input. If you are testing both configurations on the same board, then you could expect to see a difference (either negative or positive) in performance.

    There is a good section on the layout considerations starting on page 28 of the THS4271 datasheet.

    Let me know if this helps.

    Regards,
    Luke