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OPA1688: Suggested Cut-off frequency in datasheet too high?

user4991603
Prodigy 140 points
Part Number: OPA1688

The example circuit in the datasheet suggests a low-pass filter with a cut-off frequency of 4.52MHz.

For a headphone amplifier I went through one of your headphone amplifier design guides and also had a resulting circuit were the cut-off frequency of the low-pass filter had 3.68MHz.

The problem is that the circuit produced a noise-floor of -70dBu to -80dBu throughout the whole audio band. Attached you can find the circuit:

FFT of the circuit with 100k load:

Using 1k6 Ohm for Rf and 2k49 Ohm for Rg and setting all caps to 2.2nF the FFT looks like these, with a peak at 15KHz:

Can you please explain why some of your suggested headphone amplifier circuits use low pass filters with an unusually high cut-off frequency?

  • 0
    TI__Mastermind 48076 points

    Hi ,

    I do notice it as well. I will ask our system engineer and find it out for you. what is your max. BW for your audio application?

    One thing that I knew that a typical application circuit tend to be more generalized, even though the BW is a significant higher and it is said that to be applicable for Headphone Amplifier. When it comes with certain application, TI tends to have furnish with an Application or Tech. note, which is more specific to help engineer in an application design. Anyway, I will let you know. 

    Best,

    Raymond

  • 0 in reply to Raymond Zhang1
    TI__Mastermind 48076 points

    Hi ,

    The Figure 49 is a typical sound application circuit, and it is not specific for headphone audio application as I noted in the previous reply (though the figure caption is a bit confusion). 

    Best,

    Raymond