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AFE030: THD issue

Kevin Lai
Intellectual 2916 points
Part Number: AFE030
Other Parts Discussed in Thread: AFE031

Hi team,

my customer is looking to use the AFE030 and has concern for the THD performance especially with the first harmonic. please see the picture attached showing an fft of the signal before and after the power amp in the AFE030. any idea how to reduce this or why this is as high as it is?

Thanks,

Kevin

  • 0
    TI__Guru* 77541 points

    Hi Kevin,

    Is the the harmonic presented in the Transmitter side based on the notation on the graph? Can you send me the schematic to my email box?

    If it is on the transmitter side, it is related how the sine wave is generated. If it is done via DAC-mode, it is likely that PWMed square wave that generate the transmitter signal is too coarse (I recalled the DAC is 10 bit and will have large harmonic components). In addition, it has to go through LPF before the input is gained up in the programmable Tx op amp stage, and final power amplifier stage. I need to know which stages the two signals are measured. 

    If the AFE030 is implemented in PWM-mode, then your customer did not apply enough LPFs to smooth out edges of Tx Sine signal. 

    I have a feeling that your customer is running in DAC mode. Please ask them to place LPS filter to smooth out all the DAC edges. Once it is done correctly, the harmonic will be reduced significantly. Also, the LPF should have cutoff frequency around 170-200kHz range, and the harmonics may be further attenuated by the LPF. 

    BTW, AFE031 and AFE030 are pin to pin compatible. I would pick AFE031 for the application. 

    Best,

    Raymond