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ADS42LB49: harmonic

jung fang
Prodigy 80 points
Part Number: ADS42LB49
Other Parts Discussed in Thread: LMH6552

hi expert.

ADC works in AC couple mode. schematic as follows, U1 represents LMH6552, and U2 represnets ADS42LB49.

FFT plot measured in above C1 blue circle point by Agilent spectrum analyzer as follows, ADC works in 200M sample rate, input signal is 10Mhz. 

from FFT plot, we can see sample clock related spur 200M/400M. and it‘s mix product 190M/210M/390M/410M. and 10Mhz's HD2=20Mhz.

but FFT plot from ADS42LB49 sampled data。we can see many harmonic  product. like 40M, 60M,70M,90M. i don't know why exist these 40M, 60M,70M,90M harmonic.

can you help analysis it?  

power

  • 0
    TI__Mastermind 26760 points

    Hi Jung,

    What you are seeing in the FFT is to be expected. All of the harmonics are in the -90dB range or better. This lines up with the ADC datasheet performance/SFDR.

    You could try to design some additional filtering between the output of the amplifier and input of the ADC. This is called an anti-aliasing filter. This could help suppress the harmonics even further.

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    Prodigy 80 points

    hi Bob

    datasheet's harmonic performance tested when input signal is -1dbfs. pls see below 10M signal, -1dbfs input fft plot from datasheet,THD is 95dbc.

    but above fft plot from my test shows THD=-10.39-(-94.5)=84.5dbc. 

    you mentioned aliasing, and i use Agilent spectrum analyzer to capture ADC input port signal  (above C1 point). spectrum is very clean,  and 190M/200M/210M/400M spur will alias to DC and 10M as ADC works in 200M sample rate. so i don't know where 40M/60M/70M/90M comes from. is there any register setting in ADC or other way to optimize harmonic performance.

    thanks very much.

  • 0 in reply to jung fang
    TI__Mastermind 26760 points

    Hi Jung,

    Keep in mind, you have an amplifier in front of the ADC. So the combined THD won't be better than -90dBc. On top of that, the amplifier is not terminated properly. I have pointed out that you have a 50ohm differential termination. The amplifier expects a 500ohm differential load. This could be the cause for further spurious degradation.

    Regards,

    Rob