AD6688 noise Figure

Per page 7 of the data sheet,the specified NF = 24.5dB. However, per the test conditions, the specified NF is based on a decimation ratio of 8. The NF of the device doesn't change with decimation (post processing). Hence, 10log(8)=9, NF=33.5dB. Thoughts?

ADC NF = Full scale (pin) - SNR - 10log(Fs/2) - thermal Noise

Fs=3GHz,

Fs/2 = 1.5GHz

Decimation=8

(Fs/2) / 8 = 187.5MHz

SNR @ 2.6GHz -2dBfs) = 63.4dBfs

Full Scale = 1.7V p-p (+8.5dBm, 50 ohms)

ADC NF = 8.5 - 10log(187.5MHz) - 174 = 36.4dB

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  • +1
    •  Analog Employees 
    on Apr 22, 2021 2:23 PM

    Hi, thank you for bringing this to our attention. the NF value is calculated at full BW of 3GSPS. thats why it shows the 24.5dB number. the chip supports a variety of decimation options. hence the NF value will change with decimation. I will put a request to add a footnote for the datasheet to specify the conditions at which NF was calculated. 

Reply
  • +1
    •  Analog Employees 
    on Apr 22, 2021 2:23 PM

    Hi, thank you for bringing this to our attention. the NF value is calculated at full BW of 3GSPS. thats why it shows the 24.5dB number. the chip supports a variety of decimation options. hence the NF value will change with decimation. I will put a request to add a footnote for the datasheet to specify the conditions at which NF was calculated. 

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