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ADS8350: How is SNR Calculated in the DS?

Darren (FAE)
Expert 5483 points
Part Number: ADS8350

Hi guys, can anybody confirm for me the numbers used for the SNR calculation?

A customer I have is asking about how SNR was actually obtained.

I am attaching an image below for reference but this is what I have so far...

  • This is a pseudo-differential ADC, so AINM is held at VREF (2.5V), while an AC signal with DC offset at VREF(2.5V) is applied to AINP (actually only -0.5dBFS) 
  • In this case, the signal has a Vp-p of around 4.72V (-0.5dBFS for FSR of 5V), correct?
  • Thus, since the calculation for SNR is 20LOG(Vsig_rms / Vnoise_rms), we calculate Vsig_rms as (4.72 / 2√2)
  • After we find this, by using TYP SNR at 84dB (from DS), we can calculate the Vnoise_rms during the test.

Is this how SNR was calculated? (Vsig_rms as 4.72 / √2)

  • 0
    TI__Guru 63196 points

    Hi Darren,

    Thanks for your post.

    Yes - for AC signals, that is how we calculate SNR. The input signal is limited to -0.5 dBFS, which you correctly calculated as 4.72 V out of the 5-V full-scale range (+/-2.5 V). We then measure the SNR at discrete input signal frequencies. The RMS noise is integrated over the full ADC bandwidth (1/2 the sampling rate) and compared to the RMS signal amplitude (Vin_rms = Vpp / (2*sqrt(2))).

    Best regards,

  • 0 in reply to Ryan Andrews
    TI__Expert 5483 points

    Hi Ryan,

    Thanks for the reply!

    I would just like to clarify two things...

    1) The FSR you mention is +/-2.5V for the AINP pin around a DC common mode offset of VREF=2.5V, thus is 0.14V to 4.86V (AINP pin), correct?

    2) I assume you use some kind of FFT analyzer equipment to get the SNR number? Then calculate the Vnoise_rms from this, change the input waveform, do it again, until you have enough points to integrate the numbers from 10Hz or so, to 1/2 sampling rate?

    Then, you use this integrated Vnoise_rms to calculate the final SNR shown in the DS?

    Regards,

    Darren

  • 0 in reply to Darren (FAE)
    TI__Guru 63196 points

    Hi Darren,

    The full-scale range of the ADC is 5 V, based on the 2.5-V reference voltage. This is what determines the weight of the LSB (FSR / 2^n). When we test AC performance, we restrict the peak-to-peak amplitude to -0.5 dBFS to avoid other non-idealities that come from pushing the inputs all the way to the rails. In a pseudo-differential input configuration, where AINN is fixed to VREF (2.5 V), AINP would swing from 0.14 V to 4.86 V.

    The FFT is an algorithm applied to the data set collected from the ADC. We collect a relatively large 2^n number of samples with a fixed data rate and input signal frequency, then apply the FFT algorithm to analyze the frequency content of the ADC output. Depending on the frequency bin resolution, we assign a small range of bins to represent the fundamental input, the harmonics, noise, etc. Vn_rms is the total noise integrated over the ADC bandwidth (fs/2) after the DC content, fundamental, and harmonics are removed. SNR = 20*log10(Vin_rms/Vn_rms).

    For more information on AC analysis and other ADC topics, I recommend reviewing our TI Precision Labs - ADCs training on ti.com.

    Best regards,