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ADS8353: Noise Analysis

Samuel Zhang
Prodigy 10 points
Part Number: ADS8353

Hi, 

In TI training videos, they all include noise from SNR figure. My understanding is that SNR is only a concern when input is AC. 

In training, input seems to be DC. Why do we add Vn_ADC noise as input referred noise of ADC from SNR and added to the total noise? 

Video (Hands-on experiment: ADC noise) : https://www.youtube.com/watch?v=oYwT0aQvZ_c&ab_channel=TexasInstruments 

Regards,

  • +1
    TI__Guru 61826 points

    Hello Samuel,

    Thank you for your post and I hope you are enjoying the TI Precision Labs - ADC training videos!

    Noise applies to both DC and AC input signal measurements. Generally, we assume that the noise of the ADC (thermal noise and quantization noise combined) is a constant magnitude across frequency. This same noise can be measured using either DC and AC methods.

    For DC methods, we generally apply a 0V differential input and force a common-mode voltage to the input pins (i.e. both ADC input pins shorted to ground, or mid-reference, etc.). We then capture thousands of ADC output conversions and analyze the distribution, which is Gaussian, in the form of a histogram. The standard deviation of the distribution tells us the RMS noise of the ADC.

    For AC methods, we apply an input sine wave (about -20 dB relative to full-scale, or dBFS) and measure the difference between the signal magnitude and noise magnitude in an FFT. The SNR result is usually normalized back to full-scale (i.e. we add "20 dB" back to the SNR result).The noise magnitude is simply the integration of all the noise bins from DC to Fs/2. To calculate SNR, the "signal" and "noise" terms must both be described in "RMS" units, so the amplitude of the sine wave is converted using the formula Vrms = Vp / √(2). Then, Vn_rms = 1 / [10^(SNR / 20) / Vsignal_rms]

    Either method can be used to analyze the ADC RMS noise.

    Regards,

    Ryan

  • 0 in reply to Ryan Andrews
    Prodigy 10 points

    Hi Ryan, thanks for your quick response.

  • 0 in reply to Samuel Zhang
    TI__Guru 61826 points

    Theoretically, yes, as long as the ADC noise is the dominant noise source in the overall system. For example, using an AC input signal means that you might add noise from the signal source, or from the signal conditioning circuit (i.e. amplifiers, passives, etc.). Also, the larger the input signal amplitude, the more noise is added from the ADC reference voltage source.