LTC2225 W UWTI I G DIAGRA tAP N + 2 N + 4 ANALOG N INPUT N + 3 N + 5 tH N + 1 tL CLK tD D0-D11, OF N – 5 N – 4 N – 3 N – 2 N – 1 N 2225 TD01 UUWUAPPLICATIO S I FOR ATIODYNAMIC PERFORMANCEIntermodulation Distortion If the ADC input signal consists of more than one spectral Signal-to-Noise Plus Distortion Ratio component, the ADC transfer function nonlinearity can The signal-to-noise plus distortion ratio [S/(N + D)] is the produce intermodulation distortion (IMD) in addition to ratio between the RMS amplitude of the fundamental input THD. IMD is the change in one sinusoidal input caused by frequency and the RMS amplitude of all other frequency the presence of another sinusoidal input at a different components at the ADC output. The output is band limited frequency. to frequencies above DC to below half the sampling If two pure sine waves of frequencies fa and fb are applied frequency. to the ADC input, nonlinearities in the ADC transfer func- Signal-to-Noise Ratio tion can create distortion products at the sum and differ- ence frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3, The signal-to-noise ratio (SNR) is the ratio between the etc. The 3rd order intermodulation products are 2fa + fb, RMS amplitude of the fundamental input frequency and 2fb + fa, 2fa – fb and 2fb – fa. The intermodulation the RMS amplitude of all other frequency components distortion is defined as the ratio of the RMS value of either except the first five harmonics and DC. input tone to the RMS value of the largest 3rd order intermodulation product. Total Harmonic Distortion Total harmonic distortion is the ratio of the RMS sum of all Spurious Free Dynamic Range (SFDR) harmonics of the input signal to the fundamental itself. The Spurious free dynamic range is the peak harmonic or out-of-band harmonics alias into the frequency band spurious noise that is the largest spectral component between DC and half the sampling frequency. THD is excluding the input signal and DC. This value is expressed expressed as: in decibels relative to the RMS value of a full scale input THD = 20Log (√(V22 + V32 + V42 + . Vn2)/V1) signal. where V1 is the RMS amplitude of the fundamental fre- Input Bandwidth quency and V2 through Vn are the amplitudes of the second through nth harmonics. The THD calculated in this The input bandwidth is that input frequency at which the data sheet uses all the harmonics up to the fifth. amplitude of the reconstructed fundamental is reduced by 3dB for a full scale input signal. 2225fa 9
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