LTC1747 W UWTI I G DIAGRA N • ANALOG INPUT t3 t t 1 2 t0 ENC t7 t8 DATA (N – 5) DATA (N – 4) DATA DATA (N – 3) DB11 TO DB0 DB11 TO DB0 t6 CLKOUT t4 t5 t10 t9 OE t t 11 12 DATA N DATA DB11 TO DB0, OF AND CLKOUT 1747 TD UUWUAPPLICATIO S I FOR ATIODYNAMIC PERFORMANCE V22 V32 V42 Vn2 ... THD = + + + Log 20 Signal-to-Noise Plus Distortion Ratio V1 The signal-to-noise plus distortion ratio [S/(N + D)] is the where V1 is the RMS amplitude of the fundamental fre- ratio between the RMS amplitude of the fundamental input quency and V2 through Vn are the amplitudes of the frequency and the RMS amplitude of all other frequency second through nth harmonics. The THD calculated in this components at the ADC output. The output is band limited data sheet uses all the harmonics up to the fifth. to frequencies above DC to below half the sampling frequency. Intermodulation Distortion If the ADC input signal consists of more than one spectral Signal-to-Noise Ratio component, the ADC transfer function nonlinearity can The signal-to-noise ratio (SNR) is the ratio between the produce intermodulation distortion (IMD) in addition to RMS amplitude of the fundamental input frequency and THD. IMD is the change in one sinusoidal input caused by the RMS amplitude of all other frequency components the presence of another sinusoidal input at a different except the first five harmonics and DC. frequency. If two pure sine waves of frequencies fa and fb are applied Total Harmonic Distortion to the ADC input, nonlinearities in the ADC transfer func- Total harmonic distortion is the ratio of the RMS sum of all tion can create distortion products at the sum and differ- harmonics of the input signal to the fundamental itself. The ence frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3, out-of-band harmonics alias into the frequency band etc. The 3rd order intermodulation products are 2fa + fb, between DC and half the sampling frequency. THD is 2fb + fa, 2fa – fb and 2fb – fa. The intermodulation expressed as: distortion is defined as the ratio of the RMS value of either input tone to the RMS value of the largest 3rd order intermodulation product. 1747fa 10
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