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LTC1274 Datasheet(PDF) 11 Page - Linear Technology |
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LTC1274 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  11 LTC1274/LTC1277 APPLICATIONS INFORMATION using an FFT algorithm, the ADCs’ spectral content can be examined for frequencies outside the fundamental. Figures 2a and 2b show typical LTC1274 FFT plots. Signal-to-Noise Ratio The Signal-to-Noise plus Distortion Ratio [S/(N + D)] is the ratio between the RMS amplitude of the fundamental input frequency to the RMS amplitude of all other fre- quency components at the A/D output. The output is band limited to frequencies above DC and below half the sam- pling frequency. Figure 2a shows a typical spectral content with a 100kHz sampling rate and a 48.85kHz input. The dynamic performance is excellent for input frequencies well beyond Nyquist as shown in Figure 2b and Figure 3. INPUT FREQUENCY (kHz) 0 0 –20 –40 –60 –80 –100 –120 10 20 30 40 LTC1274/77 • F02a 50 fSAMPLE = 100kHz fIN = 48.85kHz Figure 2a. LTC1274 Nonaveraged, 4096 Point FFT Plot with 50kHz Input Frequency INPUT FREQUENCY (kHz) 0 0 –20 –40 –60 –80 –100 –120 10 20 30 40 LTC1274/77 • F02b 50 fSAMPLE = 100kHz fIN = 97.68kHz Figure 2b. LTC1274 Nonaveraged, 4096 Point FFT Plot with 100kHz Input Frequency Effective Number of Bits The Effective Number of Bits (ENOBs) is a measurement of the resolution of an ADC and is directly related to the S/(N + D) by the equation: N = [S/(N + D) – 1.76]/6.02 where N is the Effective Number of Bits of resolution and S/(N + D) is expressed in dB. At the maximum sampling rate of 100kHz, the LTC1274/LTC1277 maintain very good ENOBs over 300kHz. Refer to Figure 3. Total Harmonic Distortion Total Harmonic Distortion (THD) is the ratio of the RMS sum of all harmonics of the input signal to the fundamen- tal itself. The out-of-band harmonics alias into the frequency band between DC and half the sampling frequency. THD is expressed as: THD = 20log √V 2 2 + V32 + V42 ... + VN2 V1 where V1 is the RMS amplitude of the fundamental fre- quency and V2 through VN are the amplitudes of the second through Nth harmonics. THD versus input fre- INPUT FREQUENCY (Hz) 10k 12 11 10 9 8 7 6 5 4 3 2 1 0 74 68 62 56 50 100k 1M 2M LTC1274/77 • F03 fSAMPLE = 100kHz NYQUIST FREQUENCY Figure 3. ENOBs and S/(N + D) vs Input Frequency |
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