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LTC1279 Datasheet(PDF) 9 Page - Linear Technology |
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LTC1279 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  9 LTC1279 APPLICATIONS INFORMATION pared with the binary-weighted charges supplied by the capacitive DAC. Bit decisions are made by the high speed comparator. At the end of a conversion, the DAC output balances the AIN input charge. The SAR contents (a 12-bit data word) which represent the AIN are loaded into the 12-bit output latches. DYNAMIC PERFORMANCE The LTC1279 has excellent high speed sampling capabil- ity. FFT (Fast Fourier Transform) test techniques are used to test the ADC’s frequency response, distortion and noise at the rated throughput. By applying a low distor- tion sine wave and analyzing the digital output using an FFT algorithm, the ADC’s spectral content can be exam- ined for frequencies outside the fundamental. Figures 2a and 2b show typical LTC1279 FFT plots. Figure 2b. LTC1279 Nonaveraged, 4096 Point FFT Plot with 300kHz Input Frequency 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 frequency components at the A/D output. The output is band limited to frequencies above DC and below half the sampling frequency. Figure 2a shows a typical spectral content with a 600kHz sampling rate and a 100kHz input. The dynamic performance is excellent for input frequencies up to the Nyquist limit of 300kHz as shown in Figure 2b. 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 600kHz the LTC1279 maintains verygoodENOBsup to the Nyquist input frequency of 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 fundamental itself. The out-of-band harmonics alias into the frequency band between DC and half the sampling frequency. THD is expressed as: Figure 3. Effective Bits and Signal/(Noise + Distortion) vs Input Frequency FREQUENCY (Hz) 10k 100k 1M 5M 1279 G03 12 11 10 9 8 7 6 5 4 3 2 1 0 74 68 62 56 50 fSAMPLE = 600kHz NYQUIST FREQUENCY FREQUENCY (kHz) 0 50 100 150 200 1279 F02a 250 300 0 –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 –110 –120 fSAMPLE = 600kHz fIN = 97.705kHz FREQUENCY (kHz) 0 50 100 150 200 1279 F02 250 300 0 –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 –110 –120 fSAMPLE = 600kHz fIN = 292.822kHz Figure 2a. LTC1279 Nonaveraged, 4096 Point FFT Plot with 100kHz Input Frequency |
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