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LTC1418I Datasheet(PDF) 9 Page - Linear Technology |
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LTC1418I Datasheet(HTML) 9 Page - Linear Technology |
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9 / 28 page  9 LTC1418 APPLICATIONS INFORMATION Referring to Figure 1, the AIN+ and AIN– inputs are con- nected to the sample-and-hold capacitors (CSAMPLE) dur- ing the acquire phase and the comparator offset is nulled by the zeroing switches. In this acquire phase, a minimum delay of 1 µs will provide enough time for the sample-and- hold capacitors to acquire the analog signal. During the convert phase the comparator zeroing switches open, putting the comparator into compare mode. The input switches the CSAMPLE capacitors to ground, transferring the differential analog input charge onto the summing junction. This input charge is successively compared with the binary weighted charges supplied by the differential capacitive DAC. Bit decisions are made by the high speed comparator. At the end of a conversion, the differential DAC output balances the AIN + and AIN– input charges. The SAR contents (a 14-bit data word) which represent the difference of AIN+ and AIN– are loaded into the 14-bit output latches. DYNAMIC PERFORMANCE The LTC1418 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 distortion sine wave and analyzing the digital output using an FFT algo- rithm, the ADC’s spectral content can be examined for frequencies outside the fundamental. Figure 2 shows a typical LTC1418 FFT plot. 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 from above DC and below half the sampling frequency. Figure 2a shows a typical spectral content with a 200kHz sampling rate and a 10kHz input. The dynamic performance is excellent for input frequencies up to and beyond the Nyquist limit of 100kHz. FREQUENCY (kHz) 0 10 305070 90 0 –20 –40 –60 –80 –100 –120 20 40 60 80 1418 F02a 100 fSAMPLE = 200kHz fIN = 9.9609375kHz SFDR = 99.32 SINAD = 82.4 Figure 2a. LTC1418 Nonaveraged, 4096 Point FFT, Input Frequency = 10kHz FREQUENCY (kHz) 0 10 305070 90 0 –20 –40 –60 –80 –100 –120 20 40 60 80 1418 F02b 100 fSAMPLE = 200kHz fIN = 97.509765kHz SFDR = 94.29 SINAD = 81.4 Figure 2b. LTC1418 Nonaveraged, 4096 Point FFT, Input Frequency = 97.5kHz 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 200kHz the LTC1418 maintains near ideal ENOBs up to the Nyquist input frequency of 100kHz (refer to Figure 3). |
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