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LTC1273 Datasheet(PDF) 21 Page - Linear Technology |
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LTC1273 Datasheet(HTML) 21 Page - Linear Technology |
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21 / 24 page  21 LTC1273 LTC1275/LTC1276 S APPLICATI I FOR ATIO Demodulating a Signal by Undersampling with LTC1275 Figure 23 shows a 455kHz amplitude modulated input undersampled by the LTC1275. With a 227.5kHz sample rate, the converter provides a 100dB noise floor and 68dB distortion when digitizing the 455kHz AM input. Figure 24 shows an FFT of the AM signal digitized at 212.5kHz. AIN LTC1275 D11 D0 455kHz AMPLITUDE MODULATED INPUT RD RD –5V 227.5kHz SAMPLE RATE 5V LTC1273/75/76 • F23 DATA OUTPUT Figure 23. A 455kHz Amplitude Modulated Input Undersampled by the LTC1275 FREQUENCY (kHz) 0 –110 –90 –70 –50 –30 20 40 60 80 LTC1273/75/76 • F24 100 120 –10 –100 –80 –60 –40 –20 0 fSAMPLE = 212.5kHz fIN = 454.8kHz fMOD = 5.03kHz Figure 24. 455kHz Input Voltage Modulated by a 5kHz Signal A time domain view of the demodulation is shown in Figure 25. The top trace shows the 455kHz waveform modulated by a – 6dB, 5kHz signal. The bottom trace shows the demodulated signal produced by the LTC1275 recon- structed through a 12-bit DAC. The resultant frequency is 5kHz with a sample rate of 227.5kHz. There are roughly 45 points per cycle. Figure 25. 455kHz AM Signal Demodulated to 10.5 ENOBs DEMODULATED 5kHz OUTPUT 455kHz AM SIGNAL 50 µs/DIV LTC1273/75/76 • F27 1V/DIV 1V/DIV 100ps Resolution ∆Time Measurement with LTC1273 Figure 26 shows a circuit that precisely measures the difference in time between two events. It has a 400ns full scale and 100ps resolution. The start signal releases the ramp generator made up of the PNP current source and the 250pF capacitor. The circuit ramps until the stop signal shuts off the current source. The final value of the ramp represents the time between the start and stop events. The LTC1273 digitizes this final value and outputs the digital data. |
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Similar Description - LTC1273 |
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