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LTC1419ISW Datasheet(PDF) 9 Page - Linear Technology |
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LTC1419ISW Datasheet(HTML) 9 Page - Linear Technology |
9 / 20 page 9 LTC1419 APPLICATIONS INFORMATION Figure 4. Distortion vs Input Frequency Intermodulation Distortion If the ADC input signal consists of more than one spectral component, the ADC transfer function nonlinearity can produce intermodulation distortion (IMD) in addition to THD. IMD is the change in one sinusoidal input caused by the presence of another sinusoidal input at a different frequency. If two pure sine waves of frequencies fa and fb are applied to the ADC input, nonlinearities in the ADC transfer func- tion can create distortion products at the sum and differ- ence frequencies of mfa ±nfb, where m and n = 0, 1, 2, 3, etc. For example, the 2nd order IMD terms include (fa + fb). If the two input sine waves are equal in magni- tude, the value (in decibels) of the 2nd order IMD products can be expressed by the following formula: IMD fa fb Log + ()=20 Amplitude at (fa+fb) Amplitude at fa Peak Harmonic or Spurious Noise The peak harmonic or spurious noise is the largest spec- tral component excluding the input signal and DC. This value is expressed in decibels relative to the RMS value of a full-scale input signal. Full-Power and Full-Linear Bandwidth The full-power bandwidth is that input frequency at which the amplitude of the reconstructed fundamental is reduced by 3dB for a full-scale input signal. The full-linear bandwidth is the input frequency at which the S/(N + D) has dropped to 77dB (12.5 effective bits). The LTC1419 has been designed to optimize input band- width, allowing the ADC to undersample input signals with frequencies above the converter’s Nyquist Frequency. The noise floor stays very low at high frequencies; S/(N + D) becomes dominated by distortion at frequencies far beyond Nyquist. Driving the Analog Input The differential analog inputs of the LTC1419 are easy to drive. The inputs may be driven differentially or as a single- ended input (i.e., the – AIN input is grounded). The + AIN and – AIN inputs are sampled at the same instant. Any unwanted signal that is common mode to both inputs will be reduced by the common mode rejection of the sample- and-hold circuit. The inputs draw only one small current spike while charging the sample-and-hold capacitors at the end of conversion. During conversion the analog inputs draw only a small leakage current. If the source impedance of the driving circuit is low, then the LTC1419 inputs can be driven directly. As source impedance in- creases so will acquisition time (see Figure 6). For mini- mum acquisition time with high source impedance, a buffer amplifier should be used. The only requirement is Figure 5. Intermodulation Distortion Plot FREQUENCY (kHz) 0 –120 –100 –80 –60 –40 100 200 300 400 1419 G05 –20 0 50 150 250 350 fSAMPLE = 800kHz fIN1 = 95.8984375kHz fIN2 = 104.1015625kHz INPUT FREQUENCY (Hz) 0 –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 – 110 1419 G03 THD 2ND 3RD 1k 100k 1M 2M 10k |
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