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LTC1279CSW Datasheet(PDF) 10 Page - Linear Technology |
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LTC1279CSW Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  10 LTC1279 APPLICATIONS INFORMATION 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- quency is shown in Figure 4. The LTC1279 has good distortion performance up to the Nyquist frequency and beyond. 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 sum and difference frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3, etc. For example, the 2nd order IMD terms include (fa + fb) and (fa – fb) while the 3rd order IMD terms include (2fa + fb), (2fa – fb), (fa + 2fb), and (fa – 2fb). If the two input sine waves are equal in magnitude, the value (in decibels) of the 2nd order IMD products can be expressed by the following formula: IMD (fa ± fb) = 20log Amplitude at (fa ± fb) Amplitude at fa INPUT FREQUENCY (Hz) 10k 0 –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 100k 1M 1279 G06 2M THD 2ND HARMONIC 3RD HARMONIC fSAMPLE = 600kHz Figure 4. Distortion vs Input Frequency Figure 5 shows the IMD performance at a 100kHz input. Figure 5. Intermodulation Distortion Plot 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 re- duced 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 68dB (11 effective bits). The LTC1279 has been designed to optimize input bandwidth, allowing ADC to undersample input signals with frequen- cies above the converter’s Nyquist Frequency. The noise floor stays very low at high frequencies; S/(N + D) be- comes dominated by distortion at frequencies far beyond Nyquist. Driving the Analog Input The LTC1279’s analog input is easy to drive. It draws only one small current spike while charging the sample-and- hold capacitor at the end of conversion. During conversion the analog input draws no current. The only requirement is that the amplifier driving the analog input must settle after the small current spike before the next conversion starts. Any op amp that settles in 160ns to small current transients will allow maximum speed operation. If slower FREQUENCY (kHz) 0 50 100 150 200 1279 G08 250 300 0 –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 –110 –120 fSAMPLE = 600kHz fa = 94.189kHz fb = 97.705kHz (fb – fa) (2fa – fb) (2fa) (2fb) (3fa) (2fb – fa) (fa) (fb) (fa + fb) (2fa + fb) (fa + 2fb) (3fb) |
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