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LTC1412 Datasheet(PDF) 9 Page - Linear Technology |
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LTC1412 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  9 LTC1412 APPLICATIONS INFORMATION 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 f f f Amplitude at ab b + ()= ± () 20 log Amplitude at f f a a INPUT FREQUENCY (Hz) 2 4 6 8 10 1k 100k 1M 10M 1412 G01 0 10k 12 62 74 56 68 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 68dB (11 effective bits). The LTC1412 has been designed to optimize input bandwidth, allowing the ADC to undersample input signals with fre- Figure 3. Effective Bits and Signal/(Noise + Distortion) vs Input Frequency 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: THD VV = +++ 20 3 2 4 2 log V . . .V V 2 2 n 2 1 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 vs input frequency is shown in Figure 4. The LTC1412 has good distortion performance up to the Nyquist frequency and beyond. INPUT FREQUENCY (Hz) 10 –120 –40 –20 0 100 1k 10k 1412 G03 –60 –80 –100 3RD THD 2ND 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 FREQUENCY (kHz) 0 200 400 600 800 1000 1200 1400 –110 –100 –80 –70 –90 –60 –50 –40 –30 0 1412 G05 –20 –10 fSMPL = 3MHz fIN1 = 85.693359kHz fIN2 = 114.990234kHz Figure 5. Intermodulation Distortion Plot |
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