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LTC1276 Datasheet(PDF) 11 Page - Linear Technology |
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LTC1276 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  11 LTC1273 LTC1275/LTC1276 S APPLICATI I FOR ATIO 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 300kHz the LTC1273/LTC1275/LTC1276 maintain very good ENOBs up to the Nyquist input frequency of 150kHz. Refer to Figure 3. INPUT FREQUENCY (Hz) 10k 0 3 5 7 10 100k 2M LTC1273/75/76 • F03 1 4 6 9 12 11 8 2 1M 62 56 74 68 50 fSAMPLE = 300kHz Figure 3. Effective Bits and Signal to (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 = 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 LTC1273/LTC1275/ LTC1276 have good distortion performance up to Nyquist and beyond. Figure 4. Distortion vs Input Frequency INPUT FREQUENCY (Hz) –80 –60 –40 –20 0 1k 100k 1M 10M LTC1273/75/76 • F04 –100 10k –90 –70 –50 –30 –10 fSAMPLE = 300kHz THD 2nd HARMONIC 3rd HARMONIC 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 |
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