LTC1415 UUWUAPPLICATIONS INFORMATIONEffective Number of Bits band between DC and half the sampling frequency. THD is The effective number of bits (ENOBs) is a measurement of expressed as: the resolution of an ADC and is directly related to the V22 V32 V42 Vn2 S/(N + D) by the equation: THD = + + +… Log 20 V1 N = [S/(N + D) – 1.76]/6.02 where V1 is the RMS amplitude of the fundamental fre- where N is the effective number of bits of resolution and quency and V2 through Vn are the amplitudes of the S/(N + D) is expressed in dB. At the maximum sampling second through nth harmonics. THD vs input frequency is rate of 1.25MHz the LTC1415 maintains very good ENOBs shown in Figure 4. The LTC1415 has good distortion up to the Nyquist input frequency of 625kHz (refer to performance up to the Nyquist frequency and beyond. Figure 3). Intermodulation DistortionTotal Harmonic Distortion If the ADC input signal consists of more than one spectral Total Harmonic Distortion (THD) is the ratio of the RMS component, the ADC transfer function nonlinearity can sum of all harmonics of the input signal to the fundamental produce intermodulation distortion (IMD) in addition to itself. The out-of-band harmonics alias into the frequency THD. IMD is the change in one sinusoidal input caused by 12 74 0 11 68 SIGNAL/(NOISE + DISTORTION) (dB) –10 10 62 –20 9 56 –30 8 7 –40 6 –50 5 –60 EFFECTIVE BITS 4 THD –70 3 –80 2ND 2 –90 1 3RD AMPLITUDE (dB BELOW THE FUNDAMENTAL) 0 –100 1k 10k 100k 1M 2M 1k 10k 100k 1M 2M INPUT FREQUENCY (Hz) INPUT FREQUENCY (Hz) LT1415 • F03 LTC1415 • F04 Figure 3. Effective Bits and Signal/(Noise +Figure 4. Distortion vs Input FrequencyDistortion) vs Input Frequency 0 fSAMPLE = 1.25MHz f –20 IN1 = 86.97509766kHz fIN2 = 113.2202148kHz –40 fb – fa 2fb – fa fa + fb 2fa –60 + fb 2fa – fb 2fa 2fb fa + 2fb 3fa AMPLITUDE (dB) –80 3fb –100 –120 0 100k 200k 300k 400k 500k 600k FREQUENCY (Hz) LTC1415 • F05 Figure 5. Intermodulation Distortion Plot 9
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