Datasheet LTC1416 (Analog Devices) - 9

LTC1416
U U W U APPLICATIONS INFORMATION Total Harmonic Distortion
difference frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3, etc. For example, the 2nd order IMD terms include (fa Total Harmonic Distortion (THD) is the ratio of the RMS + fb). If the two input sine waves are equal in magnitude, sum of all harmonics of the input signal to the fundamental the value (in decibels) of the 2nd order IMD products can itself. The out-of-band harmonics alias into the frequency be expressed by the following formula: band between DC and half the sampling frequency. THD is expressed as: ( Amplitude at fa + f ) b IMD fa 2 2 2 ( +fb)=20 log Amplitude at fa V22 V3 V4 V ... n THD = + + + 20 log V1 where V1 is the RMS amplitude of the fundamental fre- 0 fSAMPLE = 400kHz quency and V2 through Vn are the amplitudes of the f –20 a=87.01171876kHz fb=113.1835938kHz second through Nth harmonics. THD versus input fre- –40 quency is shown in Figure 4. The LTC1416 has good distortion performance up to the Nyquist frequency and –60 beyond. –80 AMPLITUDE (dB) –100 0 –10 –120 –20 –30 –140 0 20 40 60 80 100 120 140 160 180 200 –40 FREQUENCY (Hz) –50 1416 G05 –60 –70
Figure 5. Intermodulation Distortion Plot
–80 3RD –90 THD
Peak Harmonic or Spurious Noise
–100 2ND AMPLITUDE (dB BELOW THE FUNDAMENTAL) –110 The peak harmonic or spurious noise is the largest spec- 1k 10k 100k 1M 2M INPUT FREQUENCY (Hz) tral component excluding the input signal and DC. This 1416 G03 value is expressed in decibels relative to the RMS value of
Figure 4. Distortion vs Input Frequency
a full-scale input signal.
Full-Power and Full-Linear Bandwidth Intermodulation Distortion
The full-power bandwidth is that input frequency at which If the ADC input signal consists of more than one spectral the amplitude of the reconstructed fundamental is component, the ADC transfer function nonlinearity can reduced by 3dB for a full-scale input signal. The full-linear produce intermodulation distortion (IMD) in addition to bandwidth is the input frequency at which the S/(N + D) THD. IMD is the change in one sinusoidal input caused by has dropped to 77dB (12.5 effective bits). The LTC1416 the presence of another sinusoidal input at a different has been designed to optimize input bandwidth, allowing frequency. the ADC to undersample input signals with frequencies If two pure sine waves of frequencies fa and fb are applied above the converter’s Nyquist frequency. The noise floor to the ADC input, nonlinearities in the ADC transfer stays very low at high frequencies; S/(N + D) becomes function can create distortion products at the sum and dominated by distortion at frequencies far beyond Nyquist. 9