Datasheet LTC1279 (Analog Devices) - 10

LTC1279
U U W U APPLICATIONS INFORMATION
√V 2 + V 2 + V 2 ... + V 2 Figure 5 shows the IMD performance at a 100kHz input. THD = 20log 2 3 4 N V1 0 (fa) (fb) –10 fSAMPLE = 600kHz where V fa = 94.189kHz 1 is the RMS amplitude of the fundamental fre- –20 fb = 97.705kHz quency and V2 through VN are the amplitudes of the –30 second through Nth harmonics. THD versus input fre- –40 quency is shown in Figure 4. The LTC1279 has good –50 (3fb) –60 distortion performance up to the Nyquist frequency and (fa + 2fb) –70 (2fa – fb) (2fb – fa) (fa + fb)(2fa + fb) beyond. AMPLITUDE (dB) –80 (fb – fa) (2fa) (2fb) –90 (3fa) 0 –100 f –10 SAMPLE = 600kHz –110 –20 –120 0 50 100 150 200 250 300 –30 FREQUENCY (kHz) –40 1279 G08 –50
Figure 5. Intermodulation Distortion Plot
–60 –70
Peak Harmonic or Spurious Noise
2ND HARMONIC –80 THD 3RD HARMONIC The peak harmonic or spurious noise is the largest spec- –90 AMPLITUDE (dB BELOW THE FUNDAMENTAL) tral component excluding the input signal and DC. This –10010k 100k 1M 2M value is expressed in decibels relative to the RMS value of INPUT FREQUENCY (Hz) a full scale input signal. 1279 G06
Figure 4. Distortion vs Input Frequency Full Power and Full Linear Bandwidth Intermodulation Distortion
The full power bandwidth is that input frequency at which the amplitude of the reconstructed fundamental is re- If the ADC input signal consists of more than one spectral duced by 3dB for a full scale input signal. component, the ADC transfer function nonlinearity can produce intermodulation distortion (IMD) in addition to The full linear bandwidth is the input frequency at which THD. IMD is the change in one sinusoidal input caused by the S/(N + D) has dropped to 68dB (11 effective bits). The the presence of another sinusoidal input at a different LTC1279 has been designed to optimize input bandwidth, frequency. allowing ADC to undersample input signals with frequen- cies above the converter’s Nyquist Frequency. The noise If two pure sine waves of frequencies fa and fb are applied floor stays very low at high frequencies; S/(N + D) be- to the ADC input, nonlinearities in the ADC transfer func- comes dominated by distortion at frequencies far beyond tion can create distortion products at sum and difference Nyquist. frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3, etc. For example, the 2nd order IMD terms include (fa + fb) and
Driving the Analog Input
(fa – fb) while the 3rd order IMD terms include (2fa + fb), (2fa – fb), (fa + 2fb), and (fa – 2fb). If the two input sine The LTC1279’s analog input is easy to drive. It draws only waves are equal in magnitude, the value (in decibels) of one small current spike while charging the sample-and- the 2nd order IMD products can be expressed by the hold capacitor at the end of conversion. During conversion the analog input draws no current. The only requirement following formula: is that the amplifier driving the analog input must settle after the small current spike before the next conversion Amplitude at (fa ± fb) IMD (fa ± fb) = 20log starts. Any op amp that settles in 160ns to small current Amplitude at fa transients will allow maximum speed operation. If slower 10