Datasheet LTC1419 (Analog Devices) - 9

LTC1419
U U W U APPLICATIONS INFORMATION
0 (fa + fb). If the two input sine waves are equal in magni- –10 tude, the value (in decibels) of the 2nd order IMD products –20 can be expressed by the following formula: –30 –40 –50 Amplitude at (fa + fb) IMD fa –60 ( +fb Log )=20 Amplitude at fa –70 –80 THD
Peak Harmonic or Spurious Noise
–90 2ND –100 The peak harmonic or spurious noise is the largest spec- AMPLITUDE (dB BELOW THE FUNDAMENTAL) 3RD – 110 1k 10k 100k 1M 2M tral component excluding the input signal and DC. This INPUT FREQUENCY (Hz) value is expressed in decibels relative to the RMS value of 1419 G03 a full-scale input signal.
Figure 4. Distortion vs Input Frequency Full-Power and Full-Linear Bandwidth
The full-power bandwidth is that input frequency at which
Intermodulation Distortion
the amplitude of the reconstructed fundamental is If the ADC input signal consists of more than one spectral reduced by 3dB for a full-scale input signal. component, the ADC transfer function nonlinearity can The full-linear bandwidth is the input frequency at which produce intermodulation distortion (IMD) in addition to the S/(N + D) has dropped to 77dB (12.5 effective bits). THD. IMD is the change in one sinusoidal input caused by The LTC1419 has been designed to optimize input band- the presence of another sinusoidal input at a different width, allowing the ADC to undersample input signals with frequency. frequencies above the converter’s Nyquist Frequency. The If two pure sine waves of frequencies fa and fb are applied noise floor stays very low at high frequencies; S/(N + D) to the ADC input, nonlinearities in the ADC transfer func- becomes dominated by distortion at frequencies far tion can create distortion products at the sum and differ- beyond Nyquist. ence frequencies of mfa ±nfb, where m and n = 0, 1, 2, 3,
Driving the Analog Input
etc. For example, the 2nd order IMD terms include The differential analog inputs of the LTC1419 are easy to 0 drive. The inputs may be driven differentially or as a single- fSAMPLE = 800kHz f ended input (i.e., the – A IN1 = 95.8984375kHz IN input is grounded). The + AIN – 20 fIN2 = 104.1015625kHz and – AIN inputs are sampled at the same instant. Any – 40 unwanted signal that is common mode to both inputs will be reduced by the common mode rejection of the sample- – 60 and-hold circuit. The inputs draw only one small current spike while charging the sample-and-hold capacitors at AMPLITUDE (dB) – 80 the end of conversion. During conversion, the analog –100 inputs draw only a small leakage current. If the source impedance of the driving circuit is low, then the LTC1419 –120 0 50 100 150 200 250 300 350 400 inputs can be driven directly. As source impedance in- FREQUENCY (kHz) creases so will acquisition time (see Figure 6). For mini- 1419 G05 mum acquisition time with high source impedance, a
Figure 5. Intermodulation Distortion Plot
buffer amplifier should be used. The only requirement is 1419fb 9