Datasheet LTC1410 (Analog Devices) - 9

LTC1410
U U W U APPLICATIONS INFORMATION
where V 0 1 is the RMS amplitude of the fundamental fre- (fa) (fb) fSAMPLE = 1.25MHz quency and V2 through Vn are the amplitudes of the f –20 IN1 = 88.19580078kHz f second through nth harmonics. THD vs Input Frequency is IN2 = 111.9995117kHz shown in Figure 4. The LTC1410 has good distortion –40 performance up to the Nyquist frequency and beyond. –60 (2f (2f (f a–fb) b–fa) a+fb) (2fa+fb) (f (2f (2f (f b–fa) a) b) a+2fb) 0 AMPLITUDE (dB) –80 (3fa) (3fb) –10 –100 –20 –30 –120 –40 0 100 200 300 400 500 600 –50 FREQUENCY (MHz) 1410 F05 –60 3RD THD –70
Figure 5. Intermodulation Distortion Plot
–80 2ND –90 AMPLITUDE (dB BELOW THE FUNDAMENTAL)
Peak Harmonic or Spurious Noise
–100 1k 10k 100k 1M 10M INPUT FREQUENCY (Hz) The peak harmonic or spurious noise is the largest spec- 1410 G03 tral component excluding the input signal and DC. This value is expressed in decibel 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 (IMD)
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 re- component, the ADC transfer function nonlinearity can duced by 3dB for a full-scale input signal. produce Intermodulation Distortion in addition to THD. The full linear bandwidth is the input frequency at which IMD is the change in one sinusoidal input caused by the the S/(N + D) has dropped to 68dB (11 effective bits). The presence of another sinusoidal input at a different LTC1410 has been designed to optimize input bandwidth, frequency. allowing the ADC to undersample input signals with fre- If two pure sine waves of frequencies fa and fb are applied quencies above the converter’s Nyquist frequency. The to the ADC input, nonlinearities in the ADC transfer func- noise floor stays very low at high frequencies; S/(N + D) tion can create distortion products at the sum and differ- does not become dominated by distortion until frequen- ence frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3, cies far beyond Nyquist. etc. For example, the 2nd order IMD terms include (f
Driving the Analog Input
a + fb). If the two input sine waves are equal in magnitude, the value (in decibels) of the 2nd order IMD products can The differential analog inputs of the LTC1410 are easy to be expressed by the following formula: drive. The inputs may be driven differentially or as a single-ended input (i.e., the – AIN input is grounded). The ( Amplitude at fa ± f )b + AIN and – AIN inputs are sampled at the same instant. IMD f ( +f )= a b 20 log Any unwanted signal that is common mode to both Amplitude at fa inputs will be reduced by the common mode rejection of the sample-and-hold circuit. The inputs draw only one small current spike while charging the sample-and-hold 9