Datasheet LTC1406 (Analog Devices) - 9

LTC1406
U U W U APPLICATIONS INFORMATION
0 If two pure sine waves of frequencies fa and fb are applied –10 to the ADC input, nonlinearities in the ADC transfer func- –20 tion can create distortion products at the sum and differ- –30 ence frequencies of mfa ±nfb, where m and n = 0, 1, 2, 3, etc. For example, the 2nd order IMD terms include (f –40 a ± fb). If the two input sine waves are equal in magnitude, the –50 value (in decibels) of the 2nd order IMD products can be –60 THD expressed by the following formula: –70 3RD HARMONIC 2ND HARMONIC AMPLITUDE (dB BELOW THE FUNDAMENTAL) –80 100k 1M 10M 100M ( Amplitude at fa ± f )b IMD f INPUT FREQUENCY (Hz) ( ±f ) a b = 20 log 1406 G03 Amplitude at fa
Figure 4. Distortion vs Input Frequency Peak Harmonic or Spurious Noise
The peak harmonic or spurious noise is the largest spec- 0 f –10 SAMPLE = 20MHz tral component excluding the input signal and DC. This fIN1 = 3.500977MHz –20 fIN2 = 3.598633MHz value is expressed in decibel relative to the RMS value of –30 a full-scale input signal (see Figure 6). –40 –50
Input Bandwidth
–60 AMPLITUDE (dB) The input bandwidth is that input frequency at which the –70 amplitude of the reconstructed fundamental is reduced by –80 3dB for a full-scale input signal. The LTC1406 has been –90 designed for wide input bandwidth (250MHz), allowing –100 0 1 2 3 4 5 6 7 8 9 10 the ADC to undersample input signals with frequencies FREQUENCY (MHz) above the converter’s Nyquist frequency. The noise floor 1406 G05 stays very low at high frequencies; S/(N + D) becomes
Figure 5. Intermodulation Distortion Plot
dominated by distortion at frequencies far beyond Nyquist. 70
Analog Inputs
60 The LTC1406 has a unique differential sample-and-hold circuit that allows rail-to-rail inputs. The A + – 50 IN and AIN inputs are sampled at the same time and the ADC will 40 always convert the difference of [(A + – IN ) – (AIN )] indepen- 30 dent of the common mode voltage. Any unwanted signal that is common to both inputs will be rejected by the com- 20 mon mode rejection of the sample-and-hold circuit. The 10 SPURIOUS-FREE DYNAMIC RANGE (dB) common mode rejection holds up to extremely high fre- 0 quencies (see Figure 7). 100k 1M 10M 100M INPUT FREQUENCY (Hz) The inputs can be driven differentially or single-ended. In 1406 G04 differential mode, both inputs are driven ±0.5V out of
Figure 6. Spurious-Free Dynamic Range vs
phase with each other. In single-ended mode, the nega-
Input Frequency
tive input is tied to a fixed voltage and A + IN is used as the 9