Datasheet LTC1416 (Analog Devices) - 8

LTC1416
U U W U APPLICATIONS INFORMATION DYNAMIC PERFORMANCE Signal-to-Noise Ratio
The LTC1416 has excellent high speed sampling capabil- The Signal-to-Noise plus Distortion Ratio [S/(N + D)] is the ity. FFT (Fast Fourier Transform) test techniques are used ratio between the RMS amplitude of the fundamental input to test the ADC’s frequency response, distortion and noise frequency to the RMS amplitude of all other frequency at the rated throughput. By applying a low distortion sine components at the A/D output. The output is band limited wave and analyzing the digital output using an FFT algo- to frequencies from above DC and below half the sampling rithm, the ADC’s spectral content can be examined for frequency. Figure 2a shows a typical spectral content with frequencies outside the fundamental. Figure 2 shows a a 400kHz sampling rate and a 100kHz input. The dynamic typical LTC1416 FFT plot. performance is excellent for input frequencies up to and beyond the Nyquist limit of 200kHz, Figure 2b. 0 fSAMPLE = 400kHz f –20 IN = 101.5625kHz
Effective Number of Bits
SFDR = 95.2dB SINAD = 80.5dB –40 The Effective Number of Bits (ENOBs) is a measurement of the resolution of an ADC and is directly related to the –60 S/(N + D) by the equation: –80 AMPLITUDE (dB) ENOB = [S/(N + D) – 1.76]/6.02 –100 where ENOB is the Effective Number of Bits of resolution –120 and S/(N + D) is expressed in dB. At the maximum –140 sampling rate of 400kHz, the LTC1416 maintains near 0 25 50 75 100 125 150 175 200 FREQUENCY (kHz) ideal ENOBs up to the Nyquist input frequency of 200kHz 1416 F02a (refer to Figure 3).
Figure 2a. LTC1416 Nonaveraged, 4096 Point FFT, Input Frequency = 100kHz
14 86 13 80 SIGNAL/(NOISE + DISTORTION) (dB) 0 12 74 fSAMPLE = 400kHz 11 NYQUIST 68 f FREQUENCY –20 IN = 189.9414kHz 10 62 SFDR = 94.8dB SINAD = 80.2dB 9 –40 8 7 –60 6 EFFECTIVE BITS 5 –80 4 AMPLITUDE (dB) 3 –100 2 1 fSAMPLE = 400kHz –120 0 1k 10k 100k 1M 2M –140 INPUT FREQUENCY (Hz) 0 25 50 75 100 125 150 175 200 1416 TA02 FREQUENCY (kHz) 1416 F02b
Figure 3. Effective Bits and Signal/(Noise + Distortion) vs Input Frequency Figure 2b. LTC1416 Nonaveraged, 4096 Point FFT, Input Frequency = 190kHz
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