Datasheet LTC1410 (Analog Devices) - 8

LTC1410
U U W U APPLICATIONS INFORMATION DYNAMIC PERFORMANCE
to frequencies from above DC and below half the sampling frequency. Figures 2a and 2b shows a typical spectral The LTC1410 has excellent high speed sampling capabil- content with a 1.25MHz sampling rate for 100kHz and ity. Fast Four Transform (FFT) test techniques are used to 600kHz inputs. The dynamic performance is excellent for test the ADC’s frequency response, distortion and noise at input frequencies up to the Nyquist limit of 625kHz and the rated throughput. By applying a low distortion sine beyond. wave and analyzing the digital output using an FFT algo- rithm, the ADC’s spectral content can be examined for
Effective Number of Bits
frequencies outside the fundamental. The Effective Number of Bits (ENOBs) is a measurement of the resolution of an ADC and is directly related to the 0 S/(N + D) by the equation: fSAMPLE = 1.25MHz fIN = 100.098kHz –20 N = [S/(N + D) – 1.76] /6.02 SFDR = 90.1dB SINAD = 72.4dB –40 where N is the effective number of bits of resolution and S/(N + D) is expressed in dB. At the maximum sampling –60 rate of 1.25MHz the LTC1410 maintains very good ENOBs up to the Nyquist input frequency of 625kHz and beyond. AMPLITUDE (dB) –80 Refer to Figure 3. –100 12 74 –120 68 0 100 200 300 400 500 600 10 NYQUIST 62 FREQUENCY (kHz) 1410 F02a 56 8 50 S/(N + D) (dB)
Figure 2a. LTC1410 Nonaveraged 4096 Point FFT, 100kHz Input
6 0 fSAMPLE = 1.25MHz EFFECTIVE BITS 4 fIN = 599.975kHz –20 SFDR = 84.7dB SINAD = 71.7dB 2 –40 fSAMPLE = 1.25MHz 0 1k 10k 100k 1M 10M –60 INPUT FREQUENCY (Hz) LTC1410 • TA02 AMPLITUDE (dB) –80
Figure 3. Effective Bits and Signal/(Noise + Distortion)
–100
vs Input Frequency
–120 0 100 200 300 400 500 600
Total Harmonic Distortion (THD)
FREQUENCY (kHz) 1410 F02b Total harmonic distortion is the ratio of the RMS sum of all
Figure 2b. LTC1410 Nonaveraged 4096 Point FFT, 600kHz Input
harmonics of the input signal to the fundamental itself. The out-of-band harmonics alias into the frequency band
Signal-to-Noise Ratio
between DC and half the sampling frequency. THD is expressed as: The Signal-to-Noise plus Distortion ratio [S/(N + D)] is the ratio between the RMS amplitude of the fundamental input V2 2 2 + V2 + V 2 4 + . .V frequency to the RMS amplitude of all other frequency THD = 20 3 log n components at the ADC output. The output is band limited V1 8