Datasheet LTC1279 (Analog Devices) - 9

LTC1279
U U W U APPLICATIONS INFORMATION
pared with the binary-weighted charges supplied by the
Signal-to-Noise Ratio
capacitive DAC. Bit decisions are made by the high speed The Signal-to-Noise plus Distortion Ratio [S/(N + D)] is the comparator. At the end of a conversion, the DAC output ratio between the RMS amplitude of the fundamental input balances the AIN input charge. The SAR contents (a 12-bit data word) which represent the A frequency to the RMS amplitude of all other frequency IN are loaded into the 12-bit output latches. components at the A/D output. The output is band limited to frequencies above DC and below half the sampling frequency. Figure 2a shows a typical spectral content with
DYNAMIC PERFORMANCE
a 600kHz sampling rate and a 100kHz input. The dynamic The LTC1279 has excellent high speed sampling capabil- performance is excellent for input frequencies up to the ity. FFT (Fast Fourier Transform) test techniques are used Nyquist limit of 300kHz as shown in Figure 2b. to test the ADC’s frequency response, distortion and noise at the rated throughput. By applying a low distor-
Effective Number of Bits
tion sine wave and analyzing the digital output using an The Effective Number of Bits (ENOBs) is a measurement of FFT algorithm, the ADC’s spectral content can be exam- the resolution of an ADC and is directly related to the ined for frequencies outside the fundamental. Figures 2a S/(N + D) by the equation: and 2b show typical LTC1279 FFT plots. 0 N = [S/(N + D) – 1.76]/6.02 –10 fSAMPLE = 600kHz fIN = 97.705kHz where N is the Effective Number of Bits of resolution and –20 –30 S/(N + D) is expressed in dB. At the maximum sampling –40 rate of 600kHz the LTC1279 maintains very good ENOBs up –50 to the Nyquist input frequency of 300kHz. Refer to Figure 3. –60 –70 12 74 AMPLITUDE (dB) –80 11 68 SIGNAL/(NOISE + DISTORTION) (dB) –90 10 62 NYQUIST –100 9 FREQUENCY 56 –110 8 50 –120 0 50 100 150 200 250 300 7 FREQUENCY (kHz) 1279 F02a 6 5
Figure 2a. LTC1279 Nonaveraged, 4096 Point FFT Plot
4
with 100kHz Input Frequency
3 EFFECTIVE NUMBER OF BITS 2 0 1 fSAMPLE = 600kHz –10 fSAMPLE = 600kHz 0 f –20 IN = 292.822kHz 10k 100k 1M 5M –30 FREQUENCY (Hz) 1279 G03 –40 –50
Figure 3. Effective Bits and Signal/(Noise + Distortion) vs
–60
Input Frequency
–70 AMPLITUDE (dB) –80 –90 –100
Total Harmonic Distortion
–110 Total Harmonic Distortion (THD) is the ratio of the RMS –120 0 50 100 150 200 250 300 sum of all harmonics of the input signal to the fundamental FREQUENCY (kHz) 1279 F02 itself. The out-of-band harmonics alias into the frequency
Figure 2b. LTC1279 Nonaveraged, 4096 Point FFT Plot
band between DC and half the sampling frequency. THD is
with 300kHz Input Frequency
expressed as: 9