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AD976ABRS Datasheet(PDF) 7 Page - Analog Devices |
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AD976ABRS Datasheet(HTML) 7 Page - Analog Devices |
7 / 16 page AD976/AD976A –7– REV. C APERTURE JITTER Aperture jitter is the variation in aperture delay for successive samples and is manifested as noise on the input to the A/D. TRANSIENT RESPONSE The time required for the AD976/AD976A to achieve its rated accuracy after a full-scale step function is applied to its input. OVERVOLTAGE RECOVERY The time required for the ADC to recover to full accuracy after an analog input signal 150% of full-scale is reduced to 50% of the full-scale value. Signal-to-(Noise Plus Distortion Ratio) (S/[N+D]) S/(N+D) is the measured signal-to-noise plus distortion ratio at the output of the ADC. The signal is the rms magnitude of the fundamental. Noise plus distortion is the rms sum of all of the nonfundamental signals and harmonics to half the sampling rate excluding dc. The S/(N+D) is dependent upon the number of quantization levels. The more levels, the lower the quantization noise. The theoretical S/(N+D) for a sine wave input signal can be calculated using the following: S/(N+D) = (6.02N + 1.76) dB (1) where N is the number of bits. Thus, for an ideal 16 bit converter, S/(N+D) = 98 dB. The output spectrum from the ADC is evaluated by applying a low noise, low distortion sine wave signal to the VIN pin and sampling at a 200 kHz throughput rate. By generating a Fast Fourier Transform (FFT) plot, the S/(N+D) data can then be obtained. Figure 10 shows a typical 2048-point FFT plot with an input signal of 45 kHz and a sampling rate of 200 kHz. The S/(N+D) obtained from this graph is 86.23 dB. Since the measured S/(N+D) is less than the theoretical value, it is possible to get a measure of performance expressed in effective number of bits (ENOB). ENOB = ((S/(N+D) – 1.76) / 6.02) Thus for an input signal of 45 kHz, the typical ENOB is 14. TOTAL HARMONIC DISTORTION (THD) THD is the ratio of the rms sum of the harmonics to the rms value of the fundamental. For the AD976/AD976A, THD is defined as: THD dB VVV V V V () = ++ + + 20 2 2 3 2 4 2 5 2 6 2 1 log where V1 is the rms amplitude of the fundamental, and V2, V3, V4, V5 and V6 are the rms amplitudes of the second through sixth harmonics. The THD is also derived from the FFT plot of the ADC output spectrum shown in Figure 10 and is seen there as –105.33 dB. Spurious Free Dynamic Range (SPFD) The spurious free dynamic range is defined as the difference, in dB, between the peak spurious or harmonic component in the ADC output spectrum (up to FS/2 and excluding dc) and the rms value of the fundamental. Normally, the value of this specification will be determined by the largest harmonic in the spectrum. The typical SPFD for the AD976/AD976A is –100 dB and can be seen in Figure 10. FUNCTIONAL DESCRIPTION The AD976/AD976A is a high speed, low power, 16-bit sam- pling, analog-to-digital converter that can operate from a single +5 volt power supply. The AD976/AD976A uses laser trimmed scaling input resistors to provide an industry standard ±10 volt input range. With a 100/200 kSPS throughput rate and a paral- lel interface, the AD976/AD976A is capable of connecting di- rectly to digital signal processors and microcontrollers. The AD976/AD976A employs a successive-approximation technique to determine the value of the analog input voltage. Instead of using the traditional laser-trimmed resistor-ladder approach, however, this device uses a capacitor array charge distribution technique. Binary weighted capacitors subdivide the input sample to perform the actual analog-to-digital conversion. The capacitor array eliminates variation in the linearity of the device due to temperature-induced mismatches of resistor val- ues. As a result of having an on-chip capacitor array, there is no need for additional external circuitry to perform the sample/hold function. Initial errors in capacitor matching are eliminated at the time of manufacturing. Calibration coefficients are calculated that cor- rect for capacitor mismatches and are stored in on-chip thin-film resistors that act as ROM. As a conversion is occurring, the appro- priate calibration coefficients are read out of ROM. The accumu- lated coefficients are then used to adjust and improve conversion accuracy. Any initial offset error is also trimmed out during factory calibration. With the addition of an onboard reference the AD976/AD976A provides a complete 16-bit A/D solution. |
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