Datasheet AD1674 (Analog Devices) - 9

AD1674 DEFINITION OF SPECIFICATIONS
are present in a sample sequence. The result, called Prime Coherent Sampling, is a highly accurate and repeatable measure
INTEGRAL NONLINEARITY (INL)
of the actual frequency-domain response of the converter. The ideal transfer function for an ADC is a straight line drawn between “zero” and “full scale.” The point used as “zero”
NYQUIST FREQUENCY
occurs 1/2 LSB before the first code transition. “Full scale” is An implication of the Nyquist sampling theorem, the “Nyquist defined as a level 1 1/2 LSB beyond the last code transition. Frequency” of a converter is that input frequency which is one- Integral nonlinearity is the worst-case deviation of a code from half the sampling frequency of the converter. the straight line. The deviation of each code is measured from the middle of that code.
SIGNAL-TO-NOISE AND DISTORTION (S/N+D) RATIO
S/(N+D) is the ratio of the rms value of the measured input sig-
DIFFERENTIAL NONLINEARITY (DNL)
nal to the rms sum of all other spectral components below the A specification which guarantees no missing codes requires that Nyquist frequency, including harmonics but excluding dc. The every code combination appear in a monotonic increasing value for S/(N+D) is expressed in decibels. sequence as the analog input level is increased. Thus every code must have a finite width. The AD1674 guarantees no missing
TOTAL HARMONIC DISTORTION (THD)
codes to 12-bit resolution; all 4096 codes are present over the THD is the ratio of the rms sum of the first six harmonic com- entire operating range. ponents to the rms value of a full-scale input signal and is ex- pressed as a percentage or in decibels. For input signals or
UNIPOLAR OFFSET
harmonics that are above the Nyquist frequency, the aliased The first transition should occur at a level 1/2 LSB above ana- component is used. log common. Unipolar offset is defined as the deviation of the actual transition from that point at 25°C. This offset can be
INTERMODULATION DISTORTION (IMD)
adjusted as shown in Figure 11. With inputs consisting of sine waves at two frequencies, fa and fb, any device with nonlinearities will create distortion products,
BIPOLAR OFFSET
of order (m+n), at sum and difference frequencies of mfa ± nfb, In the bipolar mode the major carry transition (0111 1111 1111 where m, n = 0, 1, 2, 3. Intermodulation terms are those for to 1000 0000 0000) should occur for an analog value 1/2 LSB which m or n is not equal to zero. For example, the second below analog common. The bipolar offset error specifies the order terms are (fa + fb) and (fa – fb) and the third order terms deviation of the actual transition from that point at 25°C. This are (2fa + fb), (2fa – fb), (fa + 2fb) and (fa – 2fb). The IMD offset can be adjusted as shown in Figure 12. products are expressed as the decibel ratio of the rms sum of the measured input signals to the rms sum of the distortion terms.
FULL-SCALE ERROR
The two signals are of equal amplitude and the peak value of The last transition (from 1111 1111 1110 to 1111 1111 1111
)
their sums is –0.5 dB from full scale. The IMD products are should occur for an analog value 1 1/2 LSB below the nominal normalized to a 0 dB input signal. full scale (9.9963 volts for 10 volts full scale). The full-scale error is the deviation of the actual level of the last transition
FULL-POWER BANDWIDTH
from the ideal level at 25°C. The full-scale error can be adjusted The full-power bandwidth is that input frequency at which the to zero as shown in Figures 11 and 12. amplitude of the reconstructed fundamental is reduced by 3 dB for a full-scale input.
TEMPERATURE DRIFT
The temperature drifts for full-scale error, unipolar offset and
FULL-LINEAR BANDWIDTH
bipolar offset specify the maximum change from the initial The full-linear bandwidth is the input frequency at which the (25°C) value to the value at TMIN or TMAX. slew rate limit of the sample-hold-amplifier (SHA) is reached. At this point, the amplitude of the reconstructed fundamental
POWER SUPPLY REJECTION
has degraded by less than –0.1 dB. Beyond this frequency, dis- The effect of power supply error on the performance of the tortion of the sampled input signal increases significantly. device will be a small change in full scale. The specifications show the maximum full-scale change from the initial value with
APERTURE DELAY
the supplies at various limits. Aperture delay is a measure of the SHA’s performance and is measured from the falling edge of Read/Convert (R/C) to when
FREQUENCY-DOMAIN TESTING
the input signal is held for conversion. The AD1674 is tested dynamically using a sine wave input and a 2048 point Fast Fourier Transform (FFT) to analyze the
APERTURE JITTER
resulting output. Coherent sampling is used, wherein the ADC Aperture jitter is the variation in aperture delay for successive sampling frequency and the analog input frequency are related samples and is manifested as noise on the input to the A/D. to each other by a ratio of integers. This ensures that an integral multiple of input cycles is captured, allowing direct FFT pro- cessing without windowing or digital filtering which could mask some of the dynamic characteristics of the device. In addition, the frequencies are chosen to he “relatively prime” (no common factors) to maximize the number of different ADC codes that –8– REV. C