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AD7450BRM-REEL Datasheet(PDF) 7 Page - Analog Devices |
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AD7450BRM-REEL Datasheet(HTML) 7 Page - Analog Devices |
7 / 22 page AD7450 –7– TERMINOLOGY Signal-to-(Noise + Distortion) Ratio This is the measured ratio of signal-to-(noise + distortion) at the output of the ADC. The signal is the rms amplitude of the fundamental. Noise is the sum of all nonfundamental signals up to half the sampling frequency (fS/2), excluding dc. The ratio is dependent on the number of quantization levels in the digitiza- tion process; the more levels, the smaller the quantization noise. The theoretical signal-to-(noise + distortion) ratio for an ideal N-bit converter with a sine wave input is given by: Signal–to–(Noise + Distortion) = (6.02 N + 1.76) dB Thus, for a 12-bit converter, this is 74 dB. Total Harmonic Distortion Total harmonic distortion (THD) is the ratio of the rms sum of harmonics to the fundamental. For the AD7450, it is defined as: THD dB VVV VV 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 to the sixth harmonics. Peak Harmonic or Spurious Noise Peak harmonic or spurious noise is defined as the ratio of the rms value of the next largest component in the ADC output spectrum (up to fS/2 and excluding dc) to the rms value of the fundamental. Normally, the value of this specification is deter- mined by the largest harmonic in the spectrum, but for ADCs where the harmonics are buried in the noise floor, it will be a noise peak. Intermodulation Distortion With inputs consisting of sine waves at two frequencies, fa and fb, any active device with nonlinearities will create distortion products at sum and difference frequencies of mfa ± nfb where m and n = 0, 1, 2, or 3. Intermodulation distortion terms are those for which neither m nor n are equal to zero. For example, the second order terms include (fa + fb) and (fa – fb), while the third order terms include (2fa + fb), (2fa – fb), (fa + 2fb), and (fa –2fb). The AD7450 is tested using the CCIF standard, where two input frequencies near the top end of the input bandwidth are used. In this case, the second order terms are usually distanced in frequency from the original sine waves, while the third order terms are usually at a frequency close to the input frequencies. As a result, the second and third order terms are specified separately. The calculation of the intermodulation distortion is as per the THD specification, where it is the ratio of the rms sum of the individual distortion products to the rms amplitude of the sum of the fundamentals expressed in dBs. Aperture Delay This is the amount of time from the leading edge of the sampling clock until the ADC actually takes the sample. Aperture Jitter This is the sample-to-sample variation in the effective point in time at which the actual sample is taken. Full Power Bandwidth The full power bandwidth of an ADC is that input frequency at which the amplitude of the reconstructed fundamental is reduced by 0.1 dB or 3 dB for a full-scale input. Common-Mode Rejection Ratio (CMRR) The common-mode rejection ratio is defined as the ratio of the power in the ADC output at full-scale frequency, f, to the power of a 200 mV p-p sine wave applied to the common-mode volt- age of VIN+ and VIN– of frequency fs: CMRR dB Pf Pfs () log ( / ) = 10 Pf is the power at the frequency f in the ADC output; Pfs is the power at frequency fs in the ADC output. Integral Nonlinearity (INL) This is the maximum deviation from a straight line passing through the endpoints of the ADC transfer function. Differential Nonlinearity (DNL) This is the difference between the measured and the ideal 1 LSB change between any two adjacent codes in the ADC. Zero Code Error This is the deviation of the midscale code transition (111...111 to 000...000) from the ideal VIN+ – VIN– (i.e., 0 LSB). Positive Gain Error This is the deviation of the last code transition (011...110 to 011...111) from the ideal VIN+ – VIN– (i.e., +VREF – 1 LSB), after the zero code error has been adjusted out. Negative Gain Error This is the deviation of the first code transition (100...000 to 100...001) from the ideal VIN+ – VIN– (i.e., –VREF + 1 LSB), after the zero code error has been adjusted out. Track and Hold Acquisition Time The track and hold acquisition time is the minimum time re- quired for the track and hold amplifier to remain in track mode for its output to reach and settle to within 0.5 LSB of the ap- plied input signal. Power Supply Rejection Ratio (PSRR) The power supply rejection ratio is defined as the ratio of the power in the ADC output at full-scale frequency, f, to the power of a 200 mV p-p sine wave applied to the ADC VDD supply of frequency fS. PSRR dB log Pf /Pfs () ( ) = 10 Pf is the power at frequency f in the ADC output; Pfs is the power at frequency fs in the ADC output. Rev. A |
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