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AD7722AS Arkusz danych(PDF) 11 Page - Analog Devices |
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AD7722AS Arkusz danych(HTML) 11 Page - Analog Devices |
11 / 24 page REV. B AD7722 –11– TERMINOLOGY 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 nonfundamental signals and harmonics to half the sampling rate (fCLKIN/128), excluding dc. The ADC is evaluated by applying a low noise, low distortion sine wave signal to the input pins. By generating a fast Fourier transform (FFT) plot, the S/(N+D) data can then be obtained from the output spectrum. Total Harmonic Distortion (THD) THD is the ratio of the rms sum of the harmonics to the rms value of the fundamental. THD is defined as THD = 20 log SQRT V 2 2 +V 3 2 +V 4 2 +V 5 2 +V 6 2 () V1 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. Spurious-Free Dynamic Range (SFDR) Defined as the difference in dB between the peak spurious or har- monic component in the ADC output spectrum (up to fCLKIN/128 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 output spectrum of the FFT. For input signals whose second harmonics occur in the stop-band region of the digital filter, a spur in the noise floor limits the SFDR. 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, n = 0, 1, 2, 3, and so on. Intermodulation distortion terms are those for which neither m nor n is 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). Testing is performed 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 fundamental, expressed in dB. Pass-Band Ripple The frequency response variation of the AD7722 in the defined pass-band frequency range. Pass-Band Frequency The frequency up to which the frequency response variation is within the pass-band ripple specification. Cutoff Frequency The frequency below which the AD7722’s frequency response will not have more than 3 dB of attenuation. Stop-Band Frequency The frequency above which the AD7722’s frequency response will be within its stop-band attenuation. Stop-Band Attenuation The AD7722’s frequency response will not have less than 90 dB of attenuation in the stated frequency band. Integral Nonlinearity This is the maximum deviation of any code from a straight line passing through the endpoints of the transfer function. The endpoints of the transfer function are minus full scale, a point 0.5 LSB below the first code transition (100 . . . 00 to 100 . . . 01 in bipolar mode, 000 . . . 00 to 000 . . . 01 in unipolar mode) and plus full scale, a point 0.5 LSB above the last code transition (011 . . . 10 to 011 . . . 11 in bipolar mode, 111 . . . 10 to 111 . . . 11 in unipolar mode). The error is expressed in LSB. Differential Nonlinearity This is the difference between the measured and the ideal 1 LSB change between two adjacent codes in the ADC. Common-Mode Rejection Ratio The ability of a device to reject the effect of a voltage applied to both input terminals simultaneously—often through variation of a ground level—is specified as a common-mode rejection ratio. CMRR is the ratio of gain for the differential signal to the gain for the common-mode signal. Unipolar Offset Error Unipolar offset error is the deviation of the first code transition (00 . . . 000 to 00 . . . 001) from the ideal differential voltage (VIN(+) – VIN(–) + 0.5 LSB) when operating in the unipolar mode. Bipolar Offset Error This is the deviation of the midscale transition code (111 . . . 11 to 000 . . . 00) from the ideal differential voltage (VIN(+) – VIN(–) – 0.5 LSB) when operating in the bipolar mode. Gain Error The first code transition should occur at an analog value 1/2 LSB above – full scale. The last transition should occur for an analog value 1 1/2 LSB below the nominal full scale. Gain error is the deviation of the actual difference between first and last code transitions and the ideal difference between first and last code transitions. |
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