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AD7829BR-REEL Datasheet(PDF) 8 Page - Analog Devices |
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AD7829BR-REEL Datasheet(HTML) 8 Page - Analog Devices |
8 / 28 page AD7822/AD7825/AD7829 Rev. C | Page 8 of 28 TERMINOLOGY Signal-to-(Noise + Distortion) Ratio The measured ratio of signal-to-(noise + distortion) at the output of the analog-to-digital converter. The signal is the rms amplitude of the fundamental. Noise is the rms sum of all nonfundamental signals up to half the sampling frequency (fS/2), excluding dc. The ratio is dependent upon the number of quantization levels in the digitization 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 an 8-bit converter, this is 50 dB. Total Harmonic Distortion (THD) The ratio of the rms sum of harmonics to the fundamental. For the AD7822/AD7825/AD7829, it is defined as 1 6 5 3 2 V V V V V V THD 2 2 2 4 2 2 log 20 (dB) + + + + = where V1 is the rms amplitude of the fundamental and V2, V3, V4, V5, and V6 are the rms amplitudes of the second through the sixth harmonics. Peak Harmonic or Spurious Noise 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 determined by the largest harmonic in the spectrum, but for parts where the harmonics are buried in the noise floor, it is a noise peak. Intermodulation Distortion With inputs consisting of sine waves at two frequencies, fa and fb, any active device with nonlinearities creates distortion products at sum and difference frequencies of mfa ± nfb, where m, n = 0, 1, 2, 3, … . Intermodulation 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), and the third-order terms include (2fa + fb), (2fa − fb), (fa + 2fb), and (fa − 2fb). The AD7822/AD7825/AD7829 are tested using the CCIF standard, where two input frequencies near the top end of the input bandwidth are used. In this case, the second- and third- order terms are of different significance. The second-order terms are usually distanced in frequency from the original sine waves, and 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 fundamental expressed in decibels (dB). Channel-to-Channel Isolation A measure of the level of crosstalk between channels. It is measured by applying a full-scale 20 kHz sine wave signal to one input channel and determining how much that signal is attenuated in each of the other channels. The figure given is the worst case across all four or eight channels of the AD7825 and AD7829, respectively. Relative Accuracy or Endpoint Nonlinearity The maximum deviation from a straight line passing through the endpoints of the ADC transfer function. Differential Nonlinearity The difference between the measured and the ideal one LSB change between any two adjacent codes in the ADC. Offset Error The deviation of the 128th code transition (01111111) to (10000000) from the ideal, that is, VMID. Offset Error Match The difference in offset error between any two channels. Zero-Scale Error The deviation of the first code transition (00000000) to (00000001) from the ideal; that is, VMID − 1.25 V + 1 LSB (VDD = 5 V ± 10%), or VMID − 1.0 V + 1 LSB (VDD = 3 V ± 10%). Full-Scale Error The deviation of the last code transition (11111110) to (11111111) from the ideal; that is, VMID + 1.25 V − 1 LSB (VDD = 5 V ± 10%), or VMID + 1.0 V − 1 LSB (VDD = 3 V ± 10%). |
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