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AD7864 Fiches technique(PDF) 9 Page - Analog Devices |
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AD7864 Fiches technique(HTML) 9 Page - Analog Devices |
9 / 28 page AD7864 Rev. D | Page 9 of 28 TERMINOLOGY Channel-to-Channel Isolation Signal-to-(Noise + Distortion) Ratio Channel-to-channel isolation is a measure of the level of crosstalk between channels. It is measured by applying a full- scale 50 kHz sine wave signal to all nonselected input channels and determining how much that signal is attenuated in the selected channel. The figure given is the worst case across all four channels. 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 rms sum of all nonfundamental signals up to half the sampling frequency (fS/2), excluding dc. The ratio depends on 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 Relative Accuracy Relative accuracy, or endpoint nonlinearity, is the maximum deviation from a straight line passing through the endpoints of the ADC transfer function. Signal-to-(Noise + Distortion) = (6.02 N + 1.76) dB Thus, for a 12-bit converter, this is 74 dB. Differential Nonlinearity Total Harmonic Distortion (THD) This is the difference between the measured and the ideal 1 LSB change between any two adjacent codes in the ADC. THD is the ratio of the rms sum of harmonics to the fundamental. For the AD7864, it is defined as Positive Full-Scale Error 1 6 5 4 3 2 V V V V V V THD 2 2 2 2 2 log 20 ) dB ( + + + + = This is the deviation of the last code transition (01...110 to 01...111) from the ideal, 4 × VREF − 3/2 LSB (AD7864-1, ±10 V), or 2 × VREF − 3/2 LSB (AD7864-1, ±5 V range), or VREF − 3/2 LSB (AD7864-3, ±2.5 V range), after the bipolar offset error has been adjusted out. 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 fifth harmonics. Positive Full-Scale Error (AD7864-2, 0 V to 2.5 V and 0 V to 5 V) Peak Harmonic or Spurious Noise This is the deviation of the last code transition (11...110 to 11...111) from the ideal 2 × VREF − 3/2 LSB (AD7864-2, 0 V to 5 V range) or VREF − 3/2 LSB (AD7864-2, 0 V to 2.5 V range), after the unipolar offset error has been adjusted out. 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 parts where the harmonics are buried in the noise floor, it is a noise peak. Bipolar Zero Error (AD7864-1, ±10 V/±5 V, AD7864-3, ±2.5 V) This is the deviation of the midscale transition (all 0s to all 1s) from the ideal, AGND − 1/2 LSB. Intermodulation Distortion Unipolar Offset Error (AD7864-2, 0 V to 2.5 V and 0 V to 5 V) 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, and so on. Intermodulation terms are those for which neither m nor n are equal to zero. For example, second-order terms include (fa + fb) and (fa − fb), whereas third-order terms include (2 fa + fb), (2 fa − fb), (fa + 2 fb), and (fa − 2 fb). This is the deviation of the first code transition (00...000 to 00...001) from the ideal, AGND + 1/2 LSB. Negative Full-Scale Error (AD7864-1, ±10 V/±5 V, and AD7864-3, ±2.5 V) This is the deviation of the first code transition (10...000 to 10...001) from the ideal, −4 × VREF + 1/2 LSB (AD7864-1, ±10 V), −2 × VREF + 1/2 LSB (AD7864-1, ±5 V range) or −VREF + 1/2 LSB (AD7864-3, ±2.5 V range), after bipolar zero error has been adjusted out. The AD7864 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- and third-order terms are of different significance. The second-order terms are usually distanced in frequency from the original sine waves, whereas 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 funda- mental expressed in decibels. Track-and-Hold Acquisition Time Track-and-hold acquisition time is the time required for the output of the track-and-hold amplifier to reach its final value, within ±1/2 LSB, after the end of a conversion (the point at which the track-and-hold returns to track mode). It also applies to situations where there is a step input change on the input voltage applied to the selected VINxA/VINxB input of the AD7864. |
Numéro de pièce similaire - AD7864_09 |
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Description similaire - AD7864_09 |
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