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AD7891 Datasheet(PDF) 6 Page  Analog Devices 

AD7891 Datasheet(HTML) 6 Page  Analog Devices 
6 / 20 page AD7891 –6– REV. A TERMINOLOGY Signal to (Noise + Distortion) Ratio This is the measured ratio of signal to (noise + distortion) at the output of the A/D 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 quan tization noise. The theoretical signal to (noise +distortion) ratio for an ideal Nbit converter with a sine wave input is given by: Signal to (Noise + Distortion) = (6.02N + 1.76) dB Thus for a 12bit 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 AD7891 it is defined as: THD dB VVV V V 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 through 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 parts 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, n = 0, 1, 2, 3, etc. Intermodulation 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 AD7891 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 differ ent significance. 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 sepa rately. 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 dBs. ChanneltoChannel Isolation Channeltochannel isolation is a measure of the level of crosstalk between channels. It is measured by applying a full scale 20 kHz (AD78911) or 100 kHz (AD78912) 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 eight channels. Relative Accuracy Relative accuracy or endpoint nonlinearity is the maximum deviation from a straight line passing through the endpoints of the ADC transfer function. Differential Nonlinearity This is the difference between the measured and the ideal 1 LSB change between any two adjacent codes in the ADC. Positive FullScale Error (AD78911, ±10 V and ± 5 V, AD78912, ±2.5 V) This is the deviation of the last code transition (01. . .110 to 01. . .111) from the ideal 4 × REF IN – 3/2 LSB (AD78911 ±10 V range), 2 × REF IN – 3/2 LSB (AD78911 ± 5 V range) or REF IN – 3/2 LSB (AD78912, ±2.5 V range), after the Bipolar Zero Error has been adjusted out. Positive FullScale Error (AD78912, 0 V to 5 V and 0 V to 2.5 V) This is the deviation of the last code transition (11. . .110 to 11. . .111) from the ideal 2 × REF IN – 3/2 LSB (0 V to 5 V range) or REF IN – 3/2 LSB (0 V to 2.5 V range), after the unipolar offset error has been adjusted out. Bipolar Zero Error (AD78911, ±10 V and ± 5 V, AD78912 , ±2.5 V) This is the deviation of the midscale transition (all 0s to all 1s) from the ideal AGND – 1/2 LSB. Unipolar Offset Error (AD78912, 0 V to 5 V and 0 V to 2.5 V) This is the deviation of the first code transition (00. . .000 to 00. . .001) from the ideal AGND + 1/2 LSB. Negative FullScale Error (AD78911, ± 10 V and ±5 V, AD78912, ±2.5 V) This is the deviation of the first code transition (10. . .000 to 10. . .001) from the ideal –4 × REF IN + 1/2 LSB (AD78911 ±10 V range), –2 × REF IN + 1/2 LSB (AD78911 ± 5 V range) or –REF IN + 1/2 LSB (AD78912, ±2.5 V range), after Bipolar Zero Error has been adjusted out. Track/Hold Acquisition Time Track/hold acquisition time is the time required for the output of the track/hold amplifier to reach its final value, within ±1/2 LSB, after the end of conversion (the point at which the track/hold returns to track mode). It also applies to situations where a change in the selected input channel takes place or where there is a step input change on the input voltage applied to the selected VIN input of the AD7891. It means that the user must wait for the duration of the track/hold acquisition time after the end of conversion or after a channel change/step input change to VIN before starting another conversion, to ensure that the part operates to specification. 

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