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AD7778 데이터시트(PDF) 10 Page - Analog Devices |
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AD7778 데이터시트(HTML) 10 Page - Analog Devices |
10 / 12 page AD7776/AD7777/AD7778 –10– REV. A Short Circuit Current This is defined as the maximum current which flows either into or out of the REFOUT pin if this pin is shorted to any potential between 0 V and VCC. This condition can be allowed for up to 10 seconds provided that the power dissipation of the package is not exceeded. Signal-to-Noise and Distortion Ratio, S/(N+D) Signal-to-noise and distortion ratio, S/(N+D), is the ratio of the rms value of the measured input signal to the rms sum of all other spectral components below the Nyquist frequency, includ- ing harmonics, but excluding dc. The value for S/(N+D) is given in decibels. Total Harmonic Distortion, THD Total harmonic distortion is the ratio of the rms sum of the first five harmonic components to the rms value of a full-scale input signal and is expressed in decibels. For the AD7776/AD7777/ AD7778, total harmonic distortion (THD) is defined as: 20 2 2 3 2 4 2 5 2 6 2 12 1 log VVV VV V ++ ++ () where V1 is the rms amplitude of the fundamental and V2, V3, V4, V5, and V6 are the rms amplitudes of the individual harmonics. Intermodulation Distortion, IMD With inputs consisting of sine waves at two frequencies, fa and fb, any active device with nonlinearities creates distortion products, of order (m + n), at sum and difference frequencies of mfa + nfb, where m, n = 0, 1, 2, 3. Intermodulation terms are those for which m or n is not equal to zero. For example, the second order terms include (fa + fb) and (fa – fb) and the third order terms include (2 fa + fb), (2 fa – fb), (fa + 2 fb), and (fa – 2 fb). Channel-to-Channel Isolation Channel-to-channel isolation is a measure of the level of cross-talk between channels. It is measured by applying a full-scale 100 kHz sine wave signal to any one of the input channels and monitoring the remaining channels. The figure given is the worst case across all channels. DIGITAL SIGNAL PROCESSING APPLICATIONS In digital signal processing (DSP) application areas like voice recognition, echo cancellation, and adaptive filtering, the dynamic characteristics S/(N+D), THD, and IMD of the ADC are critical. The AD7776/AD7777/AD7778 are specified dynamically as well as with standard dc specifications. Because the track/hold amplifier has a wide bandwidth, an antialiasing filter should be placed on the analog inputs to avoid aliasing high frequency noise back into the bands of interest. The dynamic performance of the ADC is evaluated by applying a sine wave signal of very low distortion to a single analog input which is sampled at 380.95 kHz. A fast Fourier transform (FFT) plot or histogram plot is then generated from which the signal to noise and distortion, harmonic distortion, and dynamic differential nonlinearity data can be obtained. Similarly, for intermodulation distortion, an input signal consisting of two pure sine waves at different frequencies is applied to the AD7776/AD7777/AD7778. Figure 11 shows a 2048-point FFT plot for a single channel of the AD7778 with an input signal of 99.88 kHz. The SNR is 58.71 dB. It can be seen that most of the harmonics are buried in the noise floor. It should be noted that the harmonics are taken into account when calculating the S/(N+D). 0 –90 –80 0 –60 –40 –20 99.88 FREQUENCY – kHz INPUT FREQUENCY = 99.88kHz SAMPLE FREQUENCY = 380.95kHz SNR = 58.7dB TA = 25 C Figure 11. ADC FFT Plot The relationship between S/(N+D) and resolution (n) is ex- pressed by the following equation: SN D n dB + () =+ () 602 176 .. This is for an ideal part with no differential or integral linearity errors. These errors cause a degradation in S/(N+D). By work- ing backwards from the above equation, it is possible to get a measure of ADC performance expressed in effective number of bits (n). n effective SN D dB () = + ()( ) −176 602 . . The effective number of bits plotted versus frequency for a single channel of the AD7778 is shown in Figure 12. The effec- tive number of bits is typically 9.5. 10.0 7.5 189.2 9.0 8.0 8.5 0 9.5 INPUT FREQUENCY – kHz SAMPLE FREQUENCY = 378.4kHz TA = 24 C Figure 12. Effective Number of Bits vs. Frequency |
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