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AD976ARS Datasheet(PDF) 11 Page - Analog Devices |
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AD976ARS Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page AD976/AD976A –11– REV. C VOLTAGE REFERENCE The AD976/AD976A has an on-chip temperature compensated bandgap voltage reference that is factory trimmed to 2.5 V ± 20 mV. The full-scale range of the ADC is equal to ±4V REF. Thus, the nominal range will be ±10 V. The accuracy of the AD976 over the specified temperature range is dominated by the drift performance of the voltage refer- ence. The on-chip voltage reference is laser-trimmed to provide a typical drift of 7 ppm/ °C. This typical drift characteristic is shown in Figure 13, which is a plot of the change in reference voltage (in mV) versus the change in temperature—notice the plot is normalized for zero error at +25 °C. If improved drift performance is required, an external reference such as the AD780 should be used to provide a drift as low as 3 ppm/ °C. In order to simplify the drive requirements of the voltage reference (internal or external), an onboard reference buffer is provided. The output of this buffer is provided at the CAP pin and is available to the user; however, when externally loading the refer- ence buffer, it is important to make sure that proper precautions are taken to minimize any degradation in the ADC’s perfor- mance. Figure 14 shows the load regulation of the reference buffer. Notice that this figure is also normalized so that there is zero error with no dc load. In the linear region, the output im- pedance at this point is typically 1 ohm. Because of this 1 ohm output impedance, it is important to minimize any ac or input dependent loads that will lead to increased distortion. Any dc loads will simply act as a gain error. Although the typical char- acteristic of Figure 14 shows that the AD976 is capable of driv- ing loads greater than 15 mA, it is not recommended that the steady state current exceed 2 mA. In addition to the on-chip reference, an external 2.5 V reference can be applied. When choosing an external reference for a 16-bit application, however, careful attention should be paid to noise and temperature drift. These critical specifications can have a significant effect on the ADC performance. Figure 9 shows the AD976/AD976A with the AD780 voltage reference applied to the REF pin. The AD780 is a bandgap reference that exhibits ultralow drift, low initial error, and low output noise. For low power applications, the REF192 provides a low quiescent current, high accuracy and low temperature drift solution. C4 0.1 F VANA C3 1 F VIN AGND1 REF CAP AGND2 10V INPUT R2 33.2k C2 2.2 F AD976/ AD976A R1 200 C1 2.2 F AD780 GND VOUT TEMP VIN 0.1 F +5V Figure 9. AD780 External Reference Connection to the AD976/AD976A AC PERFORMANCE The AD976/AD976A is fully specified and tested for dynamic performance specifications. The ac parameters are required for signal processing applications such as speech recognition and spectrum analysis. These applications require information on the ADC’s effect on the spectral content of the input signal. Hence, the parameters for which the AD976/AD976A is specified include: S/(N+D), THD and Spurious Free Dynamic Range. These terms are discussed in greater detail in the follow- ing sections. As a general rule, it is recommended that the results from sev- eral conversions be averaged to reduce the effects of noise, thus improving parameters such as S/(N+D) and THD. The ac per- formance of the AD976/AD976A can be optimized by operating the ADC at its maximum sampling rate of 100 kHz/200 kHz and by digitally filtering the resulting bit stream to the desired signal bandwidth. By distributing noise over a wider frequency range, the noise density in the frequency band of interest can be reduced. For example, if the required input bandwidth is 50 kHz, the AD976A could be oversampled by a factor of 2. This would yield a 3 dB improvement in the effective SNR performance. FREQUENCY – kHz 0 –10 –150 0 100 10 20 30 40 –40 –70 –130 –140 –20 –30 –60 –50 –90 –120 –80 –110 –100 50 60 70 80 90 95 5 15 25 3545 55 657585 FSAMPLE = 200kHz FIN = 45kHz SNR = 86.23dB THD = –105.33dB 100% Figure 10. FFT PLOT DC PERFORMANCE The factory calibration scheme used for the AD976/AD976A compensates for bit weight errors that may exist in the capacitor array. The mismatch in capacitor values is adjusted (using the calibration coefficients) during a conversion, resulting in excellent dc linearity performance. Figures 11, 12, 15, 16, 17 and 18, respectively, show typical INL, typical DNL, typical positive and negative INL and DNL distribution plots for the AD976/AD976A at +25 °C. A histogram test is a statistical method for deriving an A/D converter’s differential nonlinearity. A ramp input is sampled by the ADC and a large number of conversions are taken and stored. Theoretically, the codes would all be the same size and therefore have an equal number of occurrences. A code with an average number of occurrences would have a DNL of “0.” A code that is different than the average would have a DNL that was either greater or less than zero LSB. A DNL of –1 LSB indicates that there is a missing code present at the 16-bit level and that the ADC exhibits 15-bit performance. |
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