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AD1876 Datasheet(PDF) 8 Page - Analog Devices |
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AD1876 Datasheet(HTML) 8 Page - Analog Devices |
8 / 12 page AD1876 REV. A –8– VOLTAGE REFERENCE The AD1876 requires the use of an external voltage reference. The input voltage range is determined by the value of the refer- ence voltage; in general, a reference voltage of n volts produces an input range of ±n volts. Signal-to-noise performance is in- creased proportionately with input signal range. The AD1876 is specified with a 5.0 V reference and an analog input of ±5 V. In the presence of a fixed amount of system noise, increasing the LSB size (which results from increasing the reference voltage) will increase the effective S/(N+D) performance for input values below the point where input distortion occurs. Figure 11 illus- trates S/(N+D) as a function of input amplitude and reference voltage. During a conversion, the switched capacitor array of the AD1876 presents a dynamically changing current load at the voltage reference as the successive-approximation algorithm cycles through various choices of capacitor weighting. The out- put impedance of the reference circuitry must be low so that the output voltage will remain sufficiently constant as the current drive changes. In most applications, this requires that the output of the voltage reference be buffered by an amplifier with low im- pedance at relatively high frequencies. A (10 µF or larger) ca- pacitor connected between VREF and AGND will reduce the demands on the reference by decreasing the magnitude of high frequency components. The following two sections represent typical design approaches. VOLTAGE REFERENCE—AUDIO APPLICATIONS Audio applications require optimal ac performance over a rela- tively narrow temperature range, with low cost being important. Figure 4 shows one such approach towards attaining these goals. A voltage reference, consisting of a Zener diode, capacitor, resis- tor and op amp with typical component values, is shown. This simple circuit has the advantage of low cost, but the reference voltage value is sensitive to changes in the +12 V supply. Addi- tionally, changes in the Zener value due to temperature varia- tions will also be reflected in the reference voltage. ROPTION may be required for other component selections if the Zener requires more current than the op amp can supply. 1k Ω AD589 0.1 µF + R OPTION 3k Ω 1k Ω AD1876 11 REF AD711 V Figure 4. Low Cost Voltage Reference Circuit VOLTAGE REFERENCE—PRECISION MEASUREMENT APPLICATIONS In applications other than audio, parameters such as low drift over temperature and static accuracy are important. Figure 5 shows a voltage reference circuit featuring the 5 V AD586. The AD586 is a low cost reference which utilizes a buried Zener ar- chitecture to provide low noise and drift. Over the 0 °C to +70 °C range, the AD586L grade exhibits less than a 2.25 mV output change from its initial value at +25 °C. A noise-reduction capacitor, CN, reduces the broadband noise of the AD586 out- put, thereby optimizing the overall performance of the AD1876. AD1876 11 REF 8 +12V AGND 47 µF 1 µF N C 8 AD586 4 6 2 AGND + V Figure 5. For higher performance needs, the AD588 reference provides improved drift, low noise, and excellent initial accuracy. The AD588 uses a proprietary ion-implanted buried Zener diode in conjunction with laser-trimmed thin-film resistors for low offset and gain. The AD588 output is accurate to 0.65 mV from its value at +25 °C over the 0°C to +70°C range. The circuit shown in Figure 6 includes a noise-reduction network on Pins 4, 6 and 7. The 1 µF capacitors form low-pass filters with the internal re- sistance of the AD588 and external 3.9 k Ω resistor. This re- duces the wide-band (to 1 MHz) noise of the AD588, providing optimum performance of the AD1876. AD1876 AGND 47µF AD588 AGND NOISE REDUCTION 3.9k Ω 1µF 0.1µF 0.1µF –12V 1µF +12V NC VREF NC NC 2 16 5813 12 11 10 9 1 3 4 6 7 11 8 Figure 6. ANALOG INPUT As previously discussed, the analog input voltage range for the AD1876 is ±V REF. For purposes of ground drop and common- mode rejection, the VIN and VREF inputs each have their own ground. VREF is referred to the local analog system ground (AGND), and VIN is referred to the analog ground sense pin (AGND SENSE) which allows a remote ground sense for the input signal. If AGND SENSE is not used, it should be con- nected to the AGND pin at the package. The AGND SENSE pin is intended to be tied to potentials within 100 mV of AGND to maintain specified performance. The AD1876 analog inputs (VIN, VREF and AGND SENSE) exhibit dynamic characteristics. When a conversion cycle begins, each analog input is connected to an internal, discharged 50 pF capacitor which then charges to the voltage present at the OBSOLETE |
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