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AD677 bảng dữ liệu(PDF) 11 Page - Analog Devices |
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AD677 bảng dữ liệu(HTML) 11 Page - Analog Devices |
11 / 16 page AD677 REV. A –11– made for selecting one with low noise. A capacitor connected between REF IN and AGND will reduce the demands on the reference by decreasing the magnitude of high frequency com- ponents required to be sourced by the reference. Figures 6 and 7 represent typical design approaches. VIN 10µF AGND CN 1.0µF +12V AD586 AD677 6 2 4 8 VREF 0.1µF Figure 6. Figure 6 shows a voltage reference circuit featuring the 5 V out- put AD586. The AD586 is a low cost reference which utilizes a buried Zener architecture to provide low noise and drift. Over the 0 °C to +70°C range, the AD586M grade exhibits less than 1.0 mV output change from its initial value at +25 °C. A noise reduction capacitor, CN, reduces the broadband noise of the AD586 output, thereby optimizing the overall performance of the AD677. It is recommended that a 10 µF to 47 µF high qual- ity tantalum capacitor and a 0.1 µF capacitor be tied between the VREF input of the AD677 and ground to minimize the im- pedance on the reference. Using the AD677 with ±10 V input range (V REF = 10 V) typi- cally requires ±15 V supplies to drive op amps and the voltage reference. If ±12 V is not available in the system, regulators such as 78L12 and 79L12 can be used to provide power for the AD677. This is also the recommended approach (for any input range) when the ADC system is subjected to harsh environ- ments such as where the power supplies are noisy and where voltage spikes are present. Figure 7 shows an example of such a system based upon the 10 V AD587 reference, which provides a 300 µV LSB. Circuitry for additional protection against power supply disturbances has been shown. A 100 µF capacitor at each +15V +5V –15V 100µF 100µF 100µF AD677 10µF 10µF 0.1µF 0.1µF 0.1µF 78L12 79L12 0.01µF 0.01µF VIN VREF VDD VCC VEE VIN VO NR GND VIN 10µF 0.1µF 1µF AD587 10µF 4 2 6 8 0.1µF 10 Ω 10 Ω 10 Ω 10 Ω Figure 7. regulator prevents very large voltage spikes from entering the regulators. Any power line noise which the regulators cannot eliminate will be further filtered by an RC filter (10 Ω/10 µF) having a –3 dB point at 1.6 kHz. For best results the regulators should be within a few centimeters of the AD677. ANALOG INPUT As previously discussed, the analog input voltage range for the AD677 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. The AD677 analog inputs (VIN, VREF and AGND SENSE) ex- hibit 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 corre- sponding pin. The capacitor is disconnected when SAMPLE is taken LOW, and the stored charge is used in the subsequent conversion. In order to limit the demands placed on the external source by this high initial charging current, an internal buffer amplifier is employed between the input and this capacitance for a few hundred nanoseconds. During this time the input pin ex- hibits typically 20 k Ω input resistance, 10 pF input capacitance and ±40 µA bias current. Next, the input is switched directly to the now precharged capacitor and allowed to fully settle. During this time the input sees only a 50 pF capacitor. Once the sample is taken, the input is internally floated so that the external input source sees a very high input resistance and a parasitic input capacitance of typically only 2 pF. As a result, the only domi- nant input characteristic which must be considered is the high current steps which occur when the internal buffers are switched in and out. In most cases, these characteristics require the use of an external op amp to drive the input of the AD677. Care should be taken with op amp selection; even with modest loading conditions, most available op amps do not meet the low distortion require- ments necessary to match the performance capabilities of the AD677. Figure 8 represents a circuit, based upon the AD845, which will provide excellent overall performance. For applications optimized more for low distortion and low noise, the AD845 of Figure 8 may be replaced by the AD743. +12V –12V AD845 0.1µF 0.1µF AGND AGND SENSE ±5V INPUT AD677 3 7 6 VIN 4 2 1k Ω 1k Ω 499 Ω Figure 8. |
Số phần tương tự - AD677 |
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Mô tả tương tự - AD677 |
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