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AD1672 bảng dữ liệu(PDF) 9 Page - Analog Devices |
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AD1672 bảng dữ liệu(HTML) 9 Page - Analog Devices |
9 / 20 page AD1672 REV. 0 –9– APPLYING THE AD1672 ANALOG INPUTS Figure 9 shows the equivalent analog input of the AD1672. The input SHA and associated resistor network topology can be eas- ily configured for either unipolar (0 V to 2.5 V, 0 V to 5.0 V) or bipolar (–2.5 V to 2.5 V) input signals as shown in Figure 10. The nominal input resistance, RIN, of the AD1672 is 2 k Ω for a 2.5 V span and 4 k Ω for a 5 V span. The circuit topology both level shifts and inverts the analog input for the various input spans. SHAOUT AIN 1 4k Ω 2k Ω VBIAS 1.25V 4k Ω AIN 2 2k Ω Figure 9. Equivalent Analog Input Circuit 21 22 20 23 1µF AIN1 AIN2 REFOUT REFIN AD1672 VIN 21 22 20 23 1µF AIN1 AIN2 REFOUT REFIN AD1672 VIN 21 22 20 23 1µF AIN1 AIN2 REFOUT REFIN AD1672 VIN a. 0 to +2.5 V Input Range b. 0 to +5.0 V Input Range c. –2.5 to +2.5 V Input Range Figure 10. Input Range Connections In applications where ac coupling of the analog input signal is appropriate such as in a single supply system, the user can capacitively couple the input signal for a 2.5 V or 5 V span thus removing any preceding system dc offsets. Figure 11 shows the proper configurations of the AD1672 for ac coupling. Main- taining the specifications outlined in the data sheet requires care- ful selection of the component values. The most important concern is that the f –3 dB high pass corner is a function of C1 and C2 in parallel with RIN. The f -3 dB point can be approximated by the equation /f –3 dB = 1/ (2 ×π × RIN × CEQ ) where CEQ is the parallel combination of C1 and C2. Note that C1 is typically a large electrolytic or tantalum capacitor that be- comes inductive at high frequencies. Adding a small ceramic capacitor on the order of 0.1 µF that does not become inductive until negligibly higher frequencies maintains a low impedance over a wide frequency range. 21 22 25 20 23 C1 10µF C2 1.0µF 10µF 1.0µF ANALOG INPUT AIN1 AIN2 NCOMP2 REF OUT REFIN AD1672 21 22 25 20 23 C1 10µF C2 1.0µF 10µF 1.0µF ANALOG INPUT AIN1 AIN2 NCOMP2 REF OUT REFIN AD1672 2.5V Span 5.0V Span Figure 11. AC Coupled Inputs In applications requiring dc coupling, a buffer amplifier is rec- ommended for driving the AD1672 input. Any source resis- tance will contribute to both gain and offset error due to its interaction with the AD1672’s input resistance. The particular application and signal input range will determine how the buffer amplifier is configured. For example, in dc precision applica- tions, the buffer amplifier can be configured for convenient gain and offset adjustment as shown in Figure 12. In spectral analysis/signal processing applications, the buffer amplifier can be configured as a 2nd order antialiasing filter in a Sallen-Key or Multiple-Feedback topology as shown in Figure 13. VCC 500 Ω 50 Ω 500 Ω 5k Ω 50 Ω 5k Ω 0.1µF VEE ANALOG INPUT OFFSET ADJUSTMENTS GAIN ADJUSTMENT AD1672 AIN Figure 12. Offset and Gain Adjustment VIN VOUT C2 R4 C1 R2 R1 R3 VIN VOUT C2 C1 R2 R1 R3 Figure 13. Sallen-Key and Multiple-Feedback Antialiasing Filter Topologies In imaging and multiplexed data acquisition applications, the AD1672’s wide input bandwidth facilitates rapid acquisition of transient input signals: the input SHA can typically settle to 12- bit accuracy from a full scale input step in less than 150 ns. Fig- ure 14 illustrates the typical acquisition of a full scale input step. For amplifiers that are powered by supplies greater than 6.5 V, it is recommended that a clamping circuit be included at the input of AD1672. This circuit limits the input voltage to 6.5 V under a fault condition. |
Số phần tương tự - AD1672 |
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Mô tả tương tự - AD1672 |
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