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AD22050R 데이터시트(PDF) 5 Page - Analog Devices |
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AD22050R 데이터시트(HTML) 5 Page - Analog Devices |
5 / 8 page AD22050 –5– REV. C AD22050 +IN OFS +VS OUT –IN GND A1 A2 VDM VCM ANALOG OUTPUT ANALOG COMMON CORNER FREQUENCY = 1 2 C 100k THAT IS, 1.59Hz- F (C IS IN FARADS) C Figure 6. Connections for Single-Pole, Low-Pass Filter If the gain is raised using a resistor, as shown in Figure 5, the corner frequency is lowered by the same factor as the gain is raised. Thus, using a resistor of 200 k Ω (for which the gain would be doubled) the corner frequency is now 0.796 Hz- µF, (0.039 µF for a 20 Hz corner). AD22050 +IN OFS +VS OUT –IN GND A1 A2 VDM VCM ANALOG OUTPUT ANALOG COMMON CORNER FREQUENCY = 1Hz- F C C 255k Figure 7. Connections for Conveniently Scaled, Two-Pole, Low-Pass Filter A two-pole filter (with a roll-off of 40 dB/decade) can be imple- mented using the connections shown in Figure 7. This is a Sallen & Key form based on a ×2 amplifier. It is useful to remem- ber that a two-pole filter with a corner frequency f2 and a one-pole filter with a corner at f1 have the same attenuation at the frequency (f2 2/f 1). The attenuation at that frequency is 40 Log(f2/f1). This is illustrated in Figure 8. Using the standard resistor value shown, and equal capacitors (in Figure 7), the corner frequency is conveniently scaled at 1 Hz- µF (0.05 µF for a 20 Hz corner). A maximally flat response occurs when the resistor is lowered to 196 k Ω and the scaling is then 1.145 Hz- µF. The output offset is raised by about 4 mV (equivalent to 200 µV at the input pins). ATTENUATION f1 f2 40LOG (f2/f1) FREQUENCY –20dB/DECADE –40dB/DECADE (f22/f1) A 1-POLE FILTER, CORNER f1, AND A 2-POLE FILTER, CORNER f2, HAVE THE SAME ATTENUATION, –40LOG (f2/f1), AT FREQUENCY f22/f1 Figure 8. Comparative Responses of One- and Two-Pole Low-Pass Filters A three-pole filter (with roll-off 60 dB/decade) can be formed by adding a passive RC network at the output forming a real pole. A three-pole filter with a corner frequency f3 has the same attenuation a one-pole filter of corner f1 has at a frequency √f 3 3/f 1, where the attenuation is 30 Log (f3/f1) (see the graph in Figure 9). Using equal capacitor values, and a resistor of 160 k Ω, the corner-frequency calibration remains 1 Hz-µF. ATTENUATION f1 f3 30LOG (f3/f1) FREQUENCY –20dB/DECADE –60dB/DECADE (f3 3/f 1) –30LOG (f3/f1), AT FREQUENCY (f33/f1) A 1-POLE FILTER, CORNER f1, AND A 3-POLE FILTER, CORNER f3, HAVE THE SAME ATTENUATION, Figure 9. Comparative Responses of One- and Three-Pole Low-Pass Filters CURRENT SENSOR INTERFACE A typical automotive application making use of the large common-mode range is shown in Figure 10. AD22050 +IN OFS +VS OUT –IN GND A1 A2 100m SOLENOID LOAD POWER DARLINGTON CMOS DRIVER +VS (BATTERY) CHASSIS C 191k 20k +5V ANALOG OUTPUT 4V PER AMP 5% SENSOR CALIBRATION CORNER FREQUENCY = 0.796Hz- F (0.22 F FOR f = 3.6Hz) ANALOG COMMON FLYBACK DIODE Figure 10. Current Sensor Interface. Gain Is ×40, Single- Pole Low-Pass Filtering The current in a load, here shown as a solenoid, is controlled by a power transistor that is either cut off or saturated by a pulse at its base; the duty-cycle of the pulse determines the average current. This current is sensed in a small resistor. The aver- age differential voltage across this resistor is typically 100 mV, although its peak value will be higher by an amount that depends on the inductance of the load and the control fre- quency. The common-mode voltage, on the other hand, extends from roughly 1 V above ground, when the transistor is satu- rated, to about 1.5 V above the battery voltage, when the tran- sistor is cut off and the diode conducts. If the maximum battery voltage spikes up to +20 V, the common- mode voltage at the input can be as high as 21.5 V. This can be measured using even a +5 V supply for the AD22050. |
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