Datasheet AD22057 (Analog Devices) - 5

AD22057
frequency will increase; it should be calculated using the parallel A three-pole filter (with roll-off 60 dB/decade) can be formed by sum of the resistor and 100 kΩ. adding a passive RC network at the output forming a real pole. A three-pole filter with a corner frequency f3 has the same
ANALOG
attenuation a one-pole filter of corner f
OUTPUT
1 has at a frequency √f 3 3 /f1, where the attenuation is 30 Log (f3/f1) (see the graph in Figure 9). Using equal capacitor values, and a resistor of
+IN OFS +VS OUT CORNER FREQUENCY = 1 2
p
C
3
100k
V
VDM AD22057
160 kΩ, the corner-frequency calibration remains 1 Hz-µF.
–IN GND A1 A2 THAT IS, 1.59Hz-
m
F FREQUENCY (C IS IN FARADS) ATTENUATION VCM C ANALOG –20dB/DECADE –60dB/DECADE COMMON
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 kΩ (for which the gain would be doubled) the corner frequency is now 0.796 Hz-µF, (0.039 µF for a 20 Hz corner).
A 1-POLE FILTER, CORNER f1, 30LOG (f3/f1) AND A 3-POLE FILTER, CORNER f3, HAVE THE SAME ATTENUATION, –30LOG (f 3/f ANALOG 3/f1), AT FREQUENCY (f3 1) OUTPUT f 3 +IN OFS +V 1 f3 (f3 /f1) S OUT V C DM AD22057
Figure 9. Comparative Responses of One- and Three-Pole
–IN GND A1 A2 CORNER FREQUENCY = 1Hz-
m
F
Low-Pass Filters
255k
V
V CURRENT SENSOR INTERFACE CM C ANALOG
A typical automotive application making use of the large
COMMON
common-mode range is shown in Figure 10. Figure 7. Connections for Conveniently Scaled, Two-Pole, Low-Pass Filter
+VS (BATTERY) +5V
A two-pole filter (with a roll-off of 40 dB/decade) can be imple-
SOLENOID LOAD
mented using the connections shown in Figure 7. This is a
FLYBACK ANALOG OUTPUT
Sallen & Key form based on a ×2 amplifier. It is useful to remem-
DIODE 4V PER AMP
ber that a two-pole filter with a corner frequency f2 and a
+IN OFS +VS OUT 191k
V one-pole filter with a corner at f
100m
V
AD22057
6
5% SENSOR
1 have the same attenuation at
CALIBRATION
the frequency (f 2
–IN GND A1 A2
2 /f1). The attenuation at that frequency is
20k
V 40 Log(f2/f1). This is illustrated in Figure 8. Using the standard
CORNER FREQUENCY
resistor value shown, and equal capacitors (in Figure 7), the
CMOS DRIVER = 0.796Hz-
m
F C (0.22
m
F FOR f = 3.6Hz)
corner frequency is conveniently scaled at 1 Hz-µF (0.05 µF for a 20 Hz corner). A maximally flat response occurs when the
POWER DARLINGTON ANALOG COMMON CHASSIS
resistor is lowered to 196 kΩ 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). Figure 10. Current Sensor Interface. Gain Is ×40, Single- Pole Low-Pass Filtering
FREQUENCY ATTENUATION
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
–40dB/DECADE
its base; the duty-cycle of the pulse determines the average current. This current is sensed in a small resistor. The aver-
–20dB/DECADE
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-
40LOG (f2/f1)
quency. The common-mode voltage, on the other hand, extends from roughly 1 V above ground, when the transistor is satu-
A 1-POLE FILTER, CORNER f1, AND A 2-POLE FILTER, CORNER f
rated, to about 1.5 V above the battery voltage, when the tran-
2, HAVE THE SAME ATTENUATION,
sistor is cut off and the diode conducts.
–40LOG (f 2 2/f1), AT FREQUENCY f2 /f1
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
f 2 1 f2 (f2 /f1)
measured using even a +5 V supply for the AD22057. Figure 8. Comparative Responses of One- and Two-Pole Low-Pass Filters REV. B –5–