AD22050 Analog Devices, Inc., AD22050 Datasheet - Page 5

• Manufacturer Part Number: AD22050
• Description: Single-supply Sensor Interface Amplifier
• Manufacturer: Analog Devices, Inc.

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 (for which the gain
would be doubled) the corner frequency is now 0.796 Hz- F,
(0.039 F for a 20 Hz corner).
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 f
one-pole filter with a corner at f
the frequency (f
40 Log(f
resistor value shown, and equal capacitors (in Figure 7), the
corner frequency is conveniently scaled at 1 Hz- F (0.05 F for
a 20 Hz corner). A maximally flat response occurs when the
resistor is lowered to 196 k and the scaling is then 1.145 Hz-
200 V at the input pins).
Figure 8. Comparative Responses of One- and Two-Pole
Low-Pass Filters
REV. C
F. The output offset is raised by about 4 mV (equivalent to
Figure 6. Connections for Single-Pole, Low-Pass Filter
V
V
ATTENUATION
DM
CM
40LOG (f
V
V
DM
CM
2
/f
1
+IN OFS +V
–IN GND A1
). This is illustrated in Figure 8. Using the standard
2
/f
AD22050
1
)
+IN OFS +V
–IN GND A1
FREQUENCY
2
AD22050
2
/f
A 1-POLE FILTER, CORNER f
AND A 2-POLE FILTER, CORNER f
HAVE THE SAME ATTENUATION,
–40LOG (f
1
). The attenuation at that frequency is
S
C
OUT
A2
S
C
–20dB/DECADE
OUT
2
f
A2
1
/f
1
), AT FREQUENCY f
1
have the same attenuation at
255k
CORNER FREQUENCY =
THAT IS, 1.59Hz- F
f
C
2
1
,
CORNER
FREQUENCY = 1Hz- F
2
2
–40dB/DECADE
2
,
/f
(C IS IN FARADS)
1
(f
2
2
/f
ANALOG
OUTPUT
ANALOG
COMMON
1
)
2
ANALOG
OUTPUT
ANALOG
COMMON
2 C
and a
1
100k
–5–
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 f
attenuation a one-pole filter of corner f
Figure 9). Using equal capacitor values, and a resistor of
160 k , the corner-frequency calibration remains 1 Hz- F.
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.
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.
+V
CMOS DRIVER
CHASSIS
f
3
ATTENUATION
S
3
(BATTERY)
/f
30LOG (f
1
, where the attenuation is 30 Log (f
FLYBACK
DIODE
3
/f
1
)
FREQUENCY
100m
–20dB/DECADE
A 1-POLE FILTER, CORNER f
AND A 3-POLE FILTER, CORNER f
HAVE THE SAME ATTENUATION,
–30LOG (f
SOLENOID
LOAD
POWER
DARLINGTON
3
f
/f
1
1
–IN GND A1 A2
), AT FREQUENCY
+IN OFS +V
AD22050
–60dB/DECADE
S
C
OUT
1
,
1
has at a frequency
3
(f
3
/f
,
3
3
3
1
/f
) (see the graph in
1)
has the same
AD22050
f
191k
20k
CORNER FREQUENCY
3
(0.22 F FOR f = 3.6Hz)
ANALOG COMMON
ANALOG OUTPUT
4V PER AMP
= 0.796Hz- F
CALIBRATION
5% SENSOR
(f
3
3
/f
+5V
1
)