AD22057

–5–

REV. A

frequency will increase; it should be calculated using the parallel

sum of the resistor and 100 k

Ω.

AD22057

–IN GND

A1

A2

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).

AD22057

–IN GND A1

A2

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-

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

FREQUENCY

–20dB/DECADE

–40dB/DECADE

HAVE THE SAME ATTENUATION,

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.

√f

3

Figure 9). Using equal capacitor values, and a resistor of

160 k

Ω, the corner-frequency calibration remains 1 Hz-µF.

ATTENUATION

FREQUENCY

–20dB/DECADE

–60dB/DECADE

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.

AD22057

–IN GND A1

A2

100m

SOLENOID

LOAD

POWER

DARLINGTON

CMOS DRIVER

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 AD22057.