Data Sheet

AD8209

Rev. C | Page 15 of 16

FREQUENCY

20dB/DECADE

40dB/DECADE

Figure 34. Comparative Responses of Single-Pole and Two-Pole Low-Pass Filters

HIGH LINE CURRENT SENSING

WITH LPF AND GAIN ADJUSTMENT

The circuit shown in Figure 35 is similar to Figure 24, but

includes gain adjustment and low-pass filtering.

GND

NC

–IN

+IN

A1

A2

OUT

AD8209

5V

INDUCTIVE

LOAD

SWITCH

SHUNT

CLAMP

DIODE

BATTERY

NC = NO CONNECT

C

OUTPUT

4V/AMP

5% CALIBRATION RANGE

(0.22µF FOR

NULL

133k

Ω

20k

Ω

+

–

Figure 35. High Line Current-Sensor Interface;

Gain = 40 V/V, Single-Pole, Low-Pass Filter

A power device that is either on or off controls the current in

the load. The average current is proportional to the duty cycle

of the input pulse and is sensed by a small-value resistor. The

average differential voltage across the shunt is typically 100 mV,

although its peak value is higher by an amount that depends on the

inductance of the load and the control frequency. The common-

mode voltage, on the other hand, extends from roughly 1 V above

ground for the on condition to about 1.5 V above the battery

voltage in the off condition. The conduction of the clamping

diode regulates the common-mode potential applied to the device.

For example, a battery spike of 20 V may result in an applied

common-mode potential of 21.5 V to the input of the devices.

To produce a full-scale output of 4 V, a gain of 40 V/V is used,

adjustable by ±5% to absorb the tolerance in the shunt. There is

sufficient headroom to allow 10% overrange (to 4.4 V). The

roughly triangular voltage across the sense resistor is averaged

by a single-pole, low-pass filter that is set with a corner frequency

of 3.6 Hz, which provides about 30 dB of attenuation at 100 Hz.

A higher rate of attenuation can be obtained by using a two-pole

filter with a corner frequency of 20 Hz, as shown in Figure 36.

Although this circuit uses two separate capacitors, the total capaci-

tance is less than half of what is needed for the single-pole filter.

GND

NC

–IN

+IN

A1

A2

OUT

AD8209

5V

INDUCTIVE

LOAD

SWITCH

SHUNT

CLAMP

DIODE

BATTERY

NC = NO CONNECT

C

OUTPUT

(0.05µF FOR

93k

Ω

432k

Ω

C

50k

Ω

+

–

Figure 36. Two-Pole Low-Pass Filter