Data Sheet AD8208
Rev. C | Page 15 of 20
HIGH LINE CURRENT SENSING
WITH LPF AND GAIN ADJUSTMENT
The circuit shown in Figure 36 is similar to Figure 25, but
includes gain adjustment and low-pass filtering.
GND
NC
–IN
+IN
A1
V
S
A2
OUT
AD8208
INDUCTIVE
LOAD
SWITCH
SHUNT
CLAMP
DIODE
BATTERY
NC = NO CONNECT
08714-035
C
OUTPUT
4V/AMP
5% CALIBRATION RANGE
f
C
(Hz) = 0.767Hz/C(µF)
(0.22µF FOR
f
C
= 3.6Hz)
V
OS/IB
NULL
191kΩ
20kΩ
5V
+
–
Figure 36. 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 37.
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
V
S
A2
OUT
AD8208
INDUCTIVE
LOAD
SWITCH
SHUNT
CLAMP
DIODE
BATTERY
NC = NO CONNECT
08714-036
C
OUTPUT
f
C
(Hz) = 1/C(µF)
(0.05µF FOR
f
C
= 20Hz)
127kΩ
432kΩ
C
50kΩ
5V
+
–
Figure 37. Two-Pole Low-Pass Filter
