AD8202

Rev. A | Page 11 of 12

FREQUENCY

20dB/DECADE

40dB/DECADE

Figure 21. Comparative Responses of 1-Pole and 2-Pole Low-Pass Filters

HIGH-LINE CURRENT SENSING WITH LPF AND

GAIN ADJUSTMENT

Figure 22 is another refinement of Figure 2, including gain

adjustment and low-pass filtering.

GND

NC

–IN

+IN

A1

A2

OUT

AD8202

5V

INDUCTIVE

LOAD

POWER

DEVICE

4-TERM

SHUNT

CLAMP

DIODE

BATTERY

14V

NC = NO CONNECT

COMMON

C

OUT

4V/AMP

5% CALIBRATION RANGE

(0.22

µF FOR f

NULL

191k

Ω

20k

Ω

Figure 22. High-Line Current Sensor Interface;

Gain = ×40, 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 ×40 is used, adjust-

able 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 1-pole, low-pass filter, here 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 using a 2-pole filter

uses two separate capacitors, the total capacitance is less than

half that needed for the 1-pole filter.

GND

NC

–IN

+IN

A1

A2

OUT

AD8202

5V

INDUCTIVE

LOAD

POWER

DEVICE

4-TERM

SHUNT

CLAMP

DIODE

BATTERY

14V

NC = NO CONNECT

COMMON

(0.05

µF FOR f

C

OUTPUT

127k

Ω

C

432k

Ω

50k

Ω

Figure 23. 2-Pole Low-Pass Filter

DRIVING CHARGE REDISTRIBUTION ADCS

When driving CMOS ADCs such as those embedded in popular

microcontrollers, the charge injection (ΔQ) can cause a

significant deflection in the output voltage of the AD8202.

Though generally of short duration, this deflection may persist

until after the sample period of the ADC has expired due to the

relatively high open-loop output impedance of the AD8202.

Including an R-C network in the output can significantly reduce

the effect. The capacitor helps to absorb the transient charge,

effectively lowering the high frequency output impedance of the

AD8202. For these applications, the output signal should be

shown in Figure 24.

Since the perturbations from the analog-to-digital converter are

small, the output impedance of the AD8202 appears to be low. The

transient response, therefore, has a time constant governed by the

shown in Figure 24, this time constant is programmed at approxi-

mately 10 µs. Therefore, if samples are taken at several tens of

microseconds or more, there is negligible charge stack-up.

+IN

–IN

10k

Ω

10k

Ω

AD8202

5V

R

LAG

1k

Ω

C

LAG

0.01

µF

MICROPROCESSOR

A/D

A2

2

4

6

5

Figure 24. Recommended Circuit for Driving CMOS A/D