AD8202

Rev. D | Page 16 of 20

FREQUENCY

20dB/DECADE

40dB/DECADE

Figure 48. Comparative Responses of 1-Pole and 2-Pole Low-Pass Filters

HIGH LINE CURRENT SENSING WITH LPF AND

GAIN ADJUSTMENT

Figure 49 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 49. 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, conversely, 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 can 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,

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

Sufficient headroom allows 10% overrange (to 4.4 V). The

roughly triangular voltage across the sense resistor is averaged

by a 1-pole low-pass filter, set with a corner frequency of 3.6 Hz,

providing about 30 dB of attenuation at 100 Hz. A higher rate of

as shown in Figure 50. Although this circuit 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 50. 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 can persist

until after the sample period of the ADC expires due to the

relatively high open-loop output impedance (typically 21 kΩ)

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

combination, as shown in Figure 51.

Because 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

values shown in Figure 51, this time constant is programmed at

approximately 10 μs. Therefore, if samples are taken at several

tenths 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 51. Recommended Circuit for Driving CMOS A/D