REV. B

–8–

AD8200

HIGH-LINE CURRENT SENSING WITH LPF AND GAIN

ADJUSTMENT

Figure 11 is another refinement of Figure 1, including gain

adjustment and low-pass filtering.

5V

OUTPUT

4V/AMP

INDUCTIVE

LOAD

POWER

DEVICE

4 TERM

SHUNT

CLAMP

DIODE

BATTERY

14V

COMMON

C

NULL

191k

20k

5% CALIBRATION RANGE

(0.22 F FOR f = 3.6 Hz)

NC = NO CONNECT

GND

NC

–IN

+IN

A1

A2

OUT

AD8200

Figure 11. 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 will be 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, when the switch is ON, to about 1.5 V

above the battery voltage, when the device is OFF, and the

clamp 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.

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 =

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.

5V

OUTPUT

INDUCTIVE

LOAD

POWER

DEVICE

4 TERM

SHUNT

CLAMP

DIODE

BATTERY

14V

COMMON

C

127k

432k

50k

C

NC = NO CONNECT

GND

NC

–IN

+IN

A1

A2

OUT

AD8200

Figure 12. Illustration of 2-Pole Low-Pass Filtering

DRIVING CHARGE REDISTRIBUTION A/D

CONVERTERS

When driving CMOS ADCs, such as those embedded in

popular microcontrollers, the charge injection ( Q) can cause

a significant deflection in the output voltage of the AD8200.

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 AD8200.

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

AD8200. For these applications, the output signal should be

shown in Figure 13.

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

small, the output impedance of the AD8200 will appear to be

low. The transient response will, therefore, have a time constant

×

programmed at approximately 10

µs. Therefore, if samples are

taken at several tens of microseconds or more, there will be

negligible charge “stack-up.”

+IN

–IN

10k

10k

AD8200

5V

1k

0.01 F

MICROPROCESSOR

A/D

A2

Figure 13. Recommended Circuit for Driving CMOS A /D