AD8203YRMZ-R7 datasheet(16/20 Pages) AD

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AD8203

Rev. B | Page 16 of 20

40log (f2/f1)

f22/f1

FREQUENCY

A 1-POLE FILTER, CORNER f1, AND

A 2-POLE FILTER, CORNER f2, HAVE

THE SAME ATTENUATION –40log (f2/f1)

AT FREQUENCY f22/f1

20dB/DECADE

40dB/DECADE

Figure 49

by a 1-pole low-pass filter, shown in

, 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 with f

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

–IN

+IN

+VS

OUT

AD8203

INDUCTIVE

LOAD

POWER

DEVICE

4-TERM

SHUNT

CLAMP

DIODE

BATTERY

14V

NC = NO CONNECT

COMMON

OUT

4V/AMP

5% CALIBRATION RANGE

fC(Hz) = 0.767Hz/C(μF)

(0.22

μF FOR f

C = 3.6Hz)

VOS/IB

NULL

133k

20k

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 for 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

= 20 Hz, 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

–IN

+IN

+VS

OUT

AD8203

INDUCTIVE

LOAD

POWER

DEVICE

4-TERM

SHUNT

CLAMP

DIODE

BATTERY

14V

NC = NO CONNECT

COMMON

fC(Hz) = 1/C(μF)

(0.05

μF FOR f

C = 20Hz)

OUTPUT

93k

301k

50k

Figure 50. 2-Pole Low-Pass Filter

DRIVING CHARGE REDISTRIBUTION ADCS

When driving CMOS ADCs, such as those embedded in popu-

lar microcontrollers, the charge injection (ΔQ) can cause a

significant deflection in the output voltage of the AD8203.

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 (21 kΩ) of the

AD8203. Including an R-C network in the output can signifi-

cantly reduce the effect. The capacitor helps to absorb the

transient charge, effectively lowering the high frequency output

impedance of the AD8203. For these applications, the output

signal should be taken from the midpoint of the

RLAG to CLAG combination, as shown in Figure 51.

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

small, the output impedance of the AD8203 appears to be low. The

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

product of the two LAG components, CLAG × RLAG. For the values

shown in Figure 51, 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

AD8203

LAG

0.01

μF

MICROPROCESSOR

A/D

Figure 51. Recommended Circuit for Driving CMOS A/D

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