AD8224ACPZ-R7 datasheet(21/28 Pages) AD

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AD8224

Rev. 0 | Page 21 of 28

Power Supplies

A stable dc voltage should be used to power the instrumentation

amplifier. Noise on the supply pins can adversely affect

performance.

The AD8224 has two positive supply pins (Pin 5 and Pin 16)

and two negative supply pins (Pin 8 and Pin 13). While the part

functions with only one pin from each supply pair connected,

both pins should be connected for specified performance and

optimum reliability.

The AD8224 should be decoupled with 0.1 μF bypass capacitors,

one for each supply. Place the positive supply decoupling

capacitor near Pin 16, and the negative supply decoupling

capacitor near Pin 8. Each supply should also be decoupled with

a 10 μF tantalum capacitor. The tantalum capacitor can be

placed further away from the AD8224 and can generally be

shared by other precision integrated circuits. Figure 54 shows an

example layout.

AD8224

567

0.1µF

Figure 54. Example Layout

SOLDER WASH

The solder process can leave flux and other contaminants on

the board. When these contaminants are between the AD8224

leads and thermal pad, they can create leakage paths that are

larger than the AD8224 bias currents. A thorough washing

process removes these contaminants and restores the device’s

excellent bias current performance.

INPUT BIAS CURRENT RETURN PATH

The input bias current of the AD8224 must have a return path

to common. When the source, such as a transformer, cannot

provide a return current path, one should be created, as shown

in Figure 55.

INPUT PROTECTION

All terminals of the AD8224 are protected against ESD. ESD

protection is guaranteed to 4 kV (human body model). In

addition, the input structure allows for dc overload conditions

a diode drop above the positive supply and a diode drop below

the negative supply. Voltages beyond a diode drop of the

supplies cause the ESD diodes to conduct and enable current to

flow through the diode. Therefore, an external resistor should

be used in series with each of the inputs to limit current for

voltages above +Vs. In either scenario, the AD8224 safely

handles a continuous 6 mA current at room temperature.

For applications where the AD8224 encounters extreme

overload voltages, as in cardiac defibrillators, external series

resistors and low leakage diode clamps, such as BAV199L,

FJH1100, or SP720, should be used.

TRANSFORMER

+VS

REF

–VS

AD8224

INCORRECT

TRANSFORMER

+VS

REF

–VS

AD8224

CORRECT

CAPACITIVELY COUPLED

+VS

REF

–VS

AD8224

CAPACITIVELY COUPLED

+VS

REF

–VS

AD8224

fHIGH-PASS = 2πRC

Figure 55. Creating an IBIAS Path

RF INTERFERENCE

RF rectification is often a problem in applications where there are

large RF signals. The problem appears as a small dc offset voltage.

The AD8224 by its nature has a 5 pF gate capacitance (CG) at its

inputs. Matched series resistors form a natural low-pass filter that

reduces rectification at high frequency (see Figure 56).

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