AD8233ACBZ-R7 datasheet(22/30 Pages) AD

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Data Sheet

AD8233

Rev. 0 | Page 21 of 29

INPUT PROTECTION

All terminals of the AD8233 are protected against ESD. In

addition, the input structure allows dc overload conditions that

are 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, use an external resistor in

series with each of the inputs to limit current for voltages

beyond the supplies. In either scenario, the AD8233 safely

handles a continuous 5 mA current at room temperature.

For applications where the AD8233 encounters extreme over-

load voltages, such as in cardiac defibrillators, use external series

resistors and gas discharge tubes (GDT). Neon lamps are com-

monly used as an inexpensive alternative to GDTs. These devices

can handle the application of large voltages but do not maintain

the voltage below the absolute maximum ratings for the AD8233.

A complete solution includes further clamping to either supply

using additional resistors and low leakage diode clamps, such as

BAV199 or FJH1100.

As a safety measure, place a resistor between the input pin and

the electrode that is connected to the subject to ensure that the

current flow never exceeds 10 μA. Calculate the value of this

resistor to be equal to the supply voltage across the AD8233

divided by 10 μA.

RADIO FREQUENCY INTERFERENCE (RFI)

Radio frequency (RF) rectification is often a problem in

applications where there are large RF signals. The problem

appears as a dc offset voltage at the output. The AD8233 has a

15 pF gate capacitance and 10 kΩ resistors at each input. This

forms a low-pass filter on each input that reduces rectification

at high frequency (see Figure 60) without the addition of

external elements.

AD8233

IAOUT

+IN

–IN

10kΩ

Figure 60. RFI Filter Without External Capacitors

For increased filtering, additional resistors can be added in

series with each input. They must be placed as close as possible

to the instrumentation amplifier inputs. These can be the same

resistors used for overload and patient protection.

POWER SUPPLY REGULATION AND BYPASSING

The AD8233 is designed to be powered directly from a single

3 V battery, such as CR2032 type. It can also operate from

rechargeable Li-Ion batteries, but the designer must take into

account that the voltage during a charge cycle may exceed the

absolute maximum ratings of the AD8233. To avoid damage to

the device, use a power switch or a low power, low dropout

regulator, such as the ADP150 or ADP160.

In addition, excessive noise on the supply pins can adversely

affect performance. As in all linear circuits, bypass capacitors

must be used to decouple the chip power supplies. Place a 0.1 μF

capacitor close to the supply pin. A 1 μF capacitor can be used

farther away from the device. In most cases, the capacitor can

be shared by other integrated circuits. Keep in mind that excessive

decoupling capacitance increases power dissipation during

power cycling.

INPUT REFERRED OFFSETS

Because of its internal architecture, the instrumentation amplifier

must be used always with the dc blocking amplifier, shown as HPA

in Figure 50.

As described in the Theory of Operation section, the dc blocking

amplifier attenuates the input referred offsets present at the

inputs of the instrumentation amplifier; however, this is true

only when the dc blocking amplifier is used as an integrator. In

this configuration, the input offsets from the dc blocking

amplifier dominate appearing directly at the output of the

instrumentation amplifier.

If the dc blocking amplifier is used as a follower instead of its

intended function as an integrator, the input referred offsets of

the in-amp are amplified by a factor of 100.

LAYOUT RECOMMENDATIONS

It is important to follow good layout practices to optimize

system performance. In low power applications, most resistors

are of a high value to minimize additional supply current. The

challenge of using high value resistors is that high impedance

nodes become even more susceptible to noise pickup and board

parasitics, such as capacitance and surface leakages. Keep all of

the connections between high impedance nodes as short as

possible to avoid introducing additional noise and errors from

corrupting the signal.

To maintain high CMRR over frequency, keep the input traces

symmetrical and length matched. Place safety and input bias

resistors in the same position relative to each input. In addition,

the use of a ground plane significantly improves the noise

rejection of the system.

For WLCSP layout best practices, refer to the AN-617

Application Note.

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