AD8232ACPZ-R7 datasheet(20/28 Pages) AD

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AD8232

Data Sheet

Rev. A | Page 20 of 28

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 AD8232 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 53) without the addition of

external elements.

Figure 52. 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 AD8232 is designed to be powered directly from a single

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

rechargeable lithium-ion batteries, but the designer must take

into account that the voltage during a charge cycle may exceed

the absolute maximum ratings of the AD8232. To avoid

damage to the part, use a power switch or a low power, low

dropout regulator, such as ADP150.

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 part. 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

should be used always with the DC blocking amplifier, shown as

HPA in Figure 45.

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 appear 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.

AD8232

IAOUT

+IN

–IN

10kΩ

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