AD8232ACPZ Datasheet, PDF (20/28 Pages) Analog Devices – Single-Lead, Heart Rate Monitor Front End

AD8232
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.
+IN ^10kΩ
C_G
–IN ^{10kΩ AD8232}
C_G
IAOUT
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.
Data Sheet
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.
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