Datasheet

Data Sheet AD8232
Rev. A | Page 21 of 28
APPLICATIONS INFORMATION
ELIMINATING ELECTRODE OFFSETS
The instrumentation amplifier in the AD8232 is designed to
apply gain and to filter out near dc signals simultaneously. This
capability allows it to amplify a small ECG signal by a factor of
100 yet reject electrode offsets as large as ±300 mV.
To achieve offset rejection, connect an RC network between the
output of the instrumentation amplifier, HPSENSE, and
HPDRIVE, as shown in Figure 53.
Figure 53. Eliminating Electrode Offsets
This RC network forms an integrator that feeds any near dc signals
back into the instrumentation amplifier, thus eliminating the offsets
without saturating any node and maintaining high signal gain.
In addition to blocking offsets present across the inputs of the
instrumentation amplifier, this integrator also works as a high-
pass filter that minimizes the effect of slow moving signals, such
as baseline wander. The cutoff frequency of the filter is given by
the equation
RC
f
dB
 2
100
3


where
R is in ohms and C is in farads.
Note that the filter cutoff is 100 times higher than is typically
expected from a single-pole filter. Because of the feedback
architecture of the instrumentation amplifier, the typical filter
cutoff equation is modified by the gain of 100 of the
instrumentation amplifier.
Figure 54. Frequency Response of Single-Pole DC Blocking Circuit
Just like with any high-pass filter with low frequency cutoff, any
fast change in dc offset takes a long time to settle. If such change
saturates the instrumentation amplifier output, the S1 switch
briefly enables the 10 kΩ resistor path, thus moving the cutoff
frequency to
)10(2
)10(100
4
4
3
RC
R
f
dB




(1)
For values of R greater than 100 kΩ, the expression in Equation 1
can be approximated by
C
f
dB
 200
1
3


This higher cutoff reduces the settling time and enables faster
recovery of the ECG signal. For more information, see the Fast
Restore Circuit section.
HIGH-PASS FILTERING
The AD8232 can implement higher order high-pass filters. A
higher filter order yields better artifact rejection but at a cost of
increased signal distortion and more passive components on the
printed circuit board (PCB).
Two-Pole High-Pass Filter
A two-pole architecture can be implemented by adding a simple
ac coupling RC at the output of the instrumentation amplifier,
as shown in Figure 55.
Figure 55. Schematic for a Two-Pole High-Pass Filter
Note that the right side of C2 connects to the SW terminal. Just
like S1, S2 reduces the recovery time for this ac coupling network
by placing 10 kΩ in parallel with R2. See the Fast Restore
Circuit section for additional details on switch timing and
trigger conditions.
Keep in mind that if this passive network is not buffered, it
exhibits higher output impedance at the input of a subsequent
low-pass filter, such as with Sallen-Key filter topologies. Careful
component selection can yield good results without a buffer. See
the Low-Pass Filtering and Gain section for additional
information on component selection.
10kΩ
IAOUTHPSENSEHPDRIVE
S1
GM1 GM2
99R
R
IN+
IN–
V
CM
H P A
ELECTRODE
OFFSETS
C
R
= REFOUT
19
3
1 20
2
C1
10866-253
50
40
10
20
30
0
0.01 1001010.1
MAGNITUDE (dB)
FREQUENCY (Hz)
10866-153
20dB PER
DECADE
10kΩ
IAOUTHPSENSEHPDRIVE
S1
+IN
–IN
HPA
SW
10kΩ
S2
6
REFOUT 8
TO NEXT
STAGE
= REFOUT
19
3
1 20
2
C1 C2
R1
R2
10866-053