Datasheet

AD8232 Data Sheet
Rev. A | Page 22 of 28
Additional High-Pass Filtering Options
In addition to the topologies explained in the previous sections,
an additional pole may be added to the dc blocking circuit for
additional rejection of low frequency signals. This configuration
is shown in Figure 56.
Figure 56. Schematic for an Alternative Two-Pole High-Pass Filter
An extra benefit of this circuit topology is that it allows lower
cutoff frequency with lower R and C values and the resistor,
R
COMP
, can be used to control the Q of the filter to achieve narrow
band-pass filters (for heart rate detection) or maximum pass-
band flatness (for cardiac monitoring).
With this topology, the filter attenuation reverts to a single pole
roll off at very low frequencies. Because the initial roll off was 40 dB
per decade, this reversion to 20 dB per decade has little impact on
the ability of the filter to reject out-of-band low frequency signals.
The designer may choose different values to achieve the desired
filter performance. To simplify the design process, use the following
recommendations as a starting point for component value selection.
R1 = R2 ≥ 100 kΩ
C1 = C2
R
COMP
= 0.14 × R1
The cutoff frequency is located at
C2R2C1R1
f
C
 2
10

The selection of R
COMP
to be 0.14 times the value of the other two
resistors optimizes the filter for a maximally flat pass band. Reduce
its value to increase the Q and, consequently, the peaking of the
filter. Keep in mind that a very low value of R
COMP
can result in
an unstable circuit. The selection of values based on these criteria
result in a transfer function similar to the one shown in Figure 58.
When additional low frequency rejection is desired, a high-order
high-pass filter can be implemented by adding an ac coupling
network at the output of the instrumentation amplifier, as shown in
Figure 57. The SW terminal is connected to the ac coupling network
to obtain the best settling time response when fast restore engages.
Figure 57. Schematic for a Three-Pole High-Pass Filter
Figure 58. Frequency Response of Circuits in Figure 56 and Figure 57
Careful analysis and adjustment of all of the component values
in practice is recommended to optimize the filter characteristics.
A useful hint is to reduce the value of R
COMP
to increase the peaking
of the active filter to overcome the additional roll off introduced
by the ac coupling network. Proper adjustment can yield the
best pass-band flatness.
The design of the high-pass filter involves tradeoffs between signal
distortion, component count, low frequency rejection, and
component sizes. For example, a single-pole high-pass filter
results in the least distortion to the signal, but its rejection of
low-frequency artifacts is the lowest Table 4 compares the
recommended filtering options.
Table 4. Comparison of High-Pass Filtering Options
Filter Order Component Count Low Frequency Rejection Capacitor Sizes/Values Signal Distortion
1
Output Impedance
2
Figure 53 1 2 Good Large Low Low
Figure 55 2 4 Better Large Medium Higher
Figure 56 2 5 Better Smaller Medium Low
Figure 57 3 7 Best Smaller Highest Higher
1
For equivalent corner frequency location.
2
Output impedance refers to the drive capability of the high-pass filter before the low-pass filter. Low output impedance is desirable to allow flexibility in the selection
of values for a low-pass filter, as explained in the Low-Pass Filtering and Gain section.
10kΩ
IAOUTHPSENSEHPDRIVE
S1
+IN
–IN
HPA
SW
10kΩ
S2
6
REFOUT 8
TO NEXT
STAGE
= REFOUT
19
3
1 20
2
C1
R1 R2
R
COMP
C2
10866-155
10kΩ
IAOUTHPSENSEHPDRIVE
S1
+IN
–IN
HPA
SW
10kΩ
S2
6
REFOUT 8
TO NEXT
STAGE
= REFOUT
19
3
1 20
2
C1 C3
R1 R2
R
COMP
C2 R3
10866-156
60
40
20
0
–20
–40
–60
0.01 1001010.1
MAGNITUDE (dB)
FREQUENCY (Hz)
10866-157
THREE-POLE FILTER
TWO-POLE FILTER
40dB PER
DECADE
40dB PER
DECADE
20dB PER
DECADE
60dB PER
DECADE