Datasheet

Rev 03 Jan 2013 19:42 | Page 5
FILTER DESIGN
The resistor and capacitor values for the filters were selected to provide effective noise rejection in
applications that involve pulse detection while the subject is in motion. The filter parameters can be
adjusted to fit other applications.
High-Pass Filters
The instrumentation amplifier in the AD8232 applies gain and high pass filtering simultaneously. This
capability allows it to amplify a small ECG signal by 100 while rejecting electrode offsets as large as
±300 mV. The cutoff frequency of this filter is given by the following equation
f
c
= 100 / (2π R9 C6)
In this particular case, R9= 10MΩ and C6 = 0.22μF place the pole of the first high-pass at 7Hz. Note
that the filter cutoff is 100 times higher than would be typically expected, because of the feedback
architecture of the instrumentation amplifier.
Figure 4. Frequency response In-Amp filter (dc blocking feature)
An ac-coupling network (C9 and R11) at the output of the instrumentation amplifier introduces a
second pole. The cutoff frequency is that of a regular passive first-order high-pass filter
f
c
= 1 / (2π R11 C9)
This results in a 7Hz cutoff frequency for 0.22μF and 100kΩ. Both high-pass filters together yield a
total roll-off of 40dB per decade. Be aware that setting the same pole location for both high-pass
filters will result in 6dB attenuation at the corner frequency. In addition, because the output of this
filter is unbuffered, the instrumentation amplifier exhibits higher output impedance at the input of the