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,

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

The cutoff frequency is located at

C2

R2

C1

R1

f

C



2

10



resistors optimizes the filter for a maximally flat pass band. Reduce

its value to increase the Q and, consequently, the peaking of the

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.

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

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

of values for a low-pass filter, as explained in the Low-Pass Filtering and Gain section.

10kΩ

IAOUT

HPSENSE

HPDRIVE

S1

+IN

–IN

HPA

SW

10kΩ

S2

6

TO NEXT

STAGE

= REFOUT

19

3

1

20

2

C1

R1

R2

C2

10kΩ

IAOUT

HPSENSE

HPDRIVE

S1

+IN

–IN

HPA

SW

10kΩ

S2

6

TO NEXT

STAGE

= REFOUT

19

3

1

20

2

C1

C3

R1

R2

C2

R3

60

40

20

0

–20

–40

–60

0.01

100

10

1

0.1

FREQUENCY (Hz)

THREE-POLE FILTER

TWO-POLE FILTER

40dB PER

DECADE

40dB PER

DECADE

20dB PER

DECADE

60dB PER

DECADE