AD8232ACPZ-R7 datasheet(21/28 Pages) AD

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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



100





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

)

(

)

(

100









(1)

For values of R greater than 100 kΩ, the expression in Equation 1

can be approximated by



200





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

IAOUT

HPSENSE

HPDRIVE

GM1

GM2

99R

IN+

IN–

VCM

ELECTRODE

OFFSETS

= REFOUT

0.01

100

0.1

FREQUENCY (Hz)

20dB PER

DECADE

10kΩ

IAOUT

HPSENSE

HPDRIVE

+IN

–IN

HPA

10kΩ

REFOUT 8

TO NEXT

STAGE

= REFOUT

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