Data Sheet

AD8233

Rev. 0 | Page 17 of 29

THEORY OF OPERATION

10kΩ

IAOUT

HPSENSE

HPDRIVE

S1

GM1

GM2

99R

R

0.7V

INSTRUMENTATION AMPLIFIER (IA)

0.1V

REFOUT

REFIN

–IN

+IN

FR

C1

RLD SDN

LOD

SW

OPAMP+

OPAMP–

OUT

RLD

RLDFB

GND

150kΩ

10kΩ

HPA

= REFOUT

CHARGE

PUMP

SYNC

RECTIFIER

SWITCH

TIMING

A3

A2

A1

AC/DC

AC/DC

AC/DC

AC/DC

SDN

S1

S2

S2

B4

C5

B5

C4

D5

A3

D4

D3

D2

D1

A5

A4

B3

B2

B1

C1

C2

A2

A1

C3

*ALL SWITCHES SHOWN IN DC LEADS OFF DETECTION POSITION AND FAST RESTORE DISABLED

RFI

FILTER

Figure 50. Simplified Schematic Diagram

ARCHITECTURE OVERVIEW

The AD8233 is an integrated front end for signal conditioning

of cardiac biopotentials for heart rate monitoring. It consists of

a specialized instrumentation amplifier (IA), an operational

amplifier (A1), a right leg drive amplifier (A2), and a midsupply

reference buffer (A3). In addition, the AD8233 includes leads

on/off detection circuitry and an automatic fast restore circuit

that restores the signal shortly after leads are reconnected.

The AD8233 contains a specialized instrumentation amplifier

that amplifies the ECG signal while rejecting the electrode half

cell potential on the same stage. The amplification of the ECG

signal and the rejection of the electrode half cell potential are

possible with an indirect current feedback architecture, which

reduces size and power compared with traditional

implementations.

INSTRUMENTATION AMPLIFIER

The instrumentation amplifier is shown in Figure 50 as

comprised by two well matched transconductance amplifiers

(GM1 and GM2), the dc blocking amplifier (HPA), and an

integrator formed by C1 and an op amp. The transconductance

amplifier, GM1, generates a current that is proportional to the

voltage present at its inputs. When the feedback is satisfied, an

equal voltage appears across the inputs of the transconductance

amplifier, GM2, thereby matching the current generated by

GM1. The difference generates an error current that is integrated

across Capacitor C1. The resulting voltage appears at the output

of the instrumentation amplifier.

The feedback of the amplifier is applied via GM2 through two

separate paths: the two resistors divide the output signal to set

an overall gain of 100, whereas the dc blocking amplifier integrates

any deviation from the reference level. Consequently, dc offsets

as large as ±300 mV across the GM1 inputs appear inverted and

with the same magnitude across the inputs of GM2, all without

saturating the signal of interest.

To increase the common-mode voltage range of the instrumen-

tation amplifier, a charge pump boosts the supply voltage for the

two transconductance amplifiers. This boost in supply voltage

further prevents saturation of the amplifier in the presence of

large common-mode signals, such as line interference. The charge

pump runs from an internal oscillator, the frequency of which is

set around 500 kHz.

OPERATIONAL AMPLIFIER

The general-purpose operational amplifier (A1) is a rail-to-rail

device that can be used for low-pass filtering and to add additional

gain. The following sections provide details and example circuits

that use this amplifier.