Datasheet

AD8232 Data Sheet
Rev. A | Page 16 of 28
THEORY OF OPERATION
Figure 45. Simplified Schematic Diagram
ARCHITECTURE OVERVIEW
The AD8232 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 AD8232 includes leads
off detection circuitry and an automatic fast restore circuit that
brings back the signal shortly after leads are reconnected.
The AD8232 contains a specialized instrumentation amplifier
that amplifies the ECG signal while rejecting the electrode half-cell
potential on the same stage. This is 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 45 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 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
This 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.
10kΩ
IAOU
T
HPSENSEHPDRIVE
S1
GM1 GM2
99R
R
+V
S
0.7V
INSTRUMENTATION AMPLIFIER (IA)
+V – 0.05V
S
0.05V
REFOUTREFIN
–IN
+IN
FR
V
CM
C1
LOD+
LOD–
+V – 0.5V
S
SW OP
A
MP+ OPAMP–
OUT
RLD
RLDFB
GND
150kΩ
10kΩ
HPA
+V
S
= REFOUT
CHARGE
PUMP
SYNCH
RECTIFIER
SWITCH
TIMING
A3
A2
A1
AC/DC
AC/DC
AC/DC
AC/DC
SDN
S1
S2
S2
19
2
3
4
5
18
6 7 9
10
1 20
15
14
13
11
12
17
16
8
*ALL SWITCHES SHOWN IN DC LEADS-OFF DETECTION POSITION AND FAST RESTORE DISABLED
10866-045
RFI
FILTER