Datasheet

AD8420 Data Sheet
Rev. 0 | Page 24 of 28
APPLICATIONS INFORMATION
AD8420 IN ELECTROCARDIOGRAPHY (ECG)
A high-pass filter is commonly used in ECG signal conditioning
circuitry to remove electrode offset and motion artifacts. To avoid
degrading the input impedance and CMRR of the system, this
filtering is typically implemented after the instrumentation
amplifier, which limits the gain that can be applied with the
instrumentation amplifier.
With a 3-op-amp instrumentation amplifier, gain is applied in the
first stage. Because of this, the electrode offset is gained and then
must be removed afterward with a high-pass filter. In the AD8420
architecture, the offset can be accounted for in the input stage
by unbalancing the transconductance amplifier at the REF and
FB pins. In the steady state, the offset at the input is not gained
to the output, and higher frequency signals can be gained and
passed through. Using the AD8420 in this way, the offset tolerance
is nearly the differential input range of the part (±1 V).
Figure 69 shows an ECG front end that applies a gain of 100 to
the signal while rejecting dc and high frequencies. This circuit
combines the AD8420 with the AD8657, which is a low power,
low cost, dual, precision CMOS op amp.
–5V
8200pF
10M
INSTRUMENTATION
AMPLIFIER
G = +100
AD8420
+5V
–5V
+5V
3.3F
AB
C
09945-072
402k
200pF
200pF
2000pF
100k
100k
+5V
–5V
AD8657-2AD8657-1
REF
FB
100k
1k
200k
200k
0.015F
THREE-POLE LPF,
BESSEL RESPONSE
F
C
= 50Hz
INTEGRATOR PROVIDES
HIGH-PASS POLE AT 0.5Hz
110k
0.022F
500k
Figure 69. AD8420 in an ECG Front End