AD8224_16 Datasheet, PDF (25/29 Pages) Analog Devices – Precision, Dual-Channel, JFET Input, Rail-to-Rail Instrumentation Amplifier

AD8224
For applications where the AD8224 encounters extreme
overload voltages, as in cardiac defibrillators, external series
resistors and low leakage diode clamps, such as BAV199Ls,
FJH1100s, or SP720s, should be used.
INCORRECT
+V_S
CORRECT
+V_S
AD8224
REF
AD8224
REF
–V_S
TRANSFORMER
+V_S
C
–V_S
TRANSFORMER
+V_S
C
AD8224
f_HIGH-PASS
=
1
2πRC
R
AD8224
C
REF
C
REF
R
–V_S
CAPACITIVELY COUPLED
–V_S
CAPACITIVELY COUPLED
Figure 60. Creating an IBIAS Path
RF INTERFERENCE
RF rectification is often a problem in applications where there are
large RF signals. The problem appears as a small dc offset voltage.
The AD8224 by its nature has a 5 pF gate capacitance (CG) at its
inputs. Matched series resistors form a natural low-pass filter that
reduces rectification at high frequency (see Figure 61).
+15V
0.1µF
10µF ⁺
R
+IN
C_G
–V_S ^AD8224
R
C_G
–IN
–V_S
REF
V_OUT
0.1µF
10µF ₊
–15V
Figure 61. RFI Filtering Without External Capacitors
Data Sheet
The relationship between external, matched series resistors and
the internal gate capacitance is expressed as
FilterFreq_DIFF

1
2RC_G
FilterFreq_CM

1
2RC_G
To eliminate high frequency common-mode signals while using
smaller source resistors, a low-pass RC network can be placed at
the input of the instrumentation amplifier (see Figure 62). The
filter limits the input signal bandwidth according to the
following relationship:
FilterFreq_DIFF

1
2R(2 CD  CC
 C_G )
FilterFreq_CM

1
2R(CC
 C_G )
Mismatched CC capacitors result in mismatched low-pass filters.
The imbalance causes the AD8224 to treat what would have
been a common-mode signal as a differential signal. To reduce
the effect of mismatched external CC capacitors, select a value of
CD greater than 10 times CC. This sets the differential filter
frequency lower than the common-mode frequency.
+15V
0.1µF
10µF ⁺
C_C
R
4.02kΩ
C_D
R
4.02kΩ
C_C
1nF
10nF
1nF
+IN
AD8224
V_OUT
REF
–IN
0.1µF
10µF
+
–15V
Figure 62. RFI Suppression
COMMON-MODE INPUT VOLTAGE RANGE
The 3-op amp architecture of the AD8224 applies gain and then
removes the common-mode voltage. Therefore, internal nodes
in the AD8224 experience a combination of both the gained
signal and the common-mode signal. This combined signal can
be limited by the voltage supplies even when the individual input
and output signals are not. Figure 25 through Figure 28 show the
allowable common-mode input voltage ranges for various
output voltages, supply voltages, and gains.
Rev. D | Page 24 of 28