AD8224 Analog Devices, AD8224 Datasheet - Page 21

• Manufacturer Part Number: AD8224
• Description: Precision, Dual-Channel, JFET Input Rail-to-Rail Instrumentation Amplifier
• Manufacturer: Analog Devices

Power Supplies
A stable dc voltage should be used to power the instrumentation
amplifier. Noise on the supply pins can adversely affect
performance.
The AD8224 has two positive supply pins (Pin 5 and Pin 16)
and two negative supply pins (Pin 8 and Pin 13). While the part
functions with only one pin from each supply pair connected,
both pins should be connected for specified performance and
optimum reliability.
The AD8224 should be decoupled with 0.1 μF bypass capacitors,
one for each supply. Place the positive supply decoupling
capacitor near Pin 16, and the negative supply decoupling
capacitor near Pin 8. Each supply should also be decoupled with
a 10 μF tantalum capacitor. The tantalum capacitor can be
placed further away from the AD8224 and can generally be
shared by other precision integrated circuits. Figure 54 shows an
example layout.
SOLDER WASH
The solder process can leave flux and other contaminants on
the board. When these contaminants are between the AD8224
leads and thermal pad, they can create leakage paths that are
larger than the AD8224 bias currents. A thorough washing
process removes these contaminants and restores the device’s
excellent bias current performance.
R
G
0.1µF
1
2
3
4
AD8224
Figure 54. Example Layout
16
5
15
6
14
7
0.1µF
13
8
12
11
10
9
R
G
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INPUT BIAS CURRENT RETURN PATH
The input bias current of the AD8224 must have a return path
to common. When the source, such as a transformer, cannot
provide a return current path, one should be created, as shown
in Figure 55.
INPUT PROTECTION
All terminals of the AD8224 are protected against ESD. ESD
protection is guaranteed to 4 kV (human body model). In
addition, the input structure allows for dc overload conditions
a diode drop above the positive supply and a diode drop below
the negative supply. Voltages beyond a diode drop of the
supplies cause the ESD diodes to conduct and enable current to
flow through the diode. Therefore, an external resistor should
be used in series with each of the inputs to limit current for
voltages above +Vs. In either scenario, the AD8224 safely
handles a continuous 6 mA current at room temperature.
For applications where the AD8224 encounters extreme
overload voltages, as in cardiac defibrillators, external series
resistors and low leakage diode clamps, such as BAV199L,
FJH1100, or SP720, should be used.
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 (C
inputs. Matched series resistors form a natural low-pass filter that
reduces rectification at high frequency (see Figure 56).
CAPACITIVELY COUPLED
C
C
TRANSFORMER
INCORRECT
AD8224
+V
–V
AD8224
+V
–V
S
S
S
S
REF
Figure 55. Creating an I
REF
f
HIGH-PASS
=
CAPACITIVELY COUPLED
2πRC
1
BIAS
C
C
Path
R
R
TRANSFORMER
CORRECT
AD8224
AD8224
+V
–V
+V
–V
AD8224
S
S
S
S
G
) at its
REF
REF