AD8228
Rev. 0 | Page 18 of 24
References
The output voltage of the AD8228 is developed with respect to
the potential on the reference terminal. Care should be taken to
tie REF to the appropriate local ground.
Input Bias Current Return Path
The input bias current of the AD8228 must have a return path
to common. When the source, such as a thermocouple, cannot
provide a return current path, one should be created, as shown
in
Figure 46.
THERMOCOUPLE
+V
S
REF
–V
S
AD8228
CAPACITIVELY COUPLED
+V
S
REF
C
C
–V
S
AD8228
TRANSFORMER
+V
S
REF
–V
S
AD8228
INCORRECT
CAPACITIVELY COUPLED
+V
S
REF
C
R
R
C
–V
S
AD8228
1
f
HIGH-PASS
=
2πRC
THERMOCOUPLE
+V
S
REF
–V
S
10MΩ
AD8228
TRANSFORMER
+V
S
REF
–V
S
AD8228
CORRECT
07035-009
Figure 46. Creating an I
BIAS
Path
INPUT PROTECTION
All terminals of the AD8228 are protected against ESD (1 kV,
human body model). In addition, the input structure allows for
dc overload conditions of about 3.5 V beyond the supplies.
Input Voltages Beyond the Rails
For larger input voltages, an external resistor should be used in
series with each input to limit current during overload conditions.
The AD8228 can safely handle a continuous 6 mA current. The
limiting resistor can be computed from
600
mA6
−
−
≥
SUPPLY
IN
LIMIT
VV
R
For applications where the AD8228 encounters extreme overload
voltages, such as cardiac defibrillators, external series resistors
and low leakage diode clamps such as the BAV199L, the FJH1100s,
or the SP720 should be used.
Large Differential Voltages When G = 100
When operating at a gain of 100, large differential input voltages
can cause more than 6 mA of current to flow into the inputs.
This condition occurs when the voltage between +IN and –IN
exceeds 5 V. This is true for differential voltages of either polarity.
The maximum allowed differential voltage can be increased by
adding an input protection resistor in series with each input.
The value of each protection resistor should be
R
PROTECT
= (V
DIFF_MAX
− 5 V)/6 mA
RADIO FREQUENCY INTERFERENCE (RFI)
RF rectification is often a problem when amplifiers are used in
applications having strong RF signals. The disturbance can appear
as a small dc offset voltage. High frequency signals can be filtered
with a low-pass RC network placed at the input of the instru-
mentation amplifier, as shown in
Figure 47. The filter limits the
input signal bandwidth, according to the following relationship:
FilterFrequency
DIFF
=
)2(π2
1
CD CCR +
FilterFrequency
CM
=
CRCπ2
1
where C
D
≥ 10 C
C
.
R
R
AD8228
+15V
+IN
–IN
0.1µF
10µF
10µF
0.1µF
REF
V
OUT
–15V
C
D
C
C
C
C
10nF
1nF
1nF
07035-010
4.02kΩ
4.02kΩ
Figure 47. RFI Suppression
C
D
affects the difference signal, and C
C
affects the common-mode
signal. Values of R and C
C
should be chosen to minimize RFI.
Mismatch between the R × C
C
at the positive input and the R × C
C
at the negative input degrades the CMRR of the AD8228. By using
a value of C
D
one magnitude larger than C
C
, the effect of the
mismatch is reduced, and performance is improved.
