Datasheet

AD8227
Rev. 0 | Page 21 of 24
INPUT BIAS CURRENT RETURN PATH
The other AD8227 terminals should be kept within the supplies.
All terminals of the AD8227 are protected against ESD.
The input bias current of the AD8227 must have a return path to
ground. When the source, such as a thermocouple, cannot provide
a return current path, one should be created, as shown in Figure 62.
For applications where the AD8227 encounters voltages beyond
the allowed limits, external current limiting resistors and low
leakage diode clamps such as the BAV199L, the FJH1100s, or
the SP720 should be used.
THERMOCOUPLE
+V
S
REF
–V
S
AD8227
CAPACITIVELY COUPLED
+V
S
REF
C
C
–V
S
AD8227
TRANSFORMER
+V
S
REF
–V
S
AD8227
INCORRECT
CAPACITIVELY COUPLED
+V
S
REF
C
R
C
R
–V
S
AD8227
1
f
HIGH-PASS
=
2πRC
THERMOCOUPLE
+V
S
REF
–V
S
10MΩ
AD8227
TRANSFORMER
+V
S
REF
–V
S
AD8227
CORRECT
RADIO FREQUENCY INTERFERENCE (RFI)
RF rectification is often a problem when amplifiers are used in
applications that have 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 instrumentation amplifier, as shown in Figure 63. The filter
limits the input signal bandwidth, according to the following
relationship:
07759-062
Figure 62. Creating an Input Bias Current Return Path
INPUT PROTECTION
The AD8227 has very robust inputs and typically does not need
additional input protection. Input voltages can be up to 40 V
from the opposite supply rail. For example, with a +5 V positive
supply and a −8 V negative supply, the part can safely withstand
voltages from −35 V to +32 V. Unlike some other instrumentation
amplifiers, the part can handle large differential input voltages
even when the part is in high gain. Figure 17 through Figure 20
show the behavior of the part under overvoltage conditions.
)2(π2
1
C
D
DIFF
CCR
uencyFilterFreq
+
=
C
CM
RC
uencyFilterFreq
π2
1
=
where C
D
≥ 10 C
C
.
R
R
AD8227
+V
S
+IN
–IN
0.1µF
10µF
10µF
0.1µF
REF
V
OUT
–V
S
R
G
C
D
10nF
C
C
1nF
C
C
1nF
4.02kΩ
4.02kΩ
07759-063
Figure 63. RFI Suppression
C
D
affects the differential signal and C
C
affects the common-
mode signal. Values of R and C
C
should be chosen to minimize
RFI. A mismatch between R × C
C
at the positive input and
R × C
C
at the negative input degrades the CMRR of the AD8227.
By using a value of C
D
one magnitude larger than C
C
, the effect
of the mismatch is reduced, and performance is improved.