Data Sheet
AD8229
Rev. B | Page 19 of 24
INPUT BIAS CURRENT RETURN PATH
The input bias current of t
he AD8229 must have a return path to
ground. When using a floating source without a current return
path, such as a thermocouple, a current return path should be
THERMOCOUPLE
+VS
REF
–VS
AD8229
CAPACITIVELY COUPLED
+VS
REF
C
–VS
AD8229
TRANSFORMER
+VS
REF
–VS
AD8229
INCORRECT
CAPACITIVELY COUPLED
+VS
REF
C
R
C
–VS
AD8229
1
fHIGH-PASS = 2πRC
THERMOCOUPLE
+VS
REF
–VS
10M
AD8229
TRANSFORMER
+VS
REF
–VS
AD8229
CORRECT
09412-
062
Figure 60. Creating an Input Bias Current Return Path
INPUT PROTECTION
The inputs to t
he AD8229 should be kept within the ratings
be done, protection circuitry can be added in front of th
e AD8229to limit the current into the inputs to a maximum current, IMAX.
Input Voltages Beyond the Rails
If voltages beyond the rails are expected, use an external resistor in
series with each input to limit current during overload conditions.
The limiting resistor at the input can be computed from
MAX
SUPPLY
IN
PROTECT
I
V
R
|
≥
Noise-sensitive applications may require a lower protection
resistance. Low leakage diode clamps, such as the BAV199, can be
used at the inputs to shunt current away from th
e AD8229 inputs
and therefore allow smaller protection resistor values. To ensure
current flows primarily through the external protection diodes,
place a small value resistor, such as a 33 Ω, between the diodes and
SIMPLE METHOD
LOW NOISE METHOD
+VS
AD8229
RPROTECT
–VS
I
VIN+
+
–
VIN–
+
–
09412-
066
+VS
AD8229
RPROTECT
33
RPROTECT
–VS
I
VIN+
+
–
VIN–
+
–
+VS
–VS
Figure 61. Protection for Voltages Beyond the Rails
Large Differential Input Voltage at High Gain
If large differential voltages at high gain are expected, use an
external resistor in series with each input to limit current during
overload conditions. The limiting resistor at each input can be
computed from
≥
G
MAX
DIFF
PROTECT
R
I
V
R
1
|
2
1
Noise-sensitive applications may require a lower protection
resistance. Low leakage diode clamps, such as the BAV199, can be
used across the inputs to shunt current away from th
e AD8229inputs and therefore allow smaller protection resistor values.
09412-
067
AD8229
RPROTECT
I
VDIFF
+
–
AD8229
RPROTECT
I
VDIFF
+
–
SIMPLE METHOD
LOW NOISE METHOD
Figure 62. Protection for Large Differential Voltages
IMAX
The maximum current into the AD8229 inputs, IMAX, depends on both time and temperature. At room temperature, the part
can withstand a current of 10 mA for at least a day. This time is
cumulative over the life of the part. At 210°C, limit current to
2 mA for the same period. The part can withstand 5 mA at
210°C for an hour, cumulative over the life of the part.
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:
)
2
(
π
2
1
C
D
DIFF
C
R
uency
FilterFreq
+
=
C
CM
RC
uency
FilterFreq
π
2
1
=
where CD ≥ 10 CC.