Data Sheet
AD8229
place a small value resistor, such as a 33 Ω, between the diodes and
the AD8229.
INPUT BIAS CURRENT RETURN PATH
The input bias current of the 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
created, as shown in Figure 60.
+V
S
+V
+V
S
S
R
R
PROTECT
33Ω
PROTECT
I
I
+
IN+
–
+
IN+
–
V
V
–V
+V
S
AD8229
AD8229
S
INCORRECT
+V
CORRECT
+V
R
R
PROTECT
33Ω
PROTECT
S
S
+
IN–
–
+
IN–
–
–V
S
–V
S
V
V
–V
S
SIMPLE METHOD
LOW NOISE METHOD
AD8229
AD8229
Figure 61. Protection for Voltages Beyond the Rails
REF
REF
REF
REF
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
–V
–V
S
S
TRANSFORMER
TRANSFORMER
+V
S
+V
S
|VDIFF | −1V
1
2
RPROTECT
≥
− RG
IMAX
AD8229
AD8229
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 the AD8229
inputs and therefore allow smaller protection resistor values.
REF
10MΩ
–V
–V
S
S
THERMOCOUPLE
THERMOCOUPLE
R
R
PROTECT
PROTECT
I
I
+
+
+V
+V
S
S
V
V
DIFF
–
DIFF
–
AD8229
AD8229
C
C
C
R
PROTECT
R
PROTECT
R
R
1
fHIGH-PASS
=
AD8229
2πRC
AD8229
SIMPLE METHOD
LOW NOISE METHOD
C
REF
Figure 62. Protection for Large Differential Voltages
IMAX
–V
–V
S
S
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.
CAPACITIVELY COUPLED
CAPACITIVELY COUPLED
Figure 60. Creating an Input Bias Current Return Path
INPUT PROTECTION
The inputs to the AD8229 should be kept within the ratings
stated in the Absolute Maximum Ratings section. If this cannot
be done, protection circuitry can be added in front of the AD8229
RADIO FREQUENCY INTERFERENCE (RFI)
to limit the current into the inputs to a maximum current, IMAX
.
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:
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
|VIN −VSUPPLY
|
RPROTECT
≥
IMAX
1
FilterFrequencyDIFF
FilterFrequencyCM
=
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 the AD8229 inputs
and therefore allow smaller protection resistor values. To ensure
current flows primarily through the external protection diodes,
2πR(2CD +CC )
1
=
2πRCC
where CD ≥ 10 CC.
Rev. B | Page 19 of 24
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