AD8229(Rev0) 데이터 시트보기 (PDF) - Analog Devices
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AD8229 Datasheet PDF : 24 Pages
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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 59.
INCORRECT
CORRECT
+VS
+VS
AD8229
REF
–VS
TRANSFORMER
+VS
AD8229
REF
–VS
TRANSFORMER
+VS
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
the AD8229.
RPROTECT
+
VIN+
I
–
+VS
AD8229
RPROTECT
+
VIN–
–
–VS
+VS
RPROTECT
+
VIN+
–
I
–VS
+VS
RPROTECT
33Ω
33Ω
+
VIN–
–
–VS
+VS
AD8229
–VS
SIMPLE METHOD
LOW NOISE METHOD
Figure 60. Protection for Voltages Beyond the Rails
Large Differential Input Voltage at High Gain
If large differential voltages at high gain are expected, an
external resistor should be used in series with each input to limit
current during overload conditions. The limiting resistor at each
input can be computed from
AD8229
REF
–VS
THERMOCOUPLE
AD8229
REF
10MΩ
–VS
THERMOCOUPLE
+VS
C
+VS
C
AD8229
fHIGH-PASS
=
1
2πRC
R
AD8229
C
REF
C
REF
RPROTECT
≥
1
2
|
VDIFF | −1V
I MAX
− RG
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.
RPROTECT
+
I
VDIFF
–
AD8229
RPROTECT
+I
VDIFF
–
AD8229
RPROTECT
RPROTECT
R
–VS
–VS
CAPACITIVELY COUPLED
CAPACITIVELY COUPLED
Figure 59. 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 of this data
sheet. If this cannot be done, protection circuitry can be added
in front of the AD8229 to limit the current into the inputs to a
maximum current, IMAX.
Input Voltages Beyond the Rails
If voltages beyond the rails are expected, an external resistor
should be used in series with each input to limit current during
overload conditions. The limiting resistor at the input can be
computed from
RPROTECT
≥ | VIN
− VSUPPLY
I MAX
|
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
SIMPLE METHOD
LOW NOISE METHOD
Figure 61. Protection for Large Differential Voltages
IMAX
The maximum current into the AD8229 inputs, IMAX, depends
both on 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, current should be
limited to 2 mA for the same period of time. 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 62. The filter
limits the input signal bandwidth, according to the following
relationship:
FilterFrequency DIFF
=
1
2πR(2C D
+CC )
Rev. 0 | Page 19 of 24
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