ad8429 Analog Devices, Inc., ad8429 Datasheet - Page 17

• Manufacturer Part Number: ad8429
• Description: 1 Nv/hz Low Noise Instrumentation Amplifier Ad8429
• Manufacturer: Analog Devices, Inc.

INPUT BIAS CURRENT RETURN PATH
The input bias current of the AD8429 must have a return path to
ground. When using a floating source without a current return
path, such as a thermocouple, create a current return path, as
shown in Figure 50.
INPUT PROTECTION
Do not allow the inputs of the AD8429 to exceed 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 AD8429 to limit the current into the inputs to a
maximum current, I
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 condi-
tions. The limiting resistor at the input can be computed from
CAPACITIVELY COUPLED
C
C
R
THERMOCOUPLE
TRANSFORMER
PROTECT
INCORRECT
Figure 50. Creating an Input Bias Current Return Path
AD8429
+V
AD8429
–V
AD8429
+V
–V
+V
–V
≥
S
S
S
S
|
S
S
V
IN
REF
−
I
REF
MAX
REF
MAX
V
SUPPLY
.
f
HIGH-PASS
|
=
CAPACITIVELY COUPLED
2πRC
1
10MΩ
C
C
R
R
THERMOCOUPLE
TRANSFORMER
CORRECT
AD8429
AD8429
AD8429
+V
–V
+V
–V
+V
–V
S
S
S
S
S
S
REF
REF
REF
Rev. 0 | Page 17 of 20
Noise sensitive applications may require a lower protection resis-
tance. Low leakage diode clamps, such as the BAV199, can be used
at the inputs to shunt current away from the AD8429 inputs,
thereby allowing 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 AD8429.
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 by using the following equation:
Noise sensitive applications may require a lower protection resis-
tance. Low leakage diode clamps, such as the BAV199, can be used
across the inputs to shunt current away from the AD8429 inputs
and, therefore, allow smaller protection resistor values.
I
The maximum current into the AD8429 inputs, I
on time and temperature. At room temperature, the device can
withstand a current of 10 mA for at least one day. This time is
cumulative over the life of the device.
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 53.
MAX
V
V
V
+
IN+
–
+
IN–
–
DIFF
+
–
R
R
R
R
R
PROTECT
PROTECT
PROTECT
PROTECT
PROTECT
SIMPLE METHOD
SIMPLE METHOD
Figure 52. Protection for Large Differential Voltages
I
Figure 51. Protection for Voltages Beyond the Rails
I
≥
AD8429
AD8429
1
2
+V
–V
⎛
⎜
⎜
⎝
S
S
V
DIFF
I
MAX
−
1
V
V
V
V
DIFF
+
IN+
–
+
IN–
–
R
+
R
–
⎞
⎟
⎟
⎠
PROTECT
PROTECT
R
R
−
PROTECT
PROTECT
I
R
LOW NOISE METHOD
LOW NOISE METHOD
G
I
+V
–V
+V
–V
S
S
S
S
33Ω
33Ω
AD8429
AD8429
MAX
+V
–V
S
S
AD8429
, depends