Datasheet

Data Sheet AD8428
Rev. A | Page 15 of 20
INPUT BIAS CURRENT RETURN PATH
The input bias current of the AD8428 must have a return path
to ground. When the source, such as a thermocouple, cannot
provide a current return path, one should be created, as shown
in Figure 41.
THERMOCOUPLE
+V
S
REF
–V
S
AD8428
CAPACITIVELY COUPLED
+V
S
REF
C
C
–V
S
AD8428
TRANSFORMER
+V
S
REF
–V
S
AD8428
INCORRECT
CAPACITIVELY COUPLED
+V
S
REF
C
R
R
C
–V
S
AD8428
1
f
HIGH-PASS
=
2πRC
THERMOCOUPLE
+V
S
REF
–V
S
10MΩ
AD8428
TRANSFORMER
+V
S
REF
–V
S
AD8428
CORRECT
09731-046
Figure 41. Creating an Input Bias Current Return Path
INPUT PROTECTION
Do not allow the inputs of the AD8428 to exceed the ratings
stated in the Absolute Maximum Ratings section. If these ratings
cannot be adhered to, add protection circuitry in front of the
AD8428 to limit the maximum current into the inputs (see the
I
MAX
section).
I
MAX
The maximum current into the AD8428 inputs, I
MAX
, depends
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.
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 each input can be computed using
the following equation:
MAX
SUPPLY
IN
PROTECT
I
VV
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 the AD8428
inputs and, therefore, allow smaller protection resistor values.
To ensure that current flows primarily through the external
protection diodes, place a small value resistor, such as a 33 
resistor, between the diodes and the AD8428.
SIMPLE METHOD LOW NOISE METHOD
+V
S
AD8428
R
PROTECT
R
PROTECT
–V
S
I
V
IN+
+
–
V
IN–
+
–
+V
S
+
V
S
AD8428
R
PROTECT 33Ω
33Ω
R
PROTECT
–V
S
–V
S
I
V
IN+
+
–
V
IN–
+
–
+V
S
–V
S
09731-047
Figure 42. 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 using the following equation:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
−
×≥
G
MAX
DIFF
PROTECT
R
I
V
R
V1
2
1
Noise sensitive applications may require a lower protection
resistance. Low leakage diode clamps, such as the BAV199,
can be used across the AD8428 inputs to shunt current away
from the inputs and, therefore, allow smaller protection resistor
values.
AD8428
R
PROTECT
R
PROTECT
I
V
DIFF
+
–
09731-048
Figure 43. Protection for Large Differential Voltages