Datasheet

AD8421 Data Sheet
Rev. 0 | Page 22 of 28
Reference Pin
The output voltage of the AD8421 is developed with respect to
the potential on the reference terminal. Ensure that REF is tied
to the appropriate local ground.
INPUT BIAS CURRENT RETURN PATH
The input bias current of the AD8421 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 65.
THERMOCOUPLE
+V
S
REF
–V
S
AD8421
CAPACITIVELY COUPLED
+V
S
REF
C
C
–V
S
AD8421
TRANSFORMER
+V
S
REF
–V
S
AD8421
INCORRECT
CAPACITIVELY COUPLED
+V
S
REF
C
R
R
C
–V
S
AD8421
1
f
HIGH-PASS
=
2πRC
THERMOCOUPLE
+V
S
REF
–V
S
10MΩ
AD8421
TRANSFORMER
+V
S
REF
–V
S
AD8421
CORRECT
10123-061
Figure 65. Creating an Input Bias Current Return Path
INPUT VOLTAGES BEYOND THE SUPPLY RAILS
The AD8421 has very robust inputs. It typically does not need
additional input protection, as shown in Figure 66.
MOST APPLICATIONS
+
V
S
AD8421
–V
S
I
V
IN+
+
–
V
IN+
+
–
10123-062
Figure 66. Typical Application; No Input Protection Required
The AD8421 inputs are current limited; therefore, input voltages
can be up to 40 V from the opposite supply rail, with no input
protection required at all gains. For example, if +V
S
= +5 V and
−V
S
= −8 V, the part can safely withstand voltages from −35 V to
+32 V.
The remaining AD8421 terminals should be kept within the
supplies. All terminals of the AD8421 are protected against ESD.
Input Voltages Beyond the Maximum Ratings
For applications where the AD8421 encounters voltages beyond the
limits in the Absolute Maximum Ratings table, external protection
is required. This external protection depends on the duration of
the overvoltage event and the noise performance that is required.
For short-lived events, transient protectors (such as metal oxide
varistors (MOVs)), may be all that is required.
+
V
S
AD8421
–V
S
V
IN+
+
–
V
IN–
+
–
+
V
S
AD8421
R
PROTECT
R
PROTECT
–V
S
I
V
IN+
+
–
V
IN–
+
–
+V
S
+V
S
AD8421
R
PROTECT
R
PROTECT
–V
S
–V
S
I
V
IN+
+
–
V
IN–
+
–
+V
S
–V
S
+V
S
AD8421
R
PROTECT
R
PROTECT
–V
S
I
V
IN+
+
–
V
IN–
+
–
I
SIMPLE CONTINUOUS PROTECTIONTRANSIENT PROTECTION
LOW NOISE CONTINUOUS
OPTION 2
LOW NOISE CONTINUOUS
OPTION 1
10123-063
Figure 67. Input Protection Options for Input Voltages Beyond Absolute
Maximum Ratings
For longer events, use resistors in series with the inputs, combined
with diodes. To avoid degrading bias current performance, low
leakage diodes such as the BAV199 or FJH1100 are recommended.
The diodes prevent the voltage at the input of the amplifier from
exceeding the maximum ratings, and the resistors limit the current
into the diodes. Because most external diodes can easily handle
100 mA or more, resistor values do not need to be large and,
therefore, have a minimal impact on noise performance.
At the expense of some noise performance, another solution is
to use series resistors. In the case of overvoltage, current into
the AD8421 inputs is internally limited. Although the AD8421
inputs must be kept within the limits defined in the Absolute
Maximum Ratings section, the I × R drop across the protection
resistor increases the maximum voltage that the system can
withstand, as follows:
For positive input signals
V
MAX_NEW
= (40 V + Negative Supply) + I
IN
× R
PROTECT
For negative input signals
V
MIN_NEW
= (Positive Supply − 40 V) − I
OUT
× R
PROTECT