AD8421 AD [Analog Devices], AD8421 Datasheet - Page 22

• Manufacturer Part Number: AD8421
• Description: 3 nV/?Hz, Low Power
• Manufacturer: AD [Analog Devices]

AD8421
Reference Pin
The output voltage of the
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
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.
INPUT VOLTAGES BEYOND THE SUPPLY RAILS
The
additional input protection, as shown in Figure 66.
The
can be up to 40 V from the opposite supply rail, with no input
CAPACITIVELY COUPLED
C
C
AD8421
AD8421
THERMOCOUPLE
TRANSFORMER
Figure 66. Typical Application; No Input Protection Required
INCORRECT
Figure 65. Creating an Input Bias Current Return Path
AD8421
+V
AD8421
–V
AD8421
+V
–V
+V
–V
inputs are current limited; therefore, input voltages
has very robust inputs. It typically does not need
S
S
S
S
S
S
REF
V
V
REF
REF
+
IN+
+
IN+
–
–
MOST APPLICATIONS
AD8421
f
HIGH-PASS
I
AD8421
AD8421
+V
–V
=
CAPACITIVELY COUPLED
is developed with respect to
2πRC
S
S
1
10MΩ
must have a return path
C
C
R
R
THERMOCOUPLE
TRANSFORMER
CORRECT
AD8421
AD8421
AD8421
+V
–V
+V
–V
+V
–V
S
S
S
S
S
S
REF
REF
REF
Rev. 0 | Page 22 of 28
protection required at all gains. For example, if +V
−V
+32 V.
The remaining
supplies. All terminals of the
Input Voltages Beyond the Maximum Ratings
For applications where 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.
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
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
For negative input signals
V
V
Figure 67. Input Protection Options for Input Voltages Beyond Absolute
V
V
S
+
IN+
–
+
IN–
–
AD8421
+
IN+
–
+
IN–
–
= −8 V, the part can safely withstand voltages from −35 V to
V
V
TRANSIENT PROTECTION
LOW NOISE CONTINUOUS
MAX_NEW
MIN_NEW
R
R
I
PROTECT
PROTECT
I
OPTION 1
inputs is internally limited. Although the
= (Positive Supply − 40 V) − I
= (40 V + Negative Supply) + I
AD8421
AD8421
+V
–V
AD8421
S
S
+V
–V
S
S
terminals should be kept within the
Maximum Ratings
AD8421
AD8421
SIMPLE CONTINUOUS PROTECTION
V
V
V
V
+
IN+
–
+
IN–
–
R
R
+
IN+
–
+
IN–
–
PROTECT
PROTECT
encounters voltages beyond the
LOW NOISE CONTINUOUS
R
R
PROTECT
PROTECT
are protected against ESD.
I
OPTION 2
+V
–V
+V
–V
I
S
S
S
S
OUT
IN
AD8421
Data Sheet
× R
× R
AD8421
+V
–V
+V
–V
S
S
PROTECT
PROTECT
S
S
S
= +5 V and
AD8421