Data Sheet AD8422
A ground plane layer is helpful to reduce parasitic inductances.
This minimizes voltage drops with changes in current. The area
of the current path is directly proportional to the magnitude of
parasitic inductances and, therefore, the impedance of the path
at high frequencies. Large changes in currents in an inductive
decoupling path or ground return create unwanted effects due
to the coupling of such changes into the amplifier inputs.
Because load currents flow from the supplies, connect the load at
the same physical location as the bypass capacitor grounds.
Reference Pin
The output voltage of the AD8422 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 AD8422 must have a dc 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 59.
THERMOCOUPLE
+V
S
REF
–V
S
AD8422
CAPACITIVELY COUPL E D
+V
S
REF
C
C
–V
S
AD8422
TRANSFORMER
+V
S
REF
–V
S
AD8422
INCORRECT
CAPACITIVELY COUPL E D
+V
S
REF
C
R
R
C
–V
S
AD8422
1
f
HIGH-PASS
=
2πRC
THERMOCOUPLE
+V
S
REF
–V
S
10MΩ
AD8422
TRANSFORMER
+V
S
REF
–V
S
AD8422
CORRECT
11197-062
Figure 59. Creating an Input Bias Current Return Path
INPUT VOLTAGES BEYOND THE SUPPLY RAILS
Many instrumentation amplifiers specify excellent CMRR and
input impedance, but in a real system, the performance suffers
because of the external components required for input protection.
The AD8422 has very robust inputs. It typically does not need
additional input protection. Input voltages can be up to 40 V from
the opposite supply rail without damage to the part. For example,
with a +5 V positive supply and a 0 V negative supply, the part can
safely withstand voltages from −35 V to +40 V. Unlike some other
instrumentation amplifiers, the part can handle large differential
input voltages even when the part is in high gain.
MOST APPLICATIONS
+V
S
AD8422
–V
S
I
V
IN+
+
–
V
IN+
+
–
11197-063
Figure 60. Input Overvoltage Protection with no External Components
For input voltages less than 40 V from the opposite rail, no input
protection is required.
Keep the rest of the AD8422 terminals within the supplies. All
terminals of the AD8422 are protected against ESD.
Input Voltages Beyond the Maximum Ratings
For applications where the AD8422 encounters voltages beyond
the limits in the Absolute Maximum Ratings section, external
protection is required. This external protection depends on the
duration of the overvoltage event and the noise performance
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 worsening bias current performance,
low leakage diodes, such as the BAV199 or FJH1100s, are
recommended. The diodes prevent the voltage at the input of
the amplifier from exceeding the maximum ratings, while the
resistors limit the current into the diodes. Because most external
diodes can easily handle 100 mA or more, resistor values do not
have to be large. Therefore, the protection resistance has minimal
impact on noise performance.
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