ad8426 Analog Devices, Inc., ad8426 Datasheet - Page 13

• Manufacturer Part Number: ad8426
• Description: Wide Supply Range, Rail-to-rail Output Instrumentation Amplifier
• Manufacturer: Analog Devices, Inc.

Preliminary Technical Data
Common-Mode Rejection Ratio over Frequency
Poor layout can cause some of the common-mode signals to be
converted to differential signals before reaching the in-amp. Such
conversions occur when one input path has a frequency response
that is different from the other. To keep CMRR across frequency
high, the input source impedance and capacitance of each path
should be closely matched. Additional source resistance in the
input path (for example, for input protection) should be placed
close to the in-amp inputs, which minimizes their interaction
with parasitic capacitance from the PCB traces.
Parasitic capacitance at the gain setting pins can also affect CMRR
over frequency. If the board design has a component at the gain
setting pins (for example, a switch or jumper), the component
should be chosen so that the parasitic capacitance is as small as
possible.
Power Supplies
A stable dc voltage should be used to power the instrumenta-
tion amplifier. Noise on the supply pins can adversely affect
performance.
A 0.1 μF capacitor should be placed as close as possible to each
supply pin. As shown in Figure 11, a 10 μF capacitor can be used
farther away from the part. In most cases, it can be shared by
other precision integrated circuits.
Figure 11. Supply Decoupling, REF, and Output Referred to Local Ground
NOTES
1. EXPOSED LEAD FRAME TABS AT THE FOUR CORNERS
OF THE PACKAGE ARE INTERNALLY CONNECTED TO
+V
FURTHER INFORMATION ON PACKAGE AVAILABILITY.
S
. REFER TO THE OUTLINE DIMENSIONS PAGE, FOR
Figure 10. Hidden Paddle Package, Bottom View
BOTTOM VIEW
+IN
–IN
AD8426
+V
–V
S
S
0.1µF
0.1µF
REF
HIDDEN
PADDLE
EXPOSED LEAD
FRAME TABS
10µF
10µF
LOAD
V
OUT
Rev. PrD | Page 13 of 20
References
The output voltage of the
the potential on the reference terminal. Care should be taken to
tie REF to the appropriate local ground. This should also help
minimize crosstalk between the two channels.
INPUT BIAS CURRENT RETURN PATH
The input bias current of the
to ground. When the source, such as a thermocouple, cannot
provide a return current path, one should be created, as shown
in Figure 12.
INPUT PROTECTION
The
need additional input protection. Input voltages can be up to
40 V from the opposite supply rail. For example, with a +5 V
positive supply and a −8 V negative supply, the part can safely
withstand voltages from −35 V to +32 V. Unlike some other
instrumentation amplifiers, the part can handle large differen-
tial input voltages even when the part is in high gain.
The rest of the
supplies. All terminals of the
CAPACITIVELY COUPLED
C
C
AD8426
THERMOCOUPLE
TRANSFORMER
INCORRECT
Figure 12. Creating an Input Bias Current Return Path
AD8426
+V
AD8426
–V
AD8426
+V
–V
+V
–V
S
S
has very robust inputs and typically does not
S
S
S
S
AD8426
REF
REF
REF
terminals should be kept within the
AD8426
f
HIGH-PASS
AD8426
AD8426
=
is developed with respect to
CAPACITIVELY COUPLED
2πRC
are protected against ESD.
1
must have a return path
10MΩ
C
C
R
R
THERMOCOUPLE
TRANSFORMER
CORRECT
AD8426
AD8426
AD8426
+V
+V
+V
–V
–V
–V
AD8426
S
S
S
S
S
S
REF
REF
REF