Datasheet

Data Sheet AD8226
Rev. C | Page 21 of 28
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
part 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 instrumentation
amplifier. Note that noise on the supply pins can adversely affect
performance. For more information, see the PSRR performance
curves in Figure 23 and Figure 24.
A 0.1 μF capacitor should be placed as close as possible to each
supply pin. As shown in Figure 61, a 10 μF tantalum capacitor
can be used farther away from the part. In most cases, it can be
shared by other precision integrated circuits.
AD8226
+
V
S
+IN
–IN
LOAD
REF
0.1µF
10µF
0.1µF 10µF
–V
S
V
OUT
07036-006
Figure 61. Supply Decoupling, REF, and Output Referred to Local Ground
References
The output voltage of the AD8226 is developed with respect to
the potential on the reference terminal. Care should be taken to
tie REF to the appropriate local ground.
INPUT BIAS CURRENT RETURN PATH
The input bias current of the AD8226 must have a return path
to ground. When the source, such as a thermocouple, cannot
provide a return current path, one should be created, as shown
in Figure 62.
THERMOCOUPLE
+V
S
REF
–V
S
AD8226
CAPACITIVELY COUPLED
+V
S
REF
C
C
–V
S
AD8226
TRANSFORMER
+V
S
REF
–V
S
AD8226
INCORRECT
CAPACITIVELY COUPLED
+V
S
REF
C
R
R
C
–V
S
AD8226
1
f
HIGH-PASS
=
2πRC
THERMOCOUPLE
+V
S
REF
–V
S
10MΩ
AD8226
TRANSFORMER
+V
S
REF
–V
S
AD8226
CORRECT
0
7036-007
Figure 62. Creating an I
BIAS
Path