AD8228
Rev. 0 | Page 17 of 24
REFERENCE TERMINAL
The output voltage of the AD8228 is developed with respect to
the potential on the reference terminal. This is useful when the
output signal needs to be offset to a precise midsupply level. For
example, a voltage source can be tied to the REF pin to level-shift
the output so that the AD8228 can drive a single-supply ADC. The
REF pin is protected with ESD diodes and should not exceed
either +V
S
or −V
S
by more than 0.3 V.
For best performance, source impedance to the REF terminal
should be kept below 1 Ω. As shown in
Figure 41, the reference
terminal, REF, is at one end of a 10 k resistor. Additional imped-
ance at the REF terminal adds to this 10 k resistor and results
in amplification of the signal connected to the positive input.
The amplification from the additional R
REF
can be computed by
REF
REF
R
R
+
+×
k20
k102
Only the positive signal path is amplified; the negative path is
unaffected. This uneven amplification degrades the CMRR of
the amplifier.
INCORRECT
V
CORRECT
AD8228
OP1177
+
–
V
07035-005
REF
AD8228
REF
Figure 43. Driving the Reference
LAYOUT
The AD8228 is a high precision device. To ensure optimum
performance at the PCB level, care must be taken in the design
of the board layout. The AD8228 pins are arranged in a logical
manner to aid in this task.
8
7
6
5
1
2
3
4
–IN
G1
G2
+V
S
V
OUT
REF
–V
S
+IN
TOP VIEW
(Not to Scale)
AD8228
07035-044
Figure 44. Pinout Diagram
Common-Mode Rejection Ratio over Frequency
The AD8228 has a higher CMRR over frequency than typical
in-amps, which gives it greater immunity to disturbances such
as line noise and its associated harmonics. The AD8228 pinout
was designed so that the board designer can take full advantage
of this performance with a well-implemented layout.
Poor layout can cause some of the common-mode signal to be
converted to a differential signal before it reaches 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, 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. Noise on the supply pins can adversely affect perform-
ance. See the PSRR performance curves in
Figure 17 and Figure 18
for more information.
A 0.1 µF capacitor should be placed as close as possible to each
supply pin. As shown in
Figure 45, 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.
AD8228
+
S
+IN
–IN
LOAD
REF
0.1µF
10µF
0.1µF 10µF
–V
S
V
OUT
07035-006
Figure 45. Supply Decoupling, REF, and Output Referred to Local Ground
