AD8221
THEORY OF OPERATION
I
V
B
I
I
B
COMPENSATION
C1
A1
A2
C2
I
B
COMPENSATION
10kΩ
+V
S
10kΩ
OUTPUT
10kΩ
A3
+V
S
–V
S
REF
+V
S
–IN
400Ω
Q1
R1 24.7kΩ
+V
S
R
G
–V
S
–V
S
–V
S
–V
S
R2
+V
S
24.7kΩ
Q2
+V
S
400Ω
+IN
10kΩ
–V
S
Figure 42. Simplified Schematic
The AD8221 is a monolithic instrumentation amplifier based
on the classic 3-op amp topology. Input transistors Q1 and Q2
are biased at a fixed current so that any differential input signal
forces the output voltages of A1 and A2 to change accordingly.
A signal applied to the input creates a current through R
G
, R1,
and R2, such that the outputs of A1 and A2 deliver the correct
voltage. Topologically, Q1, A1, R1 and Q2, A2, R2 can be
viewed as precision current feedback amplifiers. The amplified
differential and common-mode signals are applied to a
difference amplifier that rejects the common-mode voltage
but amplifies the differential voltage. The difference amplifier
employs innovations that result in low output offset voltage as
well as low output offset voltage drift. Laser-trimmed resistors
allow for a highly accurate in-amp with gain error typically less
than 20 ppm and CMRR that exceeds 90 dB (G = 1).
Using superbeta input transistors and an I
B
compensation
scheme, the AD8221 offers extremely high input impedance,
low I
B
, low I
B
drift, low I
OS
, low input bias current noise, and
extremely low voltage noise of 8 nV/√Hz.
B
B
B
Because the input amplifiers employ a current feedback
architecture, the gain-bandwidth product of the AD8221
increases with gain, resulting in a system that does not suffer
from the expected bandwidth loss of voltage feedback
architectures at higher gains.
To maintain precision even at low input levels, special attention
was given to the design and layout of the AD8221, resulting in
an in-amp whose performance satisfies the most demanding
applications.
A unique pinout enables the AD8221 to meet a CMRR
specification of 80 dB at 10 kHz (G = 1) and 110 dB at 1 kHz
(G = 1000). The balanced pinout, shown in Figure 43, reduces
the parasitics that had, in the past, adversely affected CMRR
performance. In addition, the new pinout simplifies board
layout because associated traces are grouped together. For
example, the gain setting resistor pins are adjacent to the
inputs, and the reference pin is next to the output.
–IN
1
R
G 2
R
G 3
+IN
4
8
7
6
+V
S
V
OUT
REF
03149-043
The transfer function of the AD8221 is
G
=
1+
49.4 kΩ
R
G
AD8221
TOP VIEW
5
–V
S
Users can easily and accurately set the gain using a single
standard resistor.
Figure 43. Pinout Diagram
Rev. B | Page 16 of 24
03149-042
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