AD8221-EVAL 查看數據表（PDF） - Analog Devices
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AD8221-EVAL Datasheet PDF : 20 Pages
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THEORY OF OPERATION
AD8221
I
VB
I
IB COMPENSATION
+VS
400Ω
–IN
–VS
A1
C1
A2
C2
IB COMPENSATION
10kΩ
10kΩ
R1 24.7kΩ
Q1
+VS
R2 24.7kΩ
+VS
Q2
+VS
400Ω
+IN
RG
–VS
–VS
–VS
Figure 42. Simplified Schematic
10kΩ
A3
+VS
OUTPUT
10kΩ
+VS –VS
REF
–VS
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
will force the output voltages of A1 and A2 to change accor-
dingly. A signal applied to the input creates a current through
RG, 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 dif-
ference 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 IB compensation
scheme, the AD8221 offers extremely high input impedance,
low IB, low IB drift, low IOS, low input bias current noise, and
extremely low voltage noise of 8 nV/√Hz.
The transfer function of the AD8221 is
G = 1 + 49.4kΩ
RG
Users can easily and accurately set the gain using a single,
standard resistor.
Since the input amplifiers employ a current feedback architec-
ture, the AD8221’s gain-bandwidth product increases with gain,
resulting in a system that does not suffer from the expected
bandwidth loss of voltage feedback architectures at higher gains.
In order to maintain precision even at low input levels, special
attention was given to the AD8221’s design and layout, 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
RG 2
RG 3
+IN 4
AD8221
TOP VIEW
8 +VS
7 VOUT
6 REF
5 –VS
Figure 43. Pinout Diagram
Rev. A | Page 13 of 20
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