Datasheet

AD8222
Rev. A | Page 15 of 24
THEORY OF OPERATION
C1 C2
+V
S
+IN–IN
–V
S
–V
S
10kΩ
10kΩ
10kΩ
400Ω 400Ω
10kΩ
REF
OUTPUT
I
B
COMPENSATIONI
B
COMPENSATION
+V
S
–V
S
+V
S
–V
S
+V
S
V
B
II
R1 24.7kΩ 24.7kΩ
R
G
Q1
R2
Q2
–V
S
+V
S
–V
S
+V
S
A2A1
A3
05947-045
Figure 44. Simplified Schematic
AMPLIFIER ARCHITECTURE
The two instrumentation amplifiers of the AD8222 are based
on the classic 3-op-amp topology. Figure 44 shows a simplified
schematic of one of the amplifiers. The input transistors, Q1
and Q2, are biased at a fixed current. Any differential input
signal forces the output voltages of A1 and A2 to change so that
the differential voltage also appears across R
G
. The current that
flows through R
G
must also flow through R1 and R2, resulting
in a precisely amplified version of the differential input signal
between the outputs of A1 and A2. Topologically, Q1 + A1 + R1
and Q2 + A2 + R2 can be viewed as precision current feedback
amplifiers. The common-mode signal and the amplified differen-
tial signal are applied to a difference amplifier that rejects the
common-mode voltage. The difference amplifier employs innova-
tions that result in low output offset voltage as well as low output
offset voltage drift.
Because the input amplifiers employ a current feedback architec-
ture, the gain-bandwidth product of the AD8222 increases
with gain, resulting in a system that does not suffer from the
expected bandwidth loss of voltage feedback architectures at
higher gains.
The transfer function of the AD8222 is
V
OUT
= G(V
+IN
− V
−IN
) + V
REF
where:
GR
G
k49.4
1 
GAIN SELECTION
Placing a resistor across the R
G
terminals sets the gain of the
AD8222, which can be calculated by referring to Table 8 or by
using the following gain equation:
1
k49.4


G
R
G
Table 8. Gains Achieved Using 1% Resistors
1% Standard Table Value of R
G
(Ω) Calculated Gain
49.9 k 1.990
12.4 k 4.984
5.49 k 9.998
2.61 k 19.93
1.00 k 50.40
499 100.0
249 199.4
100 495.0
49.9 991.0
The AD8222 defaults to G = 1 when no gain resistor is used.
The tolerance and gain drift of the R
G
resistor should be added
to the AD8222’s specifications to determine the total gain
accuracy of the system. When the gain resistor is not used,
gain error and gain drift are kept to a minimum.