AD8222 Analog Devices, AD8222 Datasheet - Page 15

• Manufacturer Part Number: AD8222
• Description: Precision, Dual-Channel Instrumentation Amplifier Data Sheet (Rev. 0, 7/2006)
• Manufacturer: Analog Devices

THEORY OF OPERATION
AMPLIFIER ARCHITECTURE
The two instrumentation amplifiers of the AD8222 are based on
the classic three op amp topology. Figure 44 shows a simplified
schematic of one of the amplifiers. 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
R
amplified version of the differential input signal between the
outputs of A1 and A2. Topologically, Q1, A1, and R1 and Q2,
A2, and R2 can be viewed as precision current feedback
amplifiers. The common-mode signal and the amplified
differential signal are applied to a difference amplifier that
rejects the common-mode voltage. The difference amplifier
employs innovations that result in low output offset voltage as
well as low output offset voltage drift.
Because the input amplifiers employ a current feedback
architecture, 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
where
G
must also flow through R1 and R2, resulting in a precisely
V
G
OUT
=
1 +
= G(V
I
B
–IN
49.4
COMPENSATION
R
IN+
G
kΩ
400Ω
− V
IN−
+V
–V
S
S
) + V
G
. The current that flows through
REF
Q1
I
C1
R1 24.7kΩ
A1
+V
–V
S
S
R
G
V
B
Figure 44. Simplified Schematic
+V
–V
R2
S
S
Rev. 0 | Page 15 of 24
A2
24.7kΩ
C2
Q2
GAIN SELECTION
Placing a resistor across the R
AD8222, which can be calculated by referring to Table 8 or by
using the following gain equation.
Table 8. Gains Achieved Using 1% Resistors
1% Standard Table Value of R
49.9 k
12.4 k
5.49 k
2.61 k
1.00 k
499
249
100
49.9
The AD8222 defaults to G = 1 when no gain resistor is used.
The tolerance and gain drift of the R
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.
I
–V
I
+V
R
B
G
S
COMPENSATION
S
=
400Ω
49.4
G
−
kΩ
10kΩ
10kΩ
1
+IN
10kΩ
A3
10kΩ
G
G
terminals sets the gain of the
(Ω)
+V
–V
S
S
G
resistor should be added
+V
–V
S
S
Calculated Gain
1.990
4.984
9.998
19.93
50.40
100.0
199.4
495.0
991.0
OUTPUT
REF
AD8222