AD8222_16 Datasheet, PDF (16/25 Pages) Analog Devices – Precision, Dual-Channel Instrumentation Amplifier

Data Sheet
AD8222
THEORY OF OPERATION
I
V_B
I
I_B COMPENSATION
+V_S
–IN
400Ω
–V_S
A1
C1
A2
C2
I_B COMPENSATION
10kΩ
10kΩ
R1 24.7kΩ
Q1
+V_S
R2 24.7kΩ
+V_S
Q2
+V_S
400Ω
+IN
R_G
–V_S
–V_S
–V_S
Figure 45. Simplified Schematic
10kΩ
A3
+V_S
OUTPUT
10kΩ
+V_{S –VS}
REF
–V_S
AMPLIFIER ARCHITECTURE
The two instrumentation amplifiers of the AD8222 are based
on the classic 3-op-amp topology. Figure 45 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 RG. The current that
flows through RG 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
VOUT = G(V+IN − V−IN) + VREF
GAIN SELECTION
Placing a resistor across the RG terminals sets the gain of the
AD8222, which can be calculated by referring to Table 8 or by
using the following gain equation:
49.4 kΩ
R_G  _{G  1}
Table 8. Gains Achieved Using 1% Resistors
1% Standard Table Value of RG (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 RG resistor should be added
to the AD8222 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.
where:
49.4 kΩ
G 1
RG
Rev. B | Page 15 of 24