Datasheet

AD8227
Rev. 0 | Page 19 of 24
THEORY OF OPERATION
A3
R2
8kΩ
R1
8kΩ
A1 A2
Q2Q1
–IN
+IN
+V
S
–V
S
R3
50kΩ
R4
10kΩ
R5
10kΩ
R
B
R
B
+V
S
–V
S
V
OUT
REF
07759-058
NODE 1
NODE 2
R
G
+V
S
–V
S
V
BIAS
+V
S
–V
S
NODE 4NODE 3
R6
50kΩ
DIFFERENCE
AMPLIFIER STAGEGAIN STAGE
ESD AND
OVERVOLTAGE
PROTECTION
ESD AND
OVERVOLTAGE
PROTECTION
–V
S
Figure 58. Simplified Schematic
ARCHITECTURE
The AD8227 is based on the classic three op amp topology. This
topology has two stages: a preamplifier to provide differential
amplification followed by a difference amplifier that removes
the common-mode voltage and provides additional amplifica-
tion. Figure 58 shows a simplified schematic of the AD8227.
The first stage works as follows. To maintain a constant voltage
across the bias resistor, R
B
, Amplifier A1 must keep Node 3 at a
constant diode drop above the positive input voltage. Similarly,
Amplifier A2 keeps Node 4 at a constant diode drop above the
negative input voltage. Therefore, a replica of the differential
input voltage is placed across the gain setting resistor, R
G
. The
current that flows across this resistance must also flow through
the R1 and R2 resistors, creating a gained differential signal
between the A2 and A1 outputs. Note that, in addition to a
gained differential signal, the original common-mode signal,
shifted a diode drop up, is also still present.
The second stage is a difference amplifier, composed of
Amplifier A3 and the R3 through R6 resistors. This stage
removes the common-mode signal from the amplified
differential signal and gains it by 5.
The transfer function of the AD8227 is
V
OUT
= G × (V
IN+
− V
IN−
) + V
REF
where:
GR
G
k80
5 +=
GAIN SELECTION
Placing a resistor across the R
G
terminals sets the gain of the
AD8227. The gain can be calculated by referring to Table 7 or
by using the following gain equation:
5
k80
−
=
G
R
G
Table 7. Gains Achieved Using Common Resistor Values
Standard Table Value of R
G
Calculated Gain
No resistor 5
100 kΩ 5.8
49.9 kΩ 6.6
26.7 kΩ 8
20 kΩ 9
16 kΩ 10
10 kΩ 13
5.36 kΩ 19.9
2 kΩ 45
1.78 kΩ 49.9
1 kΩ 85
845 Ω 99.7
412 Ω 199
162 Ω 499
80.6 Ω 998
The AD8227 defaults to G = 5 when no gain resistor is used.
The tolerance and gain drift of the R
G
resistor should be added
to the specifications of the AD8227 to determine the total gain
accuracy of the system. When the gain resistor is not used, gain
error and gain drift are minimal.