Datasheet

Data Sheet AD8226
Rev. C | Page 19 of 28
THEORY OF OPERATION
A3
R2
24.7kΩ
R1
24.7kΩ
A1 A2
Q2Q1
–IN
+IN
+V
S
–V
S
R3
50kΩ
R4
50kΩ
R5
50kΩ
R
B
R
B
+V
S
–V
S
V
OUT
REF
07036-003
NODE 1
NODE 2
R
G
V
BIAS
+
V
S
–V
S
+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 AD8226 is based on the classic 3-op-amp topology. This
topology has two stages: a preamplifier to provide differential
amplification, followed by a difference amplifier to remove the
common-mode voltage. Figure 58 shows a simplified schematic
of the AD8226.
The first stage works as follows: in order to maintain a constant
voltage across the bias resistor R
B
, A1 must keep Node 3 a con-
stant diode drop above the positive input voltage. Similarly, 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 A3 and
four 50 kΩ resistors. The purpose of this stage is to remove the
common-mode signal from the amplified differential signal.
The transfer function of the AD8226 is
V
OUT
= G(V
IN+
− V
IN−
) + V
REF
where:
G
R
G
k49.4
1 
GAIN SELECTION
Placing a resistor across the R
G
terminals sets the gain of the
AD8226, which can be calculated by referring to Table 7 or by
using the following gain equation:
1
k49.4


G
R
G
Table 7. 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 AD8226 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 AD8226 specifications to determine the total gain accu-
racy of the system. When the gain resistor is not used, gain
error and gain drift are minimal.
If a gain of 5 is required and minimal gain drift is important,
consider using the AD8227. The AD8227 has a default gain of 5
that is set with internal resistors. Because all resistors are internal,
the gain drift is extremely low (<5 ppm/°C maximum).