Datasheet

AD8236
Rev. 0 | Page 14 of 20
THEORY OF OPERATION
08000-006
R
G
R
G
R
G
REF
–IN
+IN
V
OUT
210k 210k52.5k 52.5k
AD8236
2 3
+V
S
8
–V
S
5
1 4
7
6
ESD
PROTECTION
ESD
PROTECTION
ESD
PROTECTION
ESD
PROTECTION
ESD
PROTECTION
ESD
PROTECTION
OP AMP
A
OP AMP
B
Figure 34. Simplified Schematic
The AD8236 is a monolithic, 2-op-amp instrumentation
amplifier. It was designed for low power, portable applications
where size and low quiescent current are paramount. For example,
it has a rail-to-rail input and output stage to offer more dynamic
range when operating on low voltage batteries. Unlike traditional
rail-to-rail input amplifiers that use a complementary differential
pair stage and suffer from nonlinearity, the AD8236 uses a novel
architecture to internally boost the supply rail, allowing the
amplifier to operate rail to rail yet still deliver a low 0.5 ppm of
nonlinearity. In addition, the 2-op-amp instrumentation amplifier
architecture offers a wide operational common-mode voltage
range. Additional information is provided in the Common-
Mode Input Voltage Range section. Precision, laser-trimmed
resistors provide the AD8236 with a high CMRR of 86 dB
(minimum) at G = 5 and gain accuracy of 0.05% (maximum).
BASIC OPERATION
The AD8236 amplifies the difference between its positive input
(+IN) and its negative input (−IN). The REF pin allows the user
to level-shift the output signal. This is convenient when interfacing
to a filter or analog-to-digital converter (ADC). The basic setup
is shown in Figure 35. Figure 37 shows an example configuration
for operating the AD8236 with dual supplies. The equation for
the AD8236 is as follows:
V
OUT
= G × (VINP − VINM) + VREF
If no gain setting resistor is installed, the default gain, G, is 5.
The Gain Selection section describes how to program the gain, G.
AD8236
R
G
+IN
+V
S
5
V
0.1µF
–V
S
VINP
GAIN SETTING
RESISTOR
VINM
OUT
REF
V
REF
V
OUT
R
G
–IN
08000-136
Figure 35. Basic Setup
GAIN SELECTION
Placing a resistor across the R
G
terminals sets the gain of the
AD8236, which can be calculated by referring to Table 6 or by
using the gain equation
5
k420
−
=
G
R
G
Table 6. Gains Achieved Using 1% Resistors
1% Standard Table Value of R
G
(Ω) Calculated Gain
422 k 6.0
210 k 7.0
140 k 8.0
105 k 9.0
84.5 k 10.0
28 k 20.0
9.31 k 50.1
4.42 k 100.0
2.15 k 200.3
The AD8236 defaults to G = 5 when no gain resistor is used.
Gain accuracy is determined by the absolute tolerance of R
G
.
The TC of the external gain resistor increases the gain drift of
the instrumentation amplifier. Gain error and gain drift are at a
minimum when the gain resistor is not used.