AD8236

Rev. 0 | Page 14 of 20

THEORY OF OPERATION

REF

–IN

+IN

210kΩ

210kΩ

52.5kΩ

52.5kΩ

AD8236

2

3

8

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:

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

+IN

5V

0.1µF

VINP

GAIN SETTING

RESISTOR

VINM

OUT

REF

–IN

Figure 35. Basic Setup

GAIN SELECTION

AD8236, which can be calculated by referring to Table 6 or by

using the gain equation

5

kΩ

420

−

=

G

R

G

Table 6. Gains Achieved Using 1% Resistors

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.

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.