AD8235

Rev. 0 | Page 14 of 20

THEORY OF OPERATION

RG

RG

REF

210kΩ

210kΩ

52.5kΩ

52.5kΩ

AD8235

–IN

+IN

B3

C3

A1

D1

A3

D3

B1

SDN

A2

C1

ESD

PROTECTION

ESD

PROTECTION

ESD

PROTECTION

ESD

PROTECTION

ESD

PROTECTION

ESD

PROTECTION

OP AMP

A

OP AMP

B

Figure 35. Simplified Schematic

The AD8235 is a monolithic, two-op amp instrumentation

amplifier. It is designed for low power, portable applications

where size and low quiescent current are paramount. The

AD8235 is offered in a WLCSP package, minimizing layout

area. Additional features that make this part optimal for

portable applications include a rail-to-rail input and output

stage that offers 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 AD8235 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 two-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

AD8235 with a high CMRR of 90 dB (minimum) at G = 5 and

gain accuracy of 0.05% (maximum).

AD8235

RG

+IN

5V

0.1µF

VINP

SDN

GAIN SETTING

RESISTOR

VINM

OUT

REF

RG

–IN

BASIC OPERATION

The AD8235 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 36. Figure 39 shows an example configuration

for operating the AD8235 with dual supplies. The equation for

the AD8235 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.

Figure 36. Basic Setup

GAIN SELECTION

AD8235. The gain may be derived by referring to Table 6 or by

using the following equation:

5

kΩ

420

−

=

G

R

G

Table 6. Gains Achieved Using 1% Resistors

Calculated Gain

422

6.0

210

7.0

140

8.0

105

9.0

84.5

10.0

28

20.0

9.31

50.1

4.42

100.0

2.15

200.3

The AD8235 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.