AD8226ARMZ-RL(RevA) View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

AD8226ARMZ-RL
(Rev.:RevA)

Wide Supply Range, Rail-to-Rail Output Instrumentation Amplifier

Analog Devices 

AD8226

THEORY OF OPERATION

NODE 3

+VS

+VS

RG

NODE 4

ESD AND

+IN

OVERVOLTAGE

PROTECTION

R1 –VS

24.7kΩ

NODE 1

Q1

A1

–VS R2

24.7kΩ

NODE 2

ESD AND

A2

Q2

OVERVOLTAGE

PROTECTION

RB

VBIAS

RB

–VS

GAIN STAGE

Figure 58. Simplified Schematic

R4

50kΩ

R5

50kΩ

–IN

R3

50kΩ

+VS

A3

+VS

R6

–VS

50kΩ

VOUT

REF

–VS

DIFFERENCE

AMPLIFIER STAGE

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 RB, 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, RG. 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

VOUT = G(VIN+ − VIN−) + VREF

where:

49.4 kΩ

G =1+

RG

GAIN SELECTION

Placing a resistor across the RG terminals sets the gain of the

AD8226, which can be calculated by referring to Table 7 or by

using the following gain equation:

49.4 kΩ

RG = G − 1

Table 7. Gains Achieved Using 1% Resistors

1% Standard Table Value of RG (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 RG 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).

Rev. A | Page 19 of 28

Share Link: