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AD627BN
(Rev.:RevA)

Micropower, Single and Dual Supply Rail-to-Rail Instrumentation Amplifier

Analog Devices 

AD627

THEORY OF OPERATION

The AD627 is a true “instrumentation amplifier” built using

two feedback loops. Its general properties are similar to those of

the classic “two op amp” instrumentation amplifier configura-

tion, and can be regarded as such, but internally the details are

somewhat different. The AD627 uses a modified “current feed-

back” scheme which, coupled with interstage feedforward

frequency compensation, results in a much better CMRR

(Common-Mode Rejection Ratio) at frequencies above dc (no-

tably the line frequency of 50 Hz–60 Hz) than might otherwise

be expected of a low power instrumentation amplifier.

Referring to the diagram, (Figure 32), A1 completes a feedback

loop which, in conjunction with V1 and R5, forces a constant

collector current in Q1. Assume that the gain-setting resistor

(RG) is not present for the moment. Resistors R2 and R1 com-

plete the loop and force the output of A1 to be equal to the

voltage on the inverting terminal with a gain of (almost exactly)

1.25. A nearly identical feedback loop completed by A2 forces a

current in Q2 which is substantially identical to that in Q1, and

A2 also provides the output voltage. When both loops are bal-

anced, the gain from the noninverting terminal to VOUT is equal

to 5, whereas the gain from the output of A1 to VOUT is equal to

–4. The inverting terminal gain of A1, (1.25) times the gain of

A2, (–4) makes the gain from the inverting and noninverting

terminals equal.

REF

–IN

R1

100k⍀

+VS

2k⍀

EXTERNAL GAIN RESISTOR

RG

R4

100k⍀

R2

25k⍀

R3

25k⍀

+VS

2k⍀

Q1

Q2

+IN

–VS

A1

–VS

A2

OUTPUT

R5

200k⍀

V1

R6

200k⍀

–VS

Figure 32. Simplified Schematic

The differential mode gain is equal to 1 + R4/R3, nominally five

and is factory trimmed to 0.01% final accuracy. Adding an external

gain setting resistor (RG) increases the gain by an amount equal

to (R4 + R1)/RG. The output voltage of the AD627 is given by the

following equation.

VOUT = [VIN(+) – VIN(–)] × (5 + 200 kΩ/RG) + VREF

Laser trims are performed on R1 through R4 to ensure that

their values are as close as possible to the absolute values in the

gain equation. This ensures low gain error and high common-

mode rejection at all practical gains.

USING THE AD627

Basic Connections

Figure 33 shows the basic connection circuit for the AD627.

The +VS and –VS terminals are connected to the power supply.

The supply can either be bipolar (VS = ±1.1 V to ± 18 V) or

single supply (–VS = 0 V, +VS = +2.2 V to +36 V). The power

supplies should be capacitively decoupled close to the devices

power pins. For best results, use surface mount 0.1 µF ceramic

chip capacitors.

The input voltage, which can be either single ended (tie either

–IN or +IN to ground) or differential. The difference between

the voltage on the inverting and noninverting pins is amplified

by the programmed gain. The programmed gain is set by the

gain resistor (see below). The output signal appears as the volt-

age difference between the output pin and the externally applied

voltage on the REF pin (see below).

Setting the Gain

The AD627s gain is resistor programmed by RG, or more pre-

cisely, by whatever impedance appears between Pins 1 and 8.

The gain is set according to the equation:

Gain = 5 + (200 kΩ/RG)

or

RG = 200 kΩ/(Gain – 5)

It follows that the minimum achievable gain is 5 (for RG = ∞).

With an internal gain accuracy of between 0.05% and 0.7%

depending on gain and grade, a 0.1% external gain resistor

would seem appropriate to prevent significant degradation of the

overall gain error. However, 0.1% resistors are not available in a

wide range of values and are quite expensive. Table I shows

recommended gain resistor values using 1% resistors. For all

gains, the size of the gain resistor is conservatively chosen as the

closest value from the standard resistor table that is higher than

the ideal value. This results in a gain that is always slightly less

than the desired gain. This prevents clipping of the signal at the

output due to resistor tolerance.

The internal resistors on the AD627 have a negative tempera-

ture coefficient of –75 ppm/°C max for gains > 5. Using a gain

resistor that also has a negative temperature coefficient of

–75 ppm/°C or less will tend to reduce the overall circuit’s gain

drift.

+VS

+1.1V TO +18V

0.1␮F

+IN

VIN RG

–IN

RG

OUTPUT

RG REF

0.1␮F

VOUT

REF (INPUT)

+VS

+2.2V TO +36V

0.1␮F

+IN

VIN RG

–IN

RG

OUTPUT

RG REF

VOUT

REF (INPUT)

–1.1V TO –18V

–VS

GAIN = 5 + (200k⍀/RG)

Figure 33. Basic Connections for Single and Dual Supplies

–10–

REV. A

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