REV. A

Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.

AD627

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700

World Wide Web Site: http://www.analog.com

Fax: 781/326-8703

© Analog Devices, Inc., 1999

Micropower, Single and Dual Supply

Rail-to-Rail Instrumentation Amplifier

FUNCTIONAL BLOCK DIAGRAM

8-Lead Plastic DIP (N) and SOIC (R)

IN

+IN

OUTPUT

REF

AD627

FEATURES
Micropower, 85 A Max Supply Current
Wide Power Supply Range (+2.2 V to 18 V)
Easy to Use

Gain Set with One External Resistor
Gain Range 5 (No Resistor) to 1,000

Higher Performance than Discrete Designs
Rail-to-Rail Output Swing
High Accuracy DC Performance

0.10% Gain Accuracy (G = 5) (AD627A)
10 ppm Gain Drift (G = 5)
125 V Max Input Offset Voltage (AD627B)
200 V Max Input Offset Voltage (AD627A)
1 V/ C Max Input Offset Voltage Drift (AD627B)
3 V/ C Max Input Offset Voltage Drift (AD627A)
10 nA Max Input Bias Current

Noise: 38 nV/

Hz RTI Noise @ 1 kHz (G = 100)

Excellent AC Specifications

77 dB Min CMRR (G = 5) (AD627A)
83 dB Min CMRR (G = 5) (AD627B)
80 kHz Bandwidth (G = 5)
135 s Settling Time to 0.01% (G = 5, 5 V Step)

APPLICATIONS
4 mA-to-20 mA Loop Powered Applications
Low Power Medical Instrumentation--ECG, EEG
Transducer Interfacing
Thermocouple Amplifiers
Industrial Process Controls
Low Power Data Acquisition
Portable Battery Powered Instruments

PRODUCT DESCRIPTION

The AD627 is an integrated, micropower, instrumentation
amplifier that delivers rail-to-rail output swing on single and
dual (+2.2 V to

18 V) supplies. The AD627 provides the user

with excellent ac and dc specifications while operating at only
85

A max.

The AD627 offers superior user flexibility by allowing the user
to set the gain of the device with a single external resistor, and
by conforming to the 8-lead industry standard pinout configura-
tion. With no external resistor, the AD627 is configured for a
gain of 5. With an external resistor, it can be programmed for
gains of up to 1000.

Wide supply voltage range (+2.2 V to

18 V), and micropower

current consumption make the AD627 a perfect fit for a wide
range of applications. Single supply operation, low power con-
sumption and rail-to-rail output swing make the AD627 ideal
for battery powered applications. Its rail-to-rail output stage
maximizes dynamic range when operating from low supply
voltages. Dual supply operation (

15 V) and low power con-

sumption make the AD627 ideal for industrial applications,
including 4 mA-to-20 mA loop-powered systems.

The AD627 does not compromise performance, unlike other
micropower instrumentation amplifiers. Low voltage offset,
offset drift, gain error, and gain drift keep dc errors to a mini-
mum in the users system. The AD627 also holds errors over
frequency to a minimum by providing excellent CMRR over
frequency. Line noise, as well as line harmonics, will be rejected,
since the CMRR remains high up to 200 Hz.

The AD627 provides superior performance, uses less circuit
board area and does it for a lower cost than micropower discrete
designs.

FREQUENCY Hz

100

CMRR dB

100

10k

TRADITIONAL

LOW POWER

DISCRETE DESIGN

AD627

Figure 1. CMRR vs. Frequency,

5 V

, Gain = 5

REV. A

AD627SPECIFICATIONS

SINGLE SUPPLY

(typical @ +25 C Single Supply, V

= +3 V and +5 V and R

= 20 k

, unless otherwise noted)

Model

AD627A

AD627B

Specification

Conditions

Min

Typ

Max

Min

Typ

Max

Units

GAIN

G = 5 + (200 k

)

Gain Range

1000

V/V

Gain Error

OUT

= (V

) + 0.1 to (+V

) 0.15

G = 5

0.03

0.10

0.01

0.06

G = 10

0.15

0.35

0.10

0.25

G = 100

0.15

0.35

0.10

0.25

G = 1000

0.50

0.70

0.25

0.35

Nonlinearity

G = 5

100

ppm

G = 100

100

ppm

Gain vs. Temperature

G = 5

ppm/

G > 5

ppm/

VOLTAGE OFFSET

Input Offset, V

OSI

250

150

Over Temperature

= V

REF

= +V

445

215

Average TC

0.1

Output Offset, V

OSO

1000

500

Over Temperature

1650

1150

Average TC

2.5

Offset Referred to the Input

vs. Supply (PSRR)

G = 5

100

G = 10

100

120

100

120

G = 100

110

125

110

125

G = 1000

110

125

110

125

INPUT CURRENT

Input Bias Current

Over Temperature

Average TC

pA/

Input Offset Current

0.3

Over Temperature

Average TC

pA/

INPUT

Input Impedance

Differential

20 2

Common-Mode

20 2

Input Voltage Range

= +2.2 V to +36 V

) 0.1

(+V

) 1

) 0.1

(+V

) 1

Common-Mode Rejection

Ratio DC to 60 Hz with

REF

= V

1 k

Source Imbalance

G = 5

= +3 V, V

= 0 V to +1.9 V

G = 5

= +5 V, V

= 0 V to +3.7 V

OUTPUT

Output Swing

= 20 k

) + 25

(+V

) 70

) + 25

(+V

) 70

= 100 k

) + 7

(+V

) 25

) + 7

(+V

) 25

Short-Circuit Current

Short-Circuit to Ground

DYNAMIC RESPONSE

Small Signal 3 dB Bandwidth
G = 5

kHz

G = 100

kHz

G = 1000

0.4

kHz

Slew Rate

+0.05/0.07

Settling Time to 0.01%

= +3 V, +1.5 V Output Step

G = 5

G = 100

290

Settling Time to 0.01%

= +5 V, +2.5 V Output Step

G = 5

G = 100

330

Overload Recovery

50% Input Overload

NOTES

Does not include effects of external resistor R

See Table III for total RTI errors.

See Applications section for input range, gain range and common-mode range.

Specifications subject to change without notice

REV. A

AD627

DUAL SUPPLY

(typical @ +25 C Dual Supply, V

= 5 V and 15 V and R

= 20 k

, unless otherwise noted)

Model

AD627A

AD627B

Specification

Conditions

Min

Typ

Max

Min

Typ

Max

Units

GAIN

G = 5 + (200 k

)

Gain Range

1000

V/V

Gain Error

OUT

= (V

) + 0.1 to (+V

) 0.15

G = 5

0.03

0.10

0.01

0.06

G = 10

0.15

0.35

0.10

0.25

G = 100

0.15

0.35

0.10

0.25

G = 1000

0.50

0.70

0.25

0.35

Nonlinearity

G = 5

5 V/

15 V

10/25

100

10/25

100

ppm

G = 100

5 V/

15 V

10/15

100

10/15

100

ppm

Gain vs. Temperature

G = 5

ppm/

G > 5

ppm/

VOLTAGE OFFSET

Total RTI Error = V

OSI

+ V

OSO/G

Input Offset, V

OSI

200

125

Over Temperature

= V

REF

= 0 V

395

190

Average TC

0.1

Output Offset, V

OSO

1000

500

Over Temperature

1700

1100

Average TC

2.5

Offset Referred to the Input

vs. Supply (PSRR)

G = 5

100

G = 10

100

120

100

120

G = 100

110

125

110

125

G = 1000

110

125

110

125

INPUT CURRENT

Input Bias Current

Over Temperature

Average TC

pA/

Input Offset Current

0.3

Over Temperature

Average TC

pA/

INPUT

Input Impedance

Differential

20 2

Common-Mode

20 2

Input Voltage Range

1.1 V to

18 V

) 0.1

(+V

) 1

) 0.1

(+V

) 1

Common-Mode Rejection

Ratio DC to 60 Hz with

1 k

Source Imbalance

G = 51000

5 V, V

= 4 V to +3.0 V

G = 51000

15 V, V

= 12 V to +10.9 V

OUTPUT

Output Swing

= 20 k

) + 25

(+V

) 70

) + 25

(+V

) 70

= 100 k

) + 7

(+V

) 25

) + 7

(+V

) 25

Short-Circuit Current

Short Circuit to Ground

DYNAMIC RESPONSE

Small Signal 3 dB Bandwidth
G = 5

kHz

G = 100

kHz

G = 1000

0.4

kHz

Slew Rate

+0.05/0.06

Settling Time to 0.01%

5 V, +5 V Output Step

G = 5

135

G = 100

350

Settling Time to 0.01%

15 V, +15 V Output Step

G = 5

330

G = 100

560

Overload Recovery

50% Input Overload

NOTES

Does not include effects of external resistor R

See Table III for total RTI errors.

See Applications section for input range, gain range and common-mode range.

Specifications subject to change without notice.

REV. A

AD627SPECIFICATIONS

ORDERING GUIDE

Model

Temperature Range

Package Descriptions

Package Options

AD627AN

C to +85

Plastic DIP

N-8

AD627AR

C to +85

Small Outline (SOIC)

SO-8

AD627AR-REEL

C to +85

8-Lead SOIC 13" Reel

SO-8

AD627AR-REEL7

C to +85

8-Lead SOIC 7" Reel

SO-8

AD627BN

C to +85

Plastic DIP

N-8

AD627BR

C to +85

Small Outline (SOIC)

SO-8

AD627BR-REEL

C to +85

8-Lead SOIC 13" Reel

SO-8

AD627BR-REEL7

C to +85

8-Lead SOIC 7" Reel

SO-8

BOTH DUAL AND SINGLE SUPPLIES

Model

AD627A

AD627B

Specification

Conditions

Min

Typ

Max

Min

Typ

Max

Units

NOISE

Voltage Noise, 1 kHz

Total RTI Noise =

(eni)

(eno/ )

Input, Voltage Noise, eni

nV/

Output, Voltage Noise, eno

177

nV/

RTI, 0.1 Hz to 10 Hz

G = 5

1.2

V p-p

G = 1000

0.56

V p-p

Current Noise

f = 1 kHz

fA/

0.1 Hz to 10 Hz

1.0

pA p-p

REFERENCE INPUT

125

Gain to Output

Voltage Range

POWER SUPPLY

Operating Range

Dual Supply

1.1

Single Supply

2.2

Quiescent Current

Over Temperature

200

nA/

TEMPERATURE RANGE

For Specified Performance

+85

NOTES

See Applications section for input range, gain range and common-mode range.

Specifications subject to change without notice.

ABSOLUTE MAXIMUM RATINGS

Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

18 V

Internal Power Dissipation

Plastic Package (N) . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 W
Small Outline Package (R) . . . . . . . . . . . . . . . . . . . . . 0.8 W

IN, +IN . . . . . . . . . . . . . . . . . . . . . V

20 V to +V

+ 20 V

Common-Mode Input Voltage . . . . V

20 V to +V

+ 20 V

Differential Input Voltage (+IN (IN)) . . . . . . . . +V

)

Output Short Circuit Duration . . . . . . . . . . . . . . . . Indefinite
Storage Temperature Range N, R . . . . . . . . 65

C to +125

Operating Temperature Range . . . . . . . . . . . 40

C to +85

Lead Temperature Range (Soldering 10 sec) . . . . . . . . +300

CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the AD627 features proprietary ESD protection circuitry, permanent damage may
occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD
precautions are recommended to avoid performance degradation or loss of functionality.

WARNING!

ESD SENSITIVE DEVICE

NOTES

Stresses above those listed under Absolute Maximum Ratings may cause perma-

nent damage to the device. This is a stress rating only; functional operation of the
device at these or any other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute maximum rating
conditions for extended periods may affect device reliability.

Specification is for device in free air:

8-Lead Plastic DIP Package:

= 90

C/W.

8-Lead SOIC Package:

= 155

C/W.

AD627

REV. A

Typical Performance Characteristics

(@ +25 C V

= 5 V, R

= 20 k unless otherwise noted)

FREQUENCY Hz

100

NOISE nV/ Hz, RTI

100

10k

100k

GAIN = 5

GAIN = 100

GAIN = 1000

Figure 2. Voltage Noise Spectral Density vs. Frequency

FREQUENCY Hz

100

CURRENT NOISE fA/ Hz

100

10k

Figure 3. Current Noise Spectral Density vs. Frequency

COMMON-MODE INPUT Volts

3.200

2.000

15

10

INPUT BIAS CURRENT nA

3.000

2.800

2.600

2.400

2.200

Figure 4. I

BIAS

vs. CMV, V

15 V

TEMPERATURE C

5.5

3.5

1.5

60

140

40

INPUT BIAS CURRENT nA

20

100

120

5.0

4.5

2.5

2.0

4.0

3.0

= +5V

= 5V

= 15V

Figure 5. Input Bias Current vs. Temperature

TOTAL POWER SUPPLY VOLTAGE Volts

65.5

59.5

POWER SUPPLY CURRENT 

64.5

63.5

62.5

61.5

60.5

Figure 6. Supply Current vs. Supply Voltage

OUTPUT CURRENT mA

V+

OUTPUT VOLTAGE SWING Volts

(V+) 1

(V+) 2

(V+) 3

(V) +2

(V) +1

= 1.5V

SOURCING

= 2.5V

= 5V

= 15V

SINKING

= 1.5V

= 2.5V

= 5V

= 15V

Figure 7. Output Voltage Swing vs. Output Current

AD627

REV. A

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 Hz60 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
(R

) 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 V

OUT

is equal

to 5, whereas the gain from the output of A1 to V

OUT

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.

R5
200k

OUTPUT

+IN

IN

100k

REF

EXTERNAL GAIN RESISTOR

100k

25k

R6
200k

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 (R

) increases the gain by an amount equal

to (R4 + R1)/R

. The output voltage of the AD627 is given by the

following equation.

OUT

= [V

(+) V

()]

(5 + 200 k

) + V

REF

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 +V

and V

terminals are connected to the power supply.

The supply can either be bipolar (V

1.1 V to

18 V) or

single supply (V

= 0 V, +V

= +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

F 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 R

, or more pre-

cisely, by whatever impedance appears between Pins 1 and 8.
The gain is set according to the equation:

Gain = 5 + (200 k

)

= 200 k

/(Gain 5)

It follows that the minimum achievable gain is 5 (for R

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.

0.1 F

1.1V TO 18V

0.1 F

+1.1V TO +18V

+IN

IN

REF (INPUT)

OUT

OUTPUT

REF

+2.2V TO +36V

+IN

IN

REF (INPUT)

OUT

OUTPUT

REF

GAIN = 5 + (200k /R

)

0.1 F

Figure 33. Basic Connections for Single and Dual Supplies

AD627

REV. A

Table I. Recommended Values of Gain Resistors

Desired

1% Std Table

Resulting

Gain

Value of R

Gain

200 k

100 k

68.1 k

7.93

51.1 k

8.91

40.2 k

9.98

20 k

13.7 k

19.6

10 k

8.06 k

29.81

5.76 k

39.72

4.53 k

49.15

3.65 k

59.79

3.09 k

69.73

2.67 k

79.9

2.37 k

89.39

100

2.1 k

99.24

200

1.05 k

195.48

500

412

489.44

1000

205

980.61

Reference Terminal

The reference terminal potential defines the zero output voltage
and is especially useful when the load does not share a precise
ground with the rest of the system. It provides a direct means of
injecting a precise offset to the output. The reference terminal is
also useful when bipolar signals are being amplified as it can be
used to provide a virtual ground voltage.

Since the AD627 output voltage is developed with respect to the
potential on the reference terminal, it can solve many grounding
problems by simply tying the REF pin to the appropriate "local
ground." The REF pin should however be tied to a low imped-
ance point for optimal CMR.

Input Range Limitations in Single Supply Applications

In general, the maximum achievable gain is determined by the
available output signal range. However, in single supply applica-
tions where the input common mode voltage is close to or equal
to zero, some limitations on the gain can be set. While the In-
put, Output and Reference Pins have ranges that are nominally
defined on the specification pages, there is a mutual interdepen-
dence between the voltage ranges on these pins. Figure 34 shows
the simplified schematic of the AD627, driven by a differential
voltage V

DIFF

which has a common mode component, V

. The

voltage on the output of op amp A1 is a function of V

DIFF

, V

the voltage on the REF pin and the programmed gain. This
voltage is given by the equation:

= 1.25 (V

+ 0.5 V) 0.25 V

REF

DIFF

(25 k

0.625)

We can also express the voltage on A1 as a function of the ac-
tual voltages on the IN and +IN pins (V and V+)

= 1.25 (V + 0.5 V) 0.25 V

REF

(V+ V) 25 k

A1's output is capable of swinging to within 50 mV of the nega-
tive rail and to within 200 mV of the positive rail. From either of
the above equations, it is clear that an increasing V

REF

, (while it

acts as a positive offset at the output of the AD627), tends to
decrease the voltage on A1. Figures 35 and 36 show the maxi-
mum voltages that can be applied to the REF pin, for a gain of
five for both the single and dual supply cases. Raising the input
common-mode voltage will increase the voltage on the output of
A1. However, in single supply applications where the common-
mode voltage is low, a differential input voltage or a voltage on
REF that is too high can drive the output of A1 into the ground
rail. Some low side headroom is added by virtue of both inputs
being shifted upwards by about 0.5 V (i.e., by the V

of Q1

and Q2). The above equations can be used to check that the
voltage on amplifier A1 is within its operating range.

Table II gives values for the maximum gains for various single
supply input conditions. The resulting output swings shown
refer to 0 V. The voltages on the REF pins has been set to either

200k

OUTPUT

+IN

IN

100k

REF

EXTERNAL GAIN RESISTOR

100k

25k

200k

IN

+IN

V+

DIFF

Figure 34. Amplifying Differential Signals with a Common-Mode Component

Table II. Maximum Gain for Low Common-Mode Single Supply Applications

REF

Supply

(1%

Resulting

Output Swing

Pin

Voltage

Tolerance)

Max Gain

WRT 0 V

100 mV, V

= 0 V

2 V

+5 V to +15 V

28.7 k

12.0

0.8 V to 3.2 V

50 mV, V

= 0 V

2 V

+5 V to +15 V

10.7 k

23.7

0.8 V to 3.2 V

10 mV, V

= 0 V

2 V

+5 V to +15 V

1.74 k

119.9

0.8 V to 3.2 V

V = 0 V, V+ = 0 V to 1 V

1 V

+10 V to +15 V

78.7 k

7.5

1 V to 8.5 V

V = 0 V, V+ = 0 mV to 100 mV

1 V

+5 V to +15 V

7.87 k

1 V to 4.1 V

V = 0 V, V+ = 0 mV to 10 mV

1 V

+5 V to +15 V

7.87

259.1

1 V to 3.6 V

AD627

REV. A

2 V or 1 V to maximize the available gain and output swing.
Note that in most cases, there is no advantage to increasing the
single supply to greater than 5 V (the exception being an input
range of 0 V to 1 V).

() Volts

REF

 Volts

MAXIMUM V

REF

MINIMUM V

REF

Figure 35. Reference Input Voltage vs. Negative Input
Voltage, V

5 V, G = 5

() Volts

0.5

REF

 Volts

0.5

1.5

2.5

3.5

4.5

MAXIMUM V

REF

MINIMUM V

REF

Figure 36. Reference Input Voltage vs. Negative Input
Voltage, V

= +5 V, G = 5

Output Buffering

The AD627 is designed to drive loads of 20 k

or greater but

can deliver up to 20 mA to heavier loads at lower output voltage
swings (see Figure 7). If more than 20 mA of output current is
required at the output, the AD627's output should be buffered
with a precision op amp such as the OP113 as shown in Figure
37 (shown for the single supply case). This op amp can swing
from 0 V to 4 V on its output while driving a load as small as
600

0.1 F

REF

0.1 F

OUT

AD627

OP113

Figure 37. Output Buffering

INPUT AND OUTPUT OFFSET ERRORS

The low errors of the AD627 are attributed to two sources,
input and output errors. The output error is divided by G when
referred to the input. In practice, the input errors dominate at
high gains and the output errors dominate at low gains. The
total offset error for a given gain is calculated as:

Total Error RTI = Input Error + (Output Error/Gain)
Total Error RTO = (Input Error

G) + Output Error

RTI offset errors and noise voltages for different gains are shown
below in Table III.

Table III. RTI Error Sources

Max Total

RTI Offset Error

RTI Offset Drift

Total RTI Noise

V/ C

nV/

Gain AD627A

AD627B AD627A

AD627B AD627A & AD627B

450

250

350

200

300

175

3.5

1.5

270

160

3.2

1.2

100

270

155

3.1

1.1

500

252

151

1000 251

151

Make vs. Buy: A Typical Application Error Budget

The example in Figure 38 serves as a good comparison between
the errors associated with an integrated and a discrete in amp
implementation. A

100 mV signal from a resistive bridge

(common-mode voltage = +2.5 V) is to be amplified. This ex-
ample compares the resulting errors from a discrete two op
amp in amp and from the AD627. The discrete implementation
uses a four-resistor precision network (1% match, 50 ppm/

tracking).

The errors associated with each implementation are detailed in
Table IV and show the integrated in amp to be more precise,
both at ambient and over temperature. It should be noted that
the discrete implementation is also more expensive. This is pri-
marily due to the relatively high cost of the low drift precision
resistor network.

Note, the input offset current of the discrete in amp implemen-
tation is the difference in the bias currents of the two op amps,
not the offset currents of the individual op amps. Also, while the
values of the resistor network are chosen so that the inverting
and noninverting inputs of each op amp see the same impedance
(about 350

), the offset current of each op amp will add an

additional error which must be characterized.

Errors Due to AC CMRR

In Table IV, the error due to common-mode rejection is the
error that results from the common-mode voltage from the
bridge 2.5 V. The ac error due to nonideal common-mode
rejection cannot be calculated without knowing the size of the ac
common-mode voltage (usually interference from 50 Hz/60 Hz
mains frequencies).

A mismatch of 0.1% between the four gain setting resistors will
determine the low frequency CMRR of a two op amp in amp.
The plot in Figure 38 shows the practical results, at ambient
temperature, of resistor mismatch. The CMRR of the circuit in
Figure 39 (Gain = 11) was measured using four resistors which

AD627

REV. A

had a mismatch of almost exactly 0.1% (R1 = 9999.5

, R2 =

999.76

, R3 = 1000.2

, R4 = 9997.7

). As expected the

CMRR at dc was measured at about 84 dB (calculated value
is 85 dB). However, as the frequency increases, the CMRR
quickly degrades. For example, a 200 mV peak-peak harmonic
of the mains frequency at 180 Hz would result in an output
voltage of about 800

V. To put this in context, a 12-bit data

acquisition system with an input range of 0 V to 2.5 V, has an
LSB weighting of 610

By contrast, the AD627 uses precision laser trimming of internal
resistors along with patented CMR trimming to yield a higher
dc CMRR and a wider bandwidth over which the CMRR is flat
(see Figure 20).

AD296

1/2

999.76

9999.5

1000.2

9997.7

OUT

+5V

VIN+

VIN

5V

AD296

1/2

Figure 39. 0.1% Resistor Mismatch Example

+5V

40.2k

+10ppm/ C

AD627A

2.5V

OUT

350

100mV

+5V

AD627A GAIN = 9.98

(

5+(200k /R

)

1/2

350 *

3.15k *

350 *

2.5V

3.15k *

OUT

+5V

1/2

"HOMEBREW" IN AMP, G = 10

*1% REGISTER MATCH, 50ppm/ C TRACKING

LT1078IS8

Figure 38. Make vs. Buy

FREQUENCY Hz

120

CMRR dB

110

100

10k

100k

Figure 40. CMRR Over Frequency of Discrete In Amp in
Figure 39

Ground Returns for Input Bias Currents

Input bias currents are those dc currents that must flow in
order to bias the input transistors of an amplifier. These are
usually transistor base currents. When amplifying "floating"
input sources such as transformers, or ac-coupled sources,
there must be a direct dc path into each input in order that the
bias current can flow. Figure 41 shows how a bias current
path can be provided for the case of transformer coupling,
capacitive ac-coupling and for a thermocouple application.

Table IV. Make vs. Buy Error Budget

"Homebrew"

Total Error

Error Source

AD627 Circuit Calculation

Circuit Calculation

AD627-ppm

Homebrewppm

ABSOLUTE ACCURACY at T

= +25

Total RTI Offset Voltage, mV

(250

V + (1000

V/10))/100 mV

(180

2)/100 mV

3500

3600

Input Offset Current, nA

1 nA

350

/100 mV

20 nA

350

/100 mV

3.5

Internal Offset Current (Homebrew Only)

Not Applicable

0.7 nA

350

/100 mV

2.45

CMRR, dB

77 dB¡141 ppm

2.5 V/100 mV

(1% Match

2.5 V)/10/100 mV

3531

25000

Gain

0.35% + 0.1%

1% Match

13500

10000

Total Absolute Error

20535

38672

DRIFT TO +85

Gain Drift, ppm/

(75 + 10) ppm/

50 ppm/

3900

3000

Total RTI Offset Voltage, mV/

(3.0

C + (10

C/10))

3.5

C)/100 mV

C/100 mV

2600

4200

Input Offset Current, pA/

(16 pA/

350

C)/100 mV (33 pA/

350

C)/100 mV

3.5

Total Drift Error

6504

7207

Grand Total Error

27039

45879

AD627

REV. A

In dc-coupled resistive bridge applications, providing this path
is generally not necessary as the bias current simply flows from
the bridge supply, through the bridge and into the amplifier.
However, if the impedance that the two inputs see are large, and
differ by a large amount (>10 k

), the offset current of the

input stage will cause dc errors compatible with the input offset
voltage of the amplifier.

LOAD

TO POWER
SUPPLY
GROUND

INPUT

+INPUT

AD627

OUT

REFERENCE

Figure 41a. Ground Returns for Bias Currents with Trans-
former Coupled Inputs

LOAD

TO POWER
SUPPLY
GROUND

INPUT

+INPUT

AD627

OUT

REFERENCE

Figure 41b. Ground Returns for Bias Currents with Ther-
mocouple Inputs

LOAD

TO POWER
SUPPLY
GROUND

INPUT

+INPUT

AD627

OUT

REFERENCE

100k

Figure 41c. Ground Returns for Bias Currents with AC
Coupled Inputs

Layout and Grounding

The use of ground planes is recommended to minimize the
impedance of ground returns (and hence the size of dc errors).
In order to isolate low level analog signals from a noisy digital
environment, many data-acquisition components have separate
analog and digital ground returns (Figure 42). All ground pins
from mixed signal components such as analog-to-digital converters
should be returned through the "high quality" analog ground
plane. Digital ground lines of mixed signal components should
also be returned through the analog ground plane. This may
seem to break the rule of keeping analog and digital grounds
separate. However, in general, there is also a requirement to
keep the voltage difference between digital and analog grounds
on a converter as small as possible (typically <0.3 V). The
increased noise, caused by the converter's digital return currents
flowing through the analog ground plane, will generally be negli-
gible. Maximum isolation between analog and digital is achieved
by connecting the ground planes back at the supplies.

If there is only a single power supply available, it must be shared
by both digital and analog circuitry. Figure 43 shows the how to
minimize interference between the digital and analog circuitry.
As in the previous case, separate analog and digital ground
planes should be used (reasonably thick traces can be used as an
alternative to a digital ground plane). These ground planes
should be connected at the power supply's ground pin. Separate
traces (or power planes) should be run from the power supply to
the supply pins of the digital and analog circuits. Ideally each
device should have its own power supply trace, but these can be
shared by a number of devices as long as a single trace is not
used to route current to both digital and analog circuitry.

INPUT PROTECTION

As shown in the simplified schematic (Figure 32), both the
inverting and noninverting inputs are clamped to the positive
and negative supplies by ESD diodes. In addition to this a 2 k

series resistor on each input provides current limiting in the
event of an overvoltage. These ESD diodes can tolerate a maxi-
mum continuous current of 10 mA. So an overvoltage, (that is
the amount by which input voltage exceeds the supply voltage),
of

20 V can be tolerated. This is true for all gains, and for

power on and off. This last case is particularly important since
the signal source and amplifier may be powered separately.

If the overvoltage is expected to exceed 20 V, additional external
series resistors current limiting resistors should be used to keep
the diode current to below 10 mA.

DIGITAL POWER SUPPLY

0.1 F

IN1

IN2

AGND DGND

AD7892-2

ADC

AGND

PROCESSOR

0.1 F

AD627

ANALOG POWER SUPPLY

+5V

5V

GND

+5V

Figure 42. Optimal Grounding Practice for a Bipolar Supply Environment with Separate Analog and Digital Supplies

AD627

REV. A

RF INTERFERENCE

All instrumentation amplifiers can rectify high frequency out-of-
band signals. Once rectified, these signals appear as dc offset
errors at the output. The circuit of Figure 44 provides good RFI
suppression without reducing performance within the in amp's
passband. Resistor R1 and capacitor C1 (and likewise, R2 and
C2) form a low pass RC filter that has a 3 dB BW equal to:
F = 1/(2

R1C1). Using the component values shown, this

filter has a 3 dB bandwidth of approximately 8 kHz. Resistors
R1 and R2 were selected to be large enough to isolate the circuit's
input from the capacitors, but not large enough to significantly
increase the circuit's noise. To preserve common-mode rejec-
tion in the amplifier's pass band, capacitors C1 and C2 need to
be 5% mica units, or low cost 20% units can be tested and
"binned" to provide closely matched devices.

OUT

REFERENCE

0.33 F

0.01 F

0.33 F

AD627

1000pF

20k

0.022 F

1000pF

LOCATE C1C3 AS CLOSE TO

THE INPUT PINS AS POSSIBLE

+IN

IN

Figure 44. Circuit to Attenuate RF Interference

Capacitor C3 is needed to maintain common-mode rejection at
the low frequencies. R1/R2 and C1/C2 form a bridge circuit
whose output appears across the in amp's input pins. Any mis-
match between C1 and C2 will unbalance the bridge and reduce
common-mode rejection. C3 insures that any RF signals are
common mode (the same on both in amp inputs) and are not
applied differentially. This second low pass network, R1 + R2
and C3, has a 3 dB frequency equal to: 1/(2

(R1 + R2) (C3)).

Using a C3 value of 0.022

F as shown, the 3 dB signal BW of

this circuit is approximately 200 Hz. The typical dc offset shift
over frequency will be less than 1 mV and the circuit's RF signal
rejection will be better than 57 dB. The 3 dB signal bandwidth

of this circuit may be increased by reducing the value of resistors
R1 and R2. The performance is similar to that using 20 k

resistors, except that the circuitry preceding the in amp must
drive a lower impedance load.

The circuit of Figure 44 should be built using a PC board with a
ground plane on both sides. All component leads should be as
short as possible. Resistors R1 and R2 can be common 1%
metal film units but capacitors C1 and C2 need to be

tolerance devices to avoid degrading the circuit's common-
mode rejection. Either the traditional 5% silver mica units or
Panasonic

2% PPS film capacitors are recommended.

APPLICATIONS CIRCUITS
A Classic Bridge Circuit

Figure 45 shows the AD627 configured to amplify the signal
from a classic resistive bridge. This circuit will work in either
dual or single supply mode. Typically the bridge will be excited
by the same voltage as is used to power the in amp. Connecting
the bottom of the bridge to the negative supply of the in amp (usu-
ally either 0, 5 V, 12 V or 15 V), sets up an input common
mode voltage that is optimally located midway between the
supply voltages. It is also appropriate to set the voltage on the
REF pin to midway between the supplies, especially if the input
signal will be bipolar. However the voltage on the REF pin can
be varied to suit the application. A good example of this is when
the REF pin is tied to the V

REF

pin of an Analog-to-Digital

Converter (ADC) whose input range is (V

REF

). With an

available output swing on the AD627 of (V

+ 100 mV) to

(+V

150 mV) the maximum programmable gain is simply this

output range divided by the input range.

AD627

OUT

DIFF

= 200k

GAIN-5

REF

0.1 F

Figure 45. A Classic Bridge Circuit

AGND DGND

AD7892-2

ADC

DGND

PROCESSOR

0.1 F

POWER SUPPLY

+5V

GND

AD627

Figure 43. Optimal Ground Practice in a Single Supply Environment

AD627

REV. A

C3430a012/99 (rev. A)

PRINTED IN U.S.A.

A 4 mA-to-20 mA Single Supply Receiver

Figure 46 shows how a signal from a 4 mA-to-20 mA transducer
can be interfaced to the AD

C812, a 12-bit ADC with an em-

bedded microcontroller. The signal from a 4 mA-to-20 mA
transducer is single ended. This initially suggests the need for
a simple shunt resistor, to convert the current to a voltage at the
high impedance analog input of the converter. However, any
line resistance in the return path (to the transducer) will add a
current dependent offset error. So the current must be sensed
differentially.

In this example, a 24.9

shunt resistor generates a maximum

differential input voltage to the AD627 of between 100 mV (for
4 mA in) and 500 mV (for 20 mA in). With no gain resistor
present, the AD627 amplifies the 500 mV input voltage by a
factor of 5, to 2.5 V, the full-scale input voltage of the ADC.
The zero current of 4 mA corresponds to a code of 819 and the
LSB size is 4.9 mA.

A Thermocouple Amplifier

Because the common-mode input range of the AD627 extends
0.1 V below ground, it is possible to measure small differential
signals which have low, or no, common mode component. Fig-
ure 47 shows a thermocouple application where one side of the
J-type thermocouple is grounded.

AD627

0.1 F

24.9

G = 5

REF

420mA

TRANSDUCER

AGND

DGND

AIN 07

REF

+5V

AVDD

+5V

DVDD

AD C812

MicroConverter

+5V

LINE

IMPEDANCE

0.1 F

MicroConverter is a trademark of Analog Devices, Inc.

Figure 46. A 4 mA-to-20 mA Receiver Circuit

Over a temperature range from 200

C to +200

C, the J-type

thermocouple delivers a voltage ranging from 7.890 mV to
10.777 mV. A programmed gain on the AD627 of 100 (R

2.1 k

) and a voltage on the AD627 REF pin of 2 V, results in

the AD627's output voltage ranging from 1.110 V to 3.077 V
relative to ground. For a different input range or different volt-
age on the REF pin, it is important to check that the voltage on
internal node A1 (see Figure 34) is not driven below ground).
This can be checked using the equations in the section entitled
Input Range Limitations in Single Supply Applications.

2.1k

AD627

0.1 F

OUT

+5V

J-TYPE

THERMOCOUPLE

REF

Figure 47. Amplifying Bipolar Signals with Low Common-
Mode Voltage

8-Lead Plastic DIP

(N-8)

0.430 (10.92)

0.348 (8.84)

0.280 (7.11)
0.240 (6.10)

PIN 1

SEATING
PLANE

0.022 (0.558)
0.014 (0.356)

0.060 (1.52)
0.015 (0.38)

0.210 (5.33)

MAX

0.130
(3.30)
MIN

0.070 (1.77)
0.045 (1.15)

0.100

(2.54)

BSC

0.160 (4.06)
0.115 (2.93)

0.325 (8.25)
0.300 (7.62)

0.015 (0.381)
0.008 (0.204)

0.195 (4.95)
0.115 (2.93)

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

8-Lead SOIC

(SO-8)

0.0098 (0.25)
0.0075 (0.19)

0.0500 (1.27)
0.0160 (0.41)

8
0

0.0196 (0.50)
0.0099 (0.25)

0.1968 (5.00)
0.1890 (4.80)

0.2440 (6.20)
0.2284 (5.80)

PIN 1

0.1574 (4.00)
0.1497 (3.80)

0.0500 (1.27)

BSC

0.0688 (1.75)
0.0532 (1.35)

SEATING

PLANE

0.0098 (0.25)
0.0040 (0.10)

0.0192 (0.49)
0.0138 (0.35)

Ð­Ð»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: AD627

ÐÐ»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: AD627