Design of

Instrumentation

Amplifier By

Dr Ravi Kumar Jatoth

Asst Professor

ECE Department

Objectives

Upon completion of this experiment you will be able to:

measure the differential gain for a basic differential amplifier.

measure the common- mode gain and calculate the CMRR for a basic differential amplifier.

build and test a discrete three op amp instrumentation amplifier.

add an offset voltage to the reference terminal of the instrumentation amplifier

Ideal Characteristics of an

Amplifier

High common mode rejection

ratio(CMRR)

High input impedance

High slew rate

Low output impedance

Low power consumption

Low thermal and time drift

WHErE ArE In-AMPS AnDDIFFErEncE

AMPS USED?

Data Acquisition

Medical Instrumentation Monitor and

control Electronics

Software-Programmable Applications .

Audio Applications High Speed Signal

conditioning Video Applications

Power control Applications

Instrumentation Amplifiers

Passive Transducer Measurement Configuration:

For passive transducers in a bridge configuration the voltage of interest is the

differential voltage e = VB - VA

Therefore need a difference amplifier with a committed adjustable gain Ad

Want Vo = Ad(VB - VA) = Ad e

VCM =

Want to reject VCM

V V EA B

2 2R

R R

R+DR

IA

Vo = Ad

e

Instrumentation Amplifiers:

Active Transducer Measurement Configuration:

For an active transducer the differential voltage e

created by the transducer is of interest

Therefore need a difference amplifier

with a committed adjustable gain Ad

Want Vo = Ad e

Surface whose temperature

is to be measured may be at

some non-zero potential (VCM)

relative to ground

Want to reject VCM

IA

Vo = Ad e

Differential Amplifier: (Single op-amp instrumentation amplifier)

To obtain vo in terms of v1and v2 use superposition theorem

Common-Mode Voltage For a differential input amplifier, common-mode voltage is defined as

the average of the two input voltages. [2]

8

-

++

Vp

+

Vn

-

+

Vo

Vcm

=

Vp+V

n

2

Common-Mode Voltage (Alternate defn.)

• For a differential amplifier, common-mode voltage is defined as the average of the two input voltages. [2]

gain mode-Common

gain mode-alDifferenti

where

cm

dm

cmcmiddmout

A

A

VAVAV

9

-

+

Vout

Vcm

-

+

Vid

Vid/2Vid/2

-

+

IOP1

Vcm

=

Vp+V

n

2

w here

Vp=V

cm+

Vid

2

Vn=V

cm-

Vid

2

Common-Mode Voltage • Ideally a differential input amplifier only responds to a differential input voltage, not

a common-mode voltage.

10

V-

V+V-

V+

Vb 0 Va 0

-

+

Vid

Vs+ 5

Vs- 5

Vcm 0

-

++3

2

6

74 OP1

-

+

Vo 0V

0V

V-

V+

Vb 0 Va 0

-

+

Vid

Vcm 1

-

++3

2

6

74 OP1

-

+

Vo 0V

0V

V-

V+

Vb 0 Va 1m

-

+

Vid

Vcm 1

-

++3

2

6

74 OP1

-

+

Vo 3.826745V

1000uV

CMRR and CMR

Common-Mode Rejection Ratio is defined as the ratio of the differential

gain to the common-mode gain

o CMR is defined as follows [2]:

CMR and CMRR are often used interchangeably

cm

dm

A

ACMRR

CMRRdBCMR 10log20

11

Ideal Differential Amplifier CMRR

• What is the CMRR of an ideal differential input amplifier (e.g. op-amp)?

• Recall that the ideal common-mode gain of a differential input amplifier is 0.

• Voltage Amplifier Model [1]

• Also recall the differential gain of an ideal op-amp is infinity.

• So

cm

dm

cm

dmOAideal

A

A

A

ACMRR

12

Vs

Rs

Ri

-

+

-

+

VCVS

-

+

Vi Vi

Ro

Rload-

+

Vo

Source Amplif ier

Adm

->Infinity

Load

Real Op-Amp CMRR In an operational amplifier, the differential

gain is known as the open-loop gain.

The open-loop gain of an operational

amplifier is fixed and determined by its

design

CMRR of Difference Amplifiers • A difference amplifier is made up of a differential amplifier (operational amplifier)

and a resistor network as shown below.

• The circuit meets our definition of a differential amplifier

• The output is proportional to the difference between the input signals

14

-

+

R3 R4

R1 R2

+

V1

+

V2

-

+

Vo

Ri1

Ri2

Ro

2

2

:) and (Assuming

follows as sresistanceinput mode-common and

mode-aldifferenti thedefine alsocan We

21

1

4231

RRR

RR

RRRR

ic

id

2432

111

21

432

11

then

and sresistanceoutput

have sourcesinput If equal.

ynecessarilnot and finite are

sources by theseen sResistance

si

si

ss

i

i

RRRR

RRR

RR

RRR

RR

DA Derivation

Use superposition to determine

Vo as a function of V1 and V2

V1 ‘off’

Non-inverting amplifier

1

2

43

422

43

42

1

22

1

So,

1

R

R

RR

RVV

RR

RVV

R

RVV

o

p

po

18

Vp

-

+

R3 R4

R1 R2

+

V2

-

+

Vo2

Ri1

Ri2

Ro

DA Derivation

• V2 ‘off’

• Inverting amplifier

1

211

R

RVVo

19

-

+

R3 R4

R1 R2

+

V1

-

+

Vo1

Ri1

Ri2

Ro

DA Derivation

Combining the contributions of V1 and V2

Re-arranging yields

1

2

43

42

1

21

21

1R

R

RR

RV

R

RVV

VVV

o

ooo

12

1

2

4

3

2

1

4

3

2

1

12

1

2

then If

1

1

VVR

RV

R

R

R

R

R

R

R

R

VVR

RV

o

o

20

DA CMRR Let’s replace V1 and V2 with our alternate definition of the inputs (in terms

of differential-mode and common-mode signals)

It is readily observed that an ideal difference amplifier’s output should only amplify the differential-mode signal…not the common-mode signal.

24

dmo

dmcm

dmcmo

o

dmcm

dmcm

VR

RV

VV

VV

R

RV

VVR

RV

VVV

VVV

1

2

1

2

12

1

2

2

1

22

2

2

-

+

R1 R2

R1 R2

-

+

VoVcm

+

Vdm/2

+Vdm/2

DA CMRR This assumes that the operational amplifier is ideal and that the resistors

are balanced.

Keeping the assumption that the operational amplifier is ideal, let’s see

what happens when an imbalance factor (ε) is introduced.

25

-

+

R1 R2

R1

-

+

VoVcm

+

Vdm/2

+Vdm/2 R2(1- )

DA CMRR Using superposition we find that

After some algebra we find that [1]

As expected, an imbalance affects the differential and common-mode gains, which will affect CMRR!

As the error->0, Adm->R2/R1 and Acm->0.

1

11

2

1

2 21

2

21

2

1

2

RR

R

RR

RVV

R

RVVV dm

cmdm

cmo

21

2

21

21

1

2

2

21

where

RR

RA

RR

RR

R

RA

VAVAV

cm

dm

cmcmdmdmo

26

DA CMRR

Since we have equations for Acm and Adm, let’s look at CMR

If the imbalance is sufficiently small we can neglect its effect on Adm

With that and some algebra we find [1]

1

2

10

1

log20)(R

R

dBCMR

21

2

21

21

1

2

1010

2

21

log20log20)(

RR

R

RR

RR

R

R

A

AdBCMR

cm

dm

27

DA CMRR

This equation shows two very important relationships

As the gain of a difference amplifier increases (R2/R1), CMR increases

As the mismatch (ε) increases, CMR decreases

Please remember that this just shows the effects of the resistor network

and assumes an ideal amplifier

1

2

10

1

log20)(R

R

dBCMR

28

DA CMRR Another possible source for CMRR degradation is the impedance at

the reference pin.

So far we have connected this pin to low-impedance ground.

Placing and impedance here will disturb the voltage divider we come

across during superposition analysis.

This will negatively affect CMR

29

-

+

R1 R2

R1 R2

-

+

VoVcm

+

Vdm/2

+Vdm/2

Instrumentation Amplifiers:

Differential Amplifier: (Single op-amp instrumentation amplifier)

Short input to v2 (Inverting Configuration)

Instrumentation Amplifiers:

Differential Amplifier: (Single op-amp instrumentation amplifier)

Short input to v1 (Noninverting Configuration)

Instrumentation Amplifiers:

Differential Amplifier: (Single op-amp instrumentation amplifier)

To obtain vo in terms of v1and v2 use superposition theorem

Instrumentation Amplifiers:

Differential Amplifier: (Single op-amp instrumentation amplifier)

Differential Input Impedance: Rin, Rid, Zid, Zd

Zd = 2R1 Zd is limited

Instrumentation Amplifiers:

Three Op Amp Instrumentation Amplifier:

CMRR and Zin are very important attributes of an IA

Can increase Zin of difference amplifier configuration by adding unity gain buffers

or buffers with gain

Instrumentation Amplifiers:

Three Op Amp Instrumentation Amplifier:

CMRR and Zin are very important attributes of an IA

Can increase Zin of difference amplifier configuration by adding buffers

Common mode signals are not amplified if common R1 is used and

connection to ground is removed.

Instrumentation Amplifiers:

Transducer and Three Op Amp IA Circuit Diagram:

Instrumentation Amplifier

Experiment

Variable Gain

Instrumentation Amplifiers 69

Analog Devices Inc. is the largest supplier of

instrumentation amplifiers in the world. The AD620 is a low cost, high accuracy instrumentation amplifier which requires

only one external resistor to set gains of 1 to 1000. Furthermore, the AD620 offers

lower power (only 1.3 mA max supply current), making it a good fit for battery

powered, portable (or remote) applications.

The AD620, with its high accuracy of 40 ppm maximum nonlinearity, low offset

voltage of 50 µV max and offset drift of 0.6 µV/°C max, is ideal for use in precision

data acquisition systems, such as weigh scales and transducer interfaces. The

low noise, low input bias current, and low power of the AD620 also make it well

suited for medical applications such as ECG and noninvasive blood pressure

monitors.

The low input bias current of 1.0 nA max is made possible with the use of

Superbeta processing in the input stage. The AD620 works well as a preamplifier

due to its low input voltage noise of 9 nV/Hz at 1 kHz, 0.28 µV p-p in the 0.1 Hz to

10 Hz band, 0.1 pA/µHz input current noise. The AD620 is also well suited for

multiplexed applications with its settling time of 15 µs to 0.01% and its cost is low

enough to enable designs with one in amp per channel.

AD620

Specifications

70

http://products.analog.com/products/info.asp?product=AD620

common-mode rejection ratio

(CMRR): The ratio of the

common-mode interference

voltage at the input of a

circuit, to the corresponding

interference voltage at the

output.

Electrostatic Warning for the

AD620 In-Amp

71

AD620 vs opamp

72

Make vs. Buy: A Typical Bridge Application Error Budget The AD620 offers improved performance over “homebrew” three op amp IA designs, along with smaller size, fewer components and lower supply current. In the typical application, a gain of 100 is required to amplify a bridge output of 20 mV full scale over the industrial temperature range of –40°C to +85°C. Regardless of the system in which it is being used, the AD620 provides greater accuracy, and at low power and price. Note that for the homebrew circuit, the OP07 specifications for input voltage offset and noise have been multiplied by 2, because a three op amp type in-amp has two op amps at its inputs.

Error Budget

73

Op07 vs LM741

The OP-07 has very low input offset voltage (25µV max for

OP-07A) which is obtained by trimming at the wafer stage.

These low offset voltages generally eliminate any need for external nulling. The OP-07 also features low input bias

current (±2nA for OP-07A) and high open-loop gain

(300V/mV for the OP-07A). The low offsets and high open-

loop gain make the OP-07 particularly useful for high-gain

instrumentation applications.

The wide input voltage range of ±13V minimum combined

with the high CMRR of 110dB (OP-07A) and high input

impedance provides high accuracy in the non-inverting

circuit configuration. Excellent linearity and gain accuracy

can be maintained even at high closed-loop gains.

The OP-07 is available in five standard performance grades.

74

The LM741 series are general purpose operational amplifiers which

feature improved performance over industry standards like the LM709.

They are direct, plug-in replacements for the 709C, LM201, MC1439 and

748 in most applications.

Op07 vs 741

(Inexpensive

versions of each)

Op07 (Analog

Devices)

LM741 (National

Instruments)

Input Offset

Voltage

30 to 75 uV 6 to 7.5 mV

Input Offset

Current

.4 to 2.8 nA 200 to 300 nA

CMRR 110 dB Min 70 dB Min

Closed Loop BW

(gain = 1)

.6 MHz .437 MHz

Slew Rate .3 V/uSec .5 V/uSec 75

$0.44 for one LM741

25 for $8

$1.25 for one Op07

25 for $25

From Digikey