Instrumentation
Amplifier
and application
Napat
Watjanatepin
EE department
Rajamangala University of
Technology Suvarnabhumi
1
Why instrumentation Amplifier ?
Electrocardiography
Why instrumentation Amplifier ?
1. It is used for very small signal instrument. For
example human’s signal ,from heart , ECG,EKG.
2. These signal have a lot of Noises ,we need to
reject them.
3. Can not use regular Differential amp, because
of its lower input Z.
4. It is necessary for use more than 1
differential amp.
5. Instrumentation amplifier was the special
differential amp.
Instrumentation Amplifier
The differential amp
turns out to be rather
limited in its
performance because of
the low input impedance
of (R2 + R1). To improve
this, two bootstrapped
buffer amplifiers (which
are just op-amps with
unity gain) are commonly
added, which results in
the simple
instrumentation
amplifier:
Instrumentation Amplifier
AC coupling
All of these problems are
caused by very lowfrequencies (DC is zero
frequency), so we need to use
a high-pass filter with a very
low cutoff frequency. We
need to keep the cutoff
frequency very low to avoid
degrading the ECG signal, so
the capacitor needs to be
large (0.47 or 1 uF) and the
resistor similarly large
(e.g. 1 M).
Differential Amplifier
The differential
amplifier
gain (Adiff)=g
= gR/R
= V0/(V1-V2)
V2=250mV
V1=255mV
Vo = g(V1-V2)
=1(255-250)mV = 5mV
Differential Amplifier
Common mode voltage gain
Usually if V+ = VVo will be 0V
because no
different input
voltage
In practical its
impossible :
1.-input ,+input are
not symmetrical.
2. Error from in
circuit resistors.
Differential Amplifier
Common mode
voltage gain (ACM)
ACM= VOCM/VCM
Common mode input
voltage = VCM
Common mode output
voltage = VOCM
Differential Amplifier
Common mode
adjustment its use
for adjust the
lowest output
voltage
Vcm=1V
VOCM=1.33mV
Ex, Find the Common
mode gain, ACM
ACM= VOCM/VCM
= 1.33mV/1V
= 0.00133
Differential Amplifier
Common mode rejection ratio(CMRR)
To measures the tendency
of the differential amp to
reject input signals common
to both input leads.
CMRR is often important in
reducing noise on transmission
lines. For example, when
measuring a thermocouple in a
noisy environment, the noise
from the environment appears
as an offset on both input
leads.
Differential Amplifier
Common mode rejection ratio(CMRR)
CMRR = ADiff/ACM
CMRRdB = 20 log CMRR
Usually ACM is very low.
Then
ADiff is high.
CMRR is very high.(that good)
Ex, If ACM=0.00133 and
ADiff = 10 find the CMRR.
CMRR = 10/0.00133
= 7500
= 77.5 dB
Simple measurement by
Differential Amplifier
Simple measurement by
Differential Amplifier
VDiff
VDiff = heart’s signal
VCM = Noise signal
Simple measurement by
Differential Amplifier
Ex, If the heart’s signal is
VR2 the CMRRdB=100 db
find 1.The ACM and Vo
gR/R
=1M/10K
=100
CMRR=10100/20
= 100000
ACM = ADiff/CMRR
= 100/100000=0.001
VoCM = ACM VCM
= 0.001(500mV)=5uV
Vo = ADiff(V1-V2)
= 100(10mV)=1V
Instrumentation Amplifier
Three op-amp Instrumentation Amplifier
ADiff=1,g=1
Vo’=V1-V2
Vo=Vo’
Instrumentation Amplifier
ImR=(V1-V2)/mR
g=Vo’/(V1-V2)
=1+(2/m)
m=mR/R
Vo = Vo’
Hence g=1
Vo’= ImR(R+R+mR)
= [(V1-V2)/mR] (2R+mR)
Instrumentation Amplifier
Ex, Find ADiff,Vo
m=mR/R
=404/20k
=0.0202
V2=5.000V
mR=404
V1=5.025V
R=20K
ADiff=1+(2/m)
=1+2/0.0202
=100
Vo’=ADiff(V1-V2)
=100(0.025V)
=2.5V
Vo =Vo’
=2.5V
Instrumentation Amplifier
Remote voltage sensing
Voltage drop problem
Instrumentation Amplifier
Remote voltage sensing
Current boost
INA 110 instrumentation Amplifier
INA 110 instrumentation Amplifier
Zin =10Tohms
INA 110 instrumentation Amplifier
Programmable gain
by Pin connection
INA 110 instrumentation Amplifier
Programmable gain
by RG connection
RG = [40K/(g-1)]-50
Ex, If we want g=600
find the RG
RG = [40000/(600-1)]-50
= 16.8 ohms
INA 110 instrumentation Amplifier
Application
Eye motion-controlled mouse
When the eyes move,
biopotentials are generated. These
corneo-retinal potentials (from the
back of the eye to the front of the
eye) biopotentials are generated
by the retinal dipole, and variously
called the EOG (Electrooculogram), ERG (Retinogram)
or ENG (Nystagmogram). The
potentials vary with a number of
factors, e.g. light level, blinking,
and tends to
drift
INA 110 instrumentation Amplifier
3-pole Butterworth lowpass filter
The AD620 instrumentation Amplifier
The AD620 instrumentation Amplifier
http://en.wikipedia.org/wiki/Instrumentation_amplifier
Analog Devices AD620 find widespread use in
working with low-level signals with large
common-mode components in noisy
environments - just the sort of situation we
find in biomedical engineering.
The AD620 instrumentation Amplifier
Application 1
The AD620 instrumentation Amplifier
Application 2
The AD620 instrumentation Amplifier
Application 3
Instrumentation Amplifier
PSpice Simulation of
Instrumentation Amplifier
PSpice Simulation of
Instrumentation Amplifier
V2
V1
V0
PSpice Simulation of
Instrumentation Amplifier
Calculation
Simulation
V1 = 0.46505 V
V2 = 0.45536 V
Vo = 0.02909 V
The Strain gages
Strain gauge
The Strain gages
The strain gage installed for measure stress of brick wall.
The Strain gages
The strain gage used for measure tension
and compression.
Tension(แรงดึง)
Compression(แรงกด)
The Strain gages
Stress(ความเคน)
คือแรงที่กระทํากับวัตถุตอ พืน้ ที่
= force/area หนวย N/m2
The Strain gages
Strain(ความเครียด)
อัตราการเปลี่ยนรูปของวัตถุที่เกิดจาก
แรงกระทําตอหนวยความยาว = ± ∆L / L
L
The Strain gages
Strain(ความเครียด)
อัตราการเปลี่ยนรูปของวัตถุที่เกิดจาก
แรงกระทําตอหนวยความยาว = ± ∆L / L
The Strain gages
The Strain gages
How its can measure compression and tension ?
The Strain gages
Strain gage can cued for measure the strain ∆ L / L
That means we can calculate force, weight,
Pressure ect.
Types of strain gage
1.Foil strain gage
(120-350 ohms)
2.Semiconductor strain gage
The Strain gages
Strain gage resistance
Rsg = R ± ∆R
The Strain gages
Do not have
tension or
compression
Rsg=R4=350
R4=R1=R2=R3
Balance bridge
V1=V2 =Vref/2
V1-V2 = 0
The Strain gages
Case1 Tension
R4= 350 + ∆R
V1 > V2
Case2
Compression
R4= 350 − ∆R
V1<V2
∆R
V1 −V 2 =VDiff = (Vref)
4R
http://dk.coe.psu.ac.th/lecture/sst/sst_slide07force/sld018.htm
The Strain gages
Ex
The Strain gages
Gage factor
The Strain gages
Ex
Instrumentation Amplifier with
Strain gage
Instrumentation Amplifier with
Strain gage
Ex
Temperature compensation gage
or Dummy gage
End of chapter