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(ความเคน) คือแรงที่กระทํากับวัตถุตอ พืน้ ที่ = force/area หนวย N/m2 The Strain gages Strain(ความเครียด) อัตราการเปลี่ยนรูปของวัตถุที่เกิดจาก แรงกระทําตอหนวยความยาว = ± ∆L / L L The Strain gages Strain(ความเครียด) อัตราการเปลี่ยนรูปของวัตถุที่เกิดจาก แรงกระทําตอหนวยความยาว = ± ∆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