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Embedded Microcomputer Systems Lecture 8.1 by Jonathan W. Valvano Recap from last time - RTOS - Debugging/verification Lab 4 Application of RTOS - Input sound - Calculate FFT - Display amplitude versus frequency on the oLED Objectives - Designing analog circuits to run on single supply - Analog circuit design using op amps - Instrumentation amps - Noise measurements and reduction - Electret microphones Convert to single supply, Vpow = 3.3V 1) Design with +Vs -Vs 2) Assume ADC range is 0 to Vmax 3) Add an analog reference, Vref = ½ Vmax 4) Map -Vs to digital ground (ADC Vmin) Analog ground to Vref reference voltage +Vs to Vpow supply From an analog signal perspective, it behaves like a ±Vref supply From the digital signal perspective, everything is 0 to Vpow Example 1 yVIIVxyxR1R2R3VinR1R2VinV = -outR =3R +1R2R *1R2I2Iin+Vs-Vs0.1F0.1FEmbedded Microcomputer Systems Lecture 8.2 by Jonathan W. Valvano Regular design Vout = -5Vin R1 = 10k R2 = 50k R3 = 8.3k Example 2 Regular design Vout =2Vin Example 3 Vout =2Vin-1.23VEmbedded Microcomputer Systems Lecture 8.3 by Jonathan W. Valvano Noninverting amplifier with an effective -0.62 V to +2.1 V analog signal range. Note: LM4041CILPR shunt reference can be adjusted for various offsets The voltage at pin3 is Vin Due to feedback, the voltage at pin 2 is Vin Current across R1 is (Vref-Vin)/R1 Current across R2 is (Vin-Vout)/R2 These two currents are equal (Vref -Vin)/R1=(Vin-Vout)/R2 Solve (1.23-Vin)*R2/R1 = (Vin-Vout) Vout = Vin - (Vref -Vin)*R2/R1 Vout = (1+R2/R1)Vin - Vref *R2/R1 Vout = 2Vin – 1.23 Example 4 Single supply design Vout = 5(Vin-1.23) Use rail to rail op amp R1 = 10k R2 = 50k V2 = Vin, V1=1.23Embedded Microcomputer Systems Lecture 8.4 by Jonathan W. Valvano RVR12V = V - V outRR2121()12RR12V 11.2.7. Linear Mode Op Amp Circuits (EE445L review) This design example works with any analog circuit in the form Vout = A1V1 + A2V2 +…+AnVn + B designed with one op amp as shown in the following figure. VoutRfV1V2VnVrefVgreferencechipop amp{positivegains{negativegains Boiler plate circuit model for linear circuit design. The first step is to choose a reference voltage Common Error: If you use resistor divider from the +12V or +5V supply to create a voltage constant, then the power supply ripple will be added directly to your analog signal. The second step is to rewrite the design equation in terms of the reference voltage, Vref. Vout = A1V1 + A2V2 +…+AnVn + ArefVrefEmbedded Microcomputer Systems Lecture 8.5 by Jonathan W. Valvano The third step is to add a ground input to the equation. Ground is zero volts (Vg=0), but it is necessary to add this ground so that the sum of all the gains is equal to one. Vout = A1V1 + A2V2 +…+AnVn + ArefVref + AgVg Choose Ag such that A1 + A2 +…+An + Aref + Ag = 1 In other words, let Ag = 1 – ( A1 + A2 +…+An + Aref ) The fourth step is to choose a feedback resistor, Rf, in the range of 10 k to 1M. The larger the gains, the larger the value of Rf must be. Then calculate input resistors to create the desired gains. In particular, |A1| = Rf/R1 so R1 = Rf/|A1| |A2| = Rf/R2 so R2 = Rf/|A2| |An| = Rf/Rn so Rn = Rf/|An| |Aref| = Rf/Rref so Rref = Rf/|Aref| |Ag| = Rf/Rg so Rg = Rf/|Ag| The last step is to build the circuit. If the gain is positive, then the input resistor is connected to the positive terminal of the op amp. Conversely, if the gain is negative, then the input resistor is connected to the negative terminal of the op amp. For example, we will design the following analog circuit Vout = 5V1 - 3V2 + 2V3 – 10 The first step is to choose a reference voltage. The REF02 +5.00V voltage reference will be used. The second step is to rewrite the design equation in terms of the reference voltage. Vout = 5V1 - 3V2 + 2V3 - 2Vref The third step is to add a ground input to the equation so that the sum of all the gains is equal to one.Embedded Microcomputer Systems Lecture 8.6 by Jonathan W. Valvano Vout = 5V1 - 3V2 + 2V3 - 2Vref - Vg The fourth step is to choose a feedback resistor, Rf =150 k. The value is a multiple of the least common multiple of the gains: 5,3,2,1. Then calculate input resistors to create the desired gains. R1 = Rf/|A1| = 150 k/5 = 30 k R2 = Rf/|A2| = 150 k/3 = 50 k R3 = Rf/|A3| = 150 k/2 = 75 k Rref = Rf/|Aref| = 150 k/2 = 75 k Rg = Rf/|Ag| = 150 k/1 = 150 k The last step is to build the circuit. 150kVoutRfV1V2V3VrefREF02op amp75k150k30k75k50kVg A linear op amp circuit. Instrumentation Amp necessary condition (this must be true) amplify a differential voltage, shown below as V1-V2 sufficient condition (one or more of this made be true) large gain (above 100), a high input impedance, and a good common mode rejection ratio.Embedded Microcomputer Systems Lecture 8.7 by Jonathan W. Valvano Integrated instrumentation amplifier. AD627/INA122 are low-power single supply rail-to-rail instrumentation amps, Vout = Gain*(V1-V2) + Vpin5 Gain = 5+(200k/RG) Vout = Vpin5 when V1 equals V2 In EE445L Lab 7, we grounded pin 5, Vpin5 = 0 Vout = Gain *( V1 – V2) With an EKG, we connect pin 5, Vref = Vpin5 EKG data acquisition system. Normal II-lead electrocardiogram. graphical display of EKG is a qualitative data acquisition system. measurement of heart rate is quantitative parameters of an EKG amplifier include: - high input impedance (larger than 1 M), - high gain, 2500, ±1 mV to ±2.5 V - 0.05 to 100 Hz band-pass filter and - good common mode rejection ratio.Embedded Microcomputer Systems Lecture 8.8 by Jonathan W. Valvano Instrumentation amp as preamp stage - good CMRR, - high input impedance, - gain of 10. A 0.05 Hz passive high pass filter - created by R2 and C2. - low-leakage capacitor for C2 is critical - polypropylene or polystyrene would be best, - low-leakage ceramic is acceptable. 2-pole Butterworth LPF. - 153 Hz cutoff is greater than 100Hz - implemented with standard components Standard EKG amp 32184567321845674.7kgain=48+3.3VAD627ANU10.1uF+3.3V0.1uFOPA2350C3R3C4R4R5R6R73.3F1M1k47k1MU2aU2bR8 R9C5C6C749k 49k0.015uF0.015uF0.015uFC8153 Hz LPFgain = 48+1.233V0.05 Hz HPFpreampV1V2V31M1M2F2F3.3 V5.1 kLM4041CILPU3C2C1R1R2R10 Single supply rail-to-rail parts


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UT EE 345 - Embedded Microcomputer Systems

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