California University of Pennsylvania Department of Applied Engineering & Technology Electrical / Computer Engineering Technology EET 215: Introduction to Instrumentations Lab No.6 Instrumentation Amplifier Names: 1. 2. 3. Signature: Date Instrumentation Amplifier Objective of the Experiment The main objective of this experiment is to understand and troubleshot an instrumentation amplifier circuit Learning Outcomes Students will demonstrate: - the ability to build and troubleshot IA circuits the ability to determine the common mode rejection ratio of an IA CAUTION: It is imperative that students exercise care when handling the ELVIS unit. Make sure all connections are correctly made. When in doubt, check with your instructor to verify proper circuit connections. Introduction The instrumentation amplifier (IA) used here is designed with a combination of differential amplifier and op-amp buffers. Instrumentation amplifiers are used extensively when sensitivity, accuracy, and stability of circuits are needed. Examples include digital scales, ECG circuits, force and strain measurements, temperature measurements, etc. Figure -1: LM 741C Refer to class notes as a reference as you do this lab. Components Needed: - LM741C op-amp (3) - Six equal resistors (may be 10KΩ or close to this values) - refer to them as R in the circuit Potentiometer to be used as a variable resistor ( 1KΩ or 5KΩ or close) potentiometer (refer to it as RG in the circuit) - Any other two equal resistors (choose 4.7KΩ if possible) (refer to them as R1, R2 in the circuit) Experiment For all circuits, the +/- Vsupply = +/- 15Volts. Component Values are: R= R1 = R2 = ? ? A- Construct the IA circuit With the power to the board initially turned OFF (top right corner), a- Careful, some op-amps in the drawing are flipped vertically. b- Ensure all op-amps are connected to the +/-Vsupply (but power is still turned off) c- Take great care in making correct connections. This will make this experiment and the next go smoothly. d- SET RG to 500Ω (May used a fixed resistor if available) e- Again, Connect circuit very nicely. This makes it easier to troubleshoot and to understand. f- NO input signal for Vin yet. OP-AMP1 V1 R Vy 10kΩ R LM741CN 10kΩ R RG 50% R 10kΩ Vout 10kΩ 10kΩ LM741CN R R 10kΩ OP-AMP3 V2 Vx LM741CN OP-AMP2 Fig.1. IA circuit set-up B- Determining the Common Mode Gain, AC a- Turn the power to the board ON and apply Vin to both inputs as shown Vin Connect to V1 Connect to V2 Vin is a sine wave at 0.2Vpk and 1KHz. This input is to mimic a common noise b- USE the DMM tool in ELVIS’s instrument launcher to perform the following measurements: Notice: The AC voltage measurements on the DMM correspond to the RMS measurements 1- Vin(RMS) = Volts (should be about 0.1414V) This is the Common Mode input. Will call it Vin(CM). 2- Vout(RMS) = Volts (Should be VERY small, NOT zero) This is the Common Mode Output. Will call it Vo(CM) If the value for Vout value is large, then here are possible errors: c- i- check all op-amps output, make sure all are low. If one is near saturation, then there is a problem with connections or the applied power supply to that opamp. ii- double check all connections – (this is part of troubleshooting skills) Calculate the common mode gain, AC 𝑉𝑜(𝐶𝑀) AC = 𝑉 𝑖𝑛(𝐶𝑀) = (should be very small – near zero, but NOT zero) C- Determining the Differential Mode Gain, Ad a- Turn the board power off and remove the common mode input connections of step B above b- Apply the differential input as shown below and turn the board’s power ON. Vin R1 Connect to V1 4.7kΩ Connect to V2 R2 4.7kΩ Here, V2 sees the input signal and V1 sees half of it. cd- Set Vin to the same value as previously. Vin = 0.2Vpk sine wave at 1KHz. Verify correct operation by monitoring Vout on the scope. Vout should be near 4.1(pk) sine wave. If far off, then troubleshoot the circuit and fix the problem (note this part assumes using the suggested resistor values.) e- If step (d) is o.k, then record your measurements using the DMM tool in ELVIS’s instrument launcher . f- 1- V1(RMS) = Volts (should be near 0.0707V) ! 2- V2(RMS) = Volts (should be near 0.1414V) ! 3- Vout(RMS) = Volts (should be near 2.89V) ! Calculate the differential gain, Ad 𝑉 𝐴𝑑 = 𝑉 𝑜𝑢𝑡 = −𝑉 2 g- (should be about 41) 1 Calculate the theoretical value of the differential gain: 2𝑅 𝐴𝑑 = 1 + 𝑅 = 𝐺 h- Do the results in steps f and g almost agree? -------------YES, --------------NO iIf the percentage difference is more that 10%, then the work was not successful. All previous steps will have to be checked. Instructor Approval: ------------------------ D- Determining the Common Mode Rejection Ratio (CMRR) 𝐴 CMRR = 𝐴𝑑 𝑐 = CMRRdB = 20log10(CMRR) ? (should be large , possible above 800) = dB ( should be high, possible more than 60dB) E- Typical CMRR values of standard components From the web, obtain and record CMRR (dB) for each of the following components: http://www.ti.com/lit/ds/symlink/lm741.pdf http://www.analog.com/static/imported-files/data_sheets/AD620.pdf Component CMRR(dB) (minimum) CMRR(dB) (typical) LM741 AD 620 (with gain, G =1000) Instructor Approval of Completion: -------------------------------. Challenges Draw the connection diagram of an instrumentation amplifier (refer to the AD620 datasheetsee figures 1 and 2 to help understand. The symbol is shown on top of first page of the datasheet.) Also, explain, briefly, what each of the 8-pins is for? abc- Instructor approval of experiment completion: