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# Instrumentation amplifiers

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• 1. Instrumentation AmplifiersPassive Transducer Measurement Configuration:For passive transducers in a bridge configuration the voltage of interest is thedifferential voltage e = VB- VATherefore need a difference amplifier with a committed adjustable gain AdWant Vo= Ad(VB- VA) = AdeVCM=Want to reject VCMV V EA B+≈2 2 RR RR+∆RIAVo = Ad e
• 2. Instrumentation Amplifiers:Active Transducer Measurement Configuration:For an active transducer the differential voltage ecreated by the transducer is of interestTherefore need a difference amplifierwith a committed adjustable gain AdWant Vo = Ad eSurface whose temperatureis to be measured may be atsome non-zero potential (VCM)relative to groundWant to reject VCMIAVo = Ad e
• 3. Instrumentation Amplifiers:Transducer and Instrumentation Amplifier (IA) Circuit Model:IA has a committed adjustable differential gain AdIf e is the differential voltage of interest (vid)Want Vo = AdeWant a high CMRR to reject VCMWant high Zin and low ZoutZd is thedifferential input impedance (1 - 100 MΩ)ZCM is thecommon mode input impedance (100 MΩ)IA not an op-amp• Op amp open loop uncommitted gain• IA closed loop committed gain• IA has higher Zin and CMRR• IA has lower Vos and Ibias and drift with temperatureR1 and R2 are the source impedances of input transducer - R1 may not equal R2IA
• 4. Instrumentation Amplifiers:Transducer (Sensor) and Instrumentation AmplifierCommon Mode Voltage Equivalent Circuit:Unwanted parasitic differential voltage Vpproduced by VCMdue to imperfections in the transducer and/or transducer connections.If bridge is balanced Vp= 0 If bridge is not balanced Vp≠ 0Vp will contaminate Vo Vo ≠ Ad e Vo = Ad (e + Vp)Therefore even if the IA has an infinite CMRR (i.e ACM =0)still have a common mode output voltage errorABSet e = 0
• 5. Instrumentation Amplifiers:Transducer (Sensor) and Instrumentation AmplifierCommon Mode Voltage Equivalent Circuit:Assuming the worst case imbalance:R1 = 0Circuit becomes →Usually specified with a 1kΩsource impedance imbalanceABABSet e = 0
• 6. Instrumentation Amplifiers:AAdCMVVCMpIncreasing ZCMreduces VpIA CMMR =Circuit CMRR =AB
• 7. Instrumentation Amplifiers:Differential Amplifier: (Single op-amp instrumentation amplifier)To obtain vo in terms of v1and v2 use superposition theorem
• 8. Instrumentation Amplifiers:Differential Amplifier: (Single op-amp instrumentation amplifier)Short input to v2 (Inverting Configuration)
• 9. Instrumentation Amplifiers:Differential Amplifier: (Single op-amp instrumentation amplifier)Short input to v1 (Noninverting Configuration)
• 10. Instrumentation Amplifiers:Differential Amplifier: (Single op-amp instrumentation amplifier)To obtain vo in terms of v1and v2 use superposition theorem
• 11. Instrumentation Amplifiers:Differential Amplifier: (Single op-amp instrumentation amplifier)Differential Input Impedance: Rin, Rid, Zid, ZdZd = 2R1 Zd is limited
• 12. Instrumentation Amplifiers:Transducer and Differential Amplifier Circuit Model:CMRR, Zd and ZCM are important attributes of an IA.External Circuit Instrumentation AmplifierOp Amp
• 13. Instrumentation Amplifiers:Transducer and Differential AmplifierCommon Mode Voltage Equivalent Circuit:CMRR, Zd and ZCM are important attributes of an IA.ABDRS2RS1Ri1+ Rf1+ RoRi2+ Rf2ZCMCan assume Ro = 0DDAB
• 14. Instrumentation Amplifiers:Three Op Amp Instrumentation Amplifier:CMRR and Zin are very important attributes of an IACan increase Zin of difference amplifier configuration by adding unity gain buffersor buffers with gain
• 15. Instrumentation Amplifiers:Three Op Amp Instrumentation Amplifier:CMRR and Zin are very important attributes of an IACan increase Zin of difference amplifier configuration by adding buffersCommon mode signals are not amplified if common R1 is used andconnection to ground is removed.
• 16. Instrumentation Amplifiers:Transducer and Three Op Amp IA Circuit Diagram:External Circuit Instrumentation Amplifier
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