Instrumentation Amplifiers Passive Transducer Measurement Configuration: For passive transducers in a bridge configuration the voltage of interest is the differential voltage e = VB - VA Therefore need a difference amplifier with a committed adjustable gain Ad Want Vo = Ad(VB - VA) = Ad e VCM = Want to reject VCM R R+DR R R IA Vo = Ad e

IA Active Transducer Measurement Configuration: Instrumentation Amplifiers: Active Transducer Measurement Configuration: For an active transducer the differential voltage e created by the transducer is of interest Therefore need a difference amplifier with a committed adjustable gain Ad Want Vo = Ad e Surface whose temperature is to be measured may be at some non-zero potential (VCM) relative to ground Want to reject VCM IA Vo = Ad e

Transducer and Instrumentation Amplifier (IA) Circuit Model: Instrumentation Amplifiers: Transducer and Instrumentation Amplifier (IA) Circuit Model: IA has a committed adjustable differential gain Ad If e is the differential voltage of interest (vid) Want Vo = Ade Want a high CMRR to reject VCM Want high Zin and low Zout Zd is the differential input impedance (1 - 100 MW) ZCM is the common mode input impedance (100 MW) 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 temperature R1 and R2 are the source impedances of input transducer - R1 may not equal R2 IA

If bridge is balanced Vp = 0 If bridge is not balanced Vp ≠ 0 Instrumentation Amplifiers: Transducer (Sensor) and Instrumentation Amplifier Common Mode Voltage Equivalent Circuit: A Set e = 0 B Unwanted parasitic differential voltage Vp produced by VCM due to imperfections in the transducer and/or transducer connections. If bridge is balanced Vp = 0 If bridge is not balanced Vp ≠ 0 Vp 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 error

Assuming the worst case imbalance: R1 = 0 Instrumentation Amplifiers: Transducer (Sensor) and Instrumentation Amplifier Common Mode Voltage Equivalent Circuit: A Set e = 0 B A Assuming the worst case imbalance: R1 = 0 Circuit becomes → Usually specified with a 1kW source impedance imbalance B

Increasing ZCM reduces Vp Instrumentation Amplifiers: IA CMMR = A B Circuit CMRR = Increasing ZCM reduces Vp

Differential Amplifier: (Single op-amp instrumentation amplifier) Instrumentation Amplifiers: Differential Amplifier: (Single op-amp instrumentation amplifier) To obtain vo in terms of v1and v2 use superposition theorem

Differential Amplifier: (Single op-amp instrumentation amplifier) Instrumentation Amplifiers: Differential Amplifier: (Single op-amp instrumentation amplifier) Short input to v2 (Inverting Configuration)

Differential Amplifier: (Single op-amp instrumentation amplifier) Instrumentation Amplifiers: Differential Amplifier: (Single op-amp instrumentation amplifier) Short input to v1 (Noninverting Configuration)

Differential Amplifier: (Single op-amp instrumentation amplifier) Instrumentation Amplifiers: Differential Amplifier: (Single op-amp instrumentation amplifier) To obtain vo in terms of v1and v2 use superposition theorem

Differential Amplifier: (Single op-amp instrumentation amplifier) Instrumentation Amplifiers: Differential Amplifier: (Single op-amp instrumentation amplifier) Differential Input Impedance: Rin, Rid, Zid, Zd Zd = 2R1 Zd is limited

Transducer and Differential Amplifier Circuit Model: Instrumentation Amplifiers: Transducer and Differential Amplifier Circuit Model: External Circuit Instrumentation Amplifier Op Amp CMRR, Zd and ZCM are important attributes of an IA.

CMRR, Zd and ZCM are important attributes of an IA. Instrumentation Amplifiers: Transducer and Differential Amplifier Common Mode Voltage Equivalent Circuit: A A RS2 RS1 Ri1+ Rf1 + Ro Ri2+ Rf2 D D B D B ZCM Can assume Ro = 0 CMRR, Zd and ZCM are important attributes of an IA.

Three Op Amp Instrumentation Amplifier: Instrumentation Amplifiers: Three Op Amp Instrumentation Amplifier: CMRR and Zin are very important attributes of an IA Can increase Zin of difference amplifier configuration by adding unity gain buffers or buffers with gain

Three Op Amp Instrumentation Amplifier: Instrumentation Amplifiers: Three Op Amp Instrumentation Amplifier: CMRR and Zin are very important attributes of an IA Can increase Zin of difference amplifier configuration by adding buffers Common mode signals are not amplified if common R1 is used and connection to ground is removed.

Transducer and Three Op Amp IA Circuit Diagram: Instrumentation Amplifiers: Transducer and Three Op Amp IA Circuit Diagram: External Circuit Instrumentation Amplifier

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