Instrumentation Amplifiers Passive Transducer Measurement Configuration: For passive transducers in a bridge configuration the voltage of interest is the differential voltage e = V B - V A Therefore need a difference amplifier with a committed adjustable gain A d Want V o = A d (V B - V A ) = A d e V CM = Want to reject V CM R R R R+  R IA V o = A d e

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 A d Want V o = A d e Surface whose temperature is to be measured may be at some non-zero potential (V CM ) relative to ground Want to reject V CM IA V o = A d e

Instrumentation Amplifiers: Transducer and Instrumentation Amplifier (IA) Circuit Model: IA has a committed adjustable differential gain A d If e is the differential voltage of interest (v id ) Want V o = A d e Want a high CMRR to reject V CM Want high Z in and low Z out Z d is the differential input impedance ( M  ) Z CM is the common mode input impedance (100 M  ) IA not an op-amp Op amp open loop uncommitted gain IA closed loop committed gain IA has higher Z in and CMRR IA has lower V os and I bias and drift with temperature R 1 and R 2 are the source impedances of input transducer - R 1 may not equal R 2 IA

Instrumentation Amplifiers: Transducer (Sensor) and Instrumentation Amplifier Common Mode Voltage Equivalent Circuit : Unwanted parasitic differential voltage V p produced by V CM due to imperfections in the transducer and/or transducer connections. If bridge is balanced V p = 0If bridge is not balanced V p ≠ 0 V p will contaminate V o V o ≠ A d eV o = A d (e + V p ) Therefore even if the IA has an infinite CMRR (i.e A CM =0) still have a common mode output voltage error A B Set e = 0

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

Instrumentation Amplifiers: Increasing Z CM reduces V p IA CMMR = Circuit CMRR = A B

Instrumentation Amplifiers: Differential Amplifier:(Single op-amp instrumentation amplifier) To obtain v o in terms of v 1 and v 2 use superposition theorem

Instrumentation Amplifiers: Differential Amplifier: (Single op-amp instrumentation amplifier) Short input to v 2 (Inverting Configuration)

Instrumentation Amplifiers: Differential Amplifier: (Single op-amp instrumentation amplifier) Short input to v 1 (Noninverting Configuration)

Instrumentation Amplifiers: Differential Amplifier:(Single op-amp instrumentation amplifier) To obtain v o in terms of v 1 and v 2 use superposition theorem

Instrumentation Amplifiers: Differential Amplifier:(Single op-amp instrumentation amplifier) Differential Input Impedance:R in, R id, Z id, Z d Z d = 2R 1 Z d is limited

Instrumentation Amplifiers: Transducer and Differential Amplifier Circuit Model: CMRR, Z d and Z CM are important attributes of an IA. External CircuitInstrumentation Amplifier Op Amp

Instrumentation Amplifiers: Transducer and Differential Amplifier Common Mode Voltage Equivalent Circuit: CMRR, Z d and Z CM are important attributes of an IA. A B D RS2RS2 RS1RS1 R i 1 + R f 1 + R o R i 2 + R f 2 Z CM Can assume R o = 0 D D A B

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

Instrumentation Amplifiers: Three Op Amp Instrumentation Amplifier: CMRR and Z in are very important attributes of an IA Can increase Z in of difference amplifier configuration by adding buffers Common mode signals are not amplified if common R 1 is used and connection to ground is removed.

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

The Buffer Amplifier

Integrator

Differentiator

Log amplifier (Nonlinear amp)