instrumentation amplifiers

Instrumentation Amplifiers

Passive Transducer Measurement Configuration:

For passive transducers in a bridge configuration the voltage of interest is thedifferential 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

V V EA B

2 2 R

R R

R+R

IA

Vo = Ad 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 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:

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 M)

ZCM 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 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


Transducer (Sensor) and Instrumentation AmplifierCommon Mode Voltage Equivalent Circuit:

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

A

B

Set e = 0


Transducer (Sensor) and Instrumentation AmplifierCommon Mode Voltage Equivalent Circuit:

Assuming the worst case imbalance: R1 = 0

Circuit becomes →

Usually specified with a 1ksource impedance imbalance

A

B

A

B

Set e = 0


A

Ad

CM

V

VCM

p

Increasing ZCM reduces Vp

IA CMMR =

Circuit CMRR =

A

B


Differential Amplifier: (Single op-amp instrumentation amplifier)

To obtain vo in terms of v1and v2 use superposition theorem



Short input to v2 (Inverting Configuration)



Short input to v1 (Noninverting Configuration)



To obtain vo in terms of v1and v2 use superposition theorem



Differential Input Impedance: Rin, Rid, Zid, Zd

Zd = 2R1 Zd is limited


Transducer and Differential Amplifier Circuit Model:

CMRR, Zd and ZCM are important attributes of an IA.

External Circuit Instrumentation Amplifier

Op Amp


Transducer and Differential Amplifier Common Mode Voltage Equivalent Circuit:

CMRR, Zd and ZCM are important attributes of an IA.

A

BD

RS2RS1

Ri1+ Rf1

+ Ro

Ri2+ Rf2

ZCM

Can assume Ro = 0

D

D

A

B


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:

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:

External Circuit Instrumentation Amplifier

instrumentation amplifiers

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