Instrumentation amplifiers

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
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 MΩ)
ZCM is the
common 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 temperature
R1 and R2 are the source impedances of input transducer - R1 may not equal R2
IA

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

A
A
d
CM
V
V
CM
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)

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
CMRR, Zd and ZCM are important attributes of an IA.
A
BD
RS2
RS1
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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