analogangle By Ron Mancini
Don’t fall in love with one type
of instrumentation amp
T
he instrumentation amps I covered in my last column are difference amps made with internal resistors
(Reference 1). Difference amps have high CMR er-
rors when you use them with high-resistance sensors.
Their input resistance is low—
usually in the tens of kilohms—so
any mismatch in sensor resistance
causes a CMR error. Manufacturers set differential amps’ gain
because resistor-matching errors
cause CMR errors. Also, the circuit
configuration precludes inputresistance matching, so you can’t
use difference amps when the sensor is sensitive to load resistance.
Inserting a buffer in front of the
difference amp’s inputs solves the
input-resistance problem because
only the IC design and mounting
hardware limit a buffer’s input
resistance.
The circuit in Figure 1 shows an
instrumentation amp with three
op amps, and the associated transfer equation follows:
VOUT
2R
⫽1⫹ F .
V1⫺V2
RG
IC1 and IC2 are buffers that provide high-impedance inputs for
the instrumentation amp. RG provides a way to change the circuit
gain with one resistor. When you
leave RG open, the gain is 1. A simple way to visualize the circuit is to
look at RG as two identical resistors
each having half the value. You’ve
now solved the input-resistance
and gain-changing problems, but
how do you get the CMR that you
have sacrificed? The CMR comes
from the difference amp IC3, and
the sacrifice is cost of the two extra
op amps and the added resistors.
The three-op-amp instrumentation amp costs more than a difference amp, but it provides just
as good CMR, gains from 1 to
several hundred, very high
input resistance, matched input
24 edn | May 30, 2002
resistance, and good bandwidth.
The circuit in Figure 2 shows an
instrumentation amp with two op
amps, and the associated transfer
equation follows:
 R
VOUT
R 
⫽⫺1⫹ 2 ⫹2 2  .
V1⫺V 2
R
R
1
G

The theoretical minimum gain
of ⫺2 occurs when R1=R2 and
when you leave open RG. However,
in reality, the minimum gain is
usually ⫺4 or ⫺5, so you can’t use
this configuration in buffer applications. The V1 signal must propagate through two op amps, but the
V2 signal propagates through one
op amp. When input
Figure
signals contain frequencies greater than the flat portion of the op-amp gain curve
(Reference 2), the V1 signal attenuates more than the V2 signal. The
unequal attenuation causes the
signal to unbalance, and CMR
reduces at high frequencies. The
gain of the two-op-amp instrumentation amp changes with one
resistor, RG.
The two-op-amp
Figure
configuration can provide higher CMR, especially in
low-voltage, single-supply applications. Two-op-amp configurations
have simpler internal circuitry that
enables lower cost, lower quiescent
current, and smaller packages. The
INA156 and INA122 are examples
of two-op-amp instrumentation
amps targeting single-supply operation. The newest three-op-amp
instrumentation amps have overvoltage input protection for industrial applications (INA129), and
some instrumentation amps
(PGA204) include programmablegain capability. The programmable
feature enables users to always
obtain the highest dynamic range
by changing gain as the input
signal approaches a limit. The
three types of instrumentation
amps all feature higher CMR than
you can obtain with discrete components. The two- and three-opamp instrumentation amps add
high matched-input resistance and
gain-changing capability.왏
References
1. Mancini, Ron, “Mutant op
amp becomes instrumentation
amp,” EDN, April 11, 2002, pg 20.
2. Mancini, Ron, “Op-amp
bandwidth and accuracy,” EDN,
Feb 17, 2000, pg 28.
3. Albaugh, Neil, The Instrumentation Amplifier Handbook, Texas
Instruments, January 2000.
V2
+
IC1
_
1
R
R
RF
RG
_
RF
IC3
R
+
_
VOUT
R
IC2
V1
+
The three-op-amp instrumentation amplifier has
one gain-setting resistor, RG, and suits gains as
low as 1.
RG
2
R2
R1
R1
R2
_
_
+
V1
VOUT
+
V2
The two-op-amp instrumentation amplifier has
one gain-setting resistor, RG, and suits gains of 2
or higher.
Ron Mancini is staff scientist
at Texas Instruments. You can
reach him at 1-352-569-9401,
rmancini@ti.com.
www.ednmag.com