Texas Instruments | Instrumentational Amplifiers (See also LMP8358) | Application notes | Texas Instruments Instrumentational Amplifiers (See also LMP8358) Application notes

Texas Instruments Instrumentational Amplifiers (See also LMP8358) Application notes
LM108,LM113,LM318
Instrumentational Amplifiers (See also LMP8358)
Literature Number: SNVA580
National Semiconductor
Linear Brief 21
June 1973
Introduction
Referred to the output, the common mode error is independent of gain and fixed by the resistor mismatch. For 1%
match the error is 0.5%, and for 0.1% match the error is
0.05%. These errors are not trivial in high precision systems.
An instrumentation amplifier is shown here that compares
favorably with multiple op amp designs, yet does not require
precisely matched resistors. Further, the design allows a
single resistor to adjust the gain. In comparing this instrumentation amp to multiple op amp types there are of course
some drawbacks. The gain linearity and accuracy are not as
good as the multiple op amp circuits.
The errors appearing in multiple op amp circuits are independent of the output signal level. For example, a common
mode error at the output of 0.5% of full scale is a 33% error
if the desired output signal is only 1.5% of full scale. With the
new circuit maximum errors at full scale output and the
percentage of output error decreases at lower output levels.
Figure 1 shows a general purpose instrumentation amplifier
optimized for wide bandwidth. It can provide gains from
under 1 to over 1000 with a single resistor adjustment. Gain
linearity is worst for unity-gain at 0.4%, and gain stability is
better than 1.5% from −55˚C to +125˚C. Typically over a 0˚C
to +70˚C range gain stability is 0.2%. Common mode rejection ratio is about 100 dB — independent of gain.
One of the most useful analog subsystems is the true instrumentation amplifier. It can faithfully amplify low level signals
in the presence of high common mode noise. This aspect of
its performance makes it especially useful as the input amplifier of a signal processing system. Other features of the
instrumentation amplifier are high input impedance, low input
current, and good linearity.
It has never been easy to design a high performance instrumentation amplifier; however, the availability of high performance IC’s considerably simplifies the problem. IC op amps
are available today that can give very low drifts as well as
low bias currents; however, most of the circuits have some
drawbacks.
The most commonly used instrumentation amplifier designs
utilize either 2 or 3 op amps and several precision resistors.
These are capable of excellent performance; however, for
high performance they require very precisely matched resistors. The common mode rejection of these designs depends
on resistor matching and overall gain. Since op amps are
now available with exceedingly high CMRR, this is no longer
a problem. The CMRR of the instrumentation amplifier is
approximately equal to half resistor mismatch plus the gain.
For a 1% resistor mismatch the CMRR is limited to 46 dB
plus the gain — referred to the input.
Instrumentational Amplifiers
Instrumentational
Amplifiers
00872701
Note: Since the LM114 is an obsolete part, substitution of the LM194 is recommended, along with the removal of the two LM194 diodes. This circuit has
not been tested with the LM194 included.
Also, the LM185-1.2 could be substituted for the LM113.
© 2002 National Semiconductor Corporation
AN008727
LB-21
FIGURE 1. Instrumentation Amplifier
www.national.com
Instrumentational Amplifiers
Transistor pair, Q1 and Q2, are operated open-loop as the
input stage to give a floating, fully differential input. Current
sources, Q3 and Q4, set the operating current of the input
pair. To obtain good linearity the output current of Q3 and Q4
are set at about twice the current in R8 at full differential
voltage. The temperature sensitivity of the transconductance
of Q1 and Q2 is compensated by making their operating
current directly proportional to absolute temperature. It has
been shown that by biasing the base of transistor current
sources at 1.22V, the output current varies as absolute temperature. The LM113 diode provides a constant 1.22V to the
current sources. Both the compensated gm of Q1 and Q2
and the large degeneration from R8 give the amplifier stable
gain over a wide temperature range.
In operation, transistors Q1 and Q2 convert a differential
input voltage to a differential output current at their collectors. This is fed into a standard differential amplifier to obtain
a single ended output voltage. Since the diff amp does not
see the common mode input voltage, 1% resistors are adequate. Gain is set by the ratio of R8 (plus the re of Q1 and
Q2) to the sum of R6 and R7.
As mentioned previously this circuit is optimized for wide
bandwidth: however, it is easily modified for other applications. If low bias current is needed, all resistors can be
increased by a factor of 100 and an LM108 substituted for
the LM318. Other possible improvements are cascaded current sources and a modified Darlington input stage.
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LB-21
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National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
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