Amplifier can be used for many applications requiring high input impedance and common mode rejection, low drift, and gain accuracy on order of one percent. Performance of inexpensive amplifier approaches that of some commercial instrumentation amplifiers in many specifications

Sturman, J. C.

English

NASA Technical Reports Server

June 1974	 B74-10015 
NASA TECH BRIEF 
Lewis Research Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed 16 the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Low Cost Instrumentation Amplifier +18V 20K 	 20K -'--O.02 MC 1456 
-HH MC 1456 1 0.02 +18V 100 K 0— 2N5452 A 1 	 2 , 	 1 HOO K 7	 6,0 >_ + 10 K K 4.47 18V 18V 
- 40K - 
9/  2 
1j 
2/ 5\ 3 " 0 10.02 CA3O18A - -18V 
The Problem: How It's Done: 
In instrumentation and other applications, it is often As shown in the figure, the amplifier consists basically 
necessary to amplify small signals in the presence of of two stages. The first, comprising the dual FET and 
unwanted common mode signals or noise. Too often the operational amplifier Al, determines most of the ampli. 
solution	 to	 this	 problem	 is	 to	 use	 a high	 quality fier's characteristics. The first stage gain is 100 and is as 
instrumentation amplifier, at considerable cost, even if its accurate as the ratios of the resistors used. Variable gain 
high precision is not required, can be provided by adding a potentiometer at R3. A 
moderate gain change in this fashion will not alter the 
The Solution: performance	 of	 the	 amplifier, particularly	 the input 
An inexpensive amplifier that can be used for many characteristics. 
•	 applications requiring high input impedance and common 
mode rejection, low drift, and gain accuracy on the order
Input impedance of the amplifier is determined almost 
entirely by resistors R 1 and R2 . Their only function is to 
of one percent. (continued overleaf) 
This document was prepared under the sponsorship of the National Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States information contained in this document, or warrants that such use 
Government nor any person acting on behalf of theUniled States will be free from privately owned rights.
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provide a ground return for true floating inputs. For 
inputs that provide a conduction path to ground, these 
two resistors can be eliminated to provide the highest 
possible input impedance. 
The second stage provides a means of tailoring the gain, 
bandwidth, and output power to meet many possible 
requirements. Gain changes or bandwidth limiting using 
capacitor C 1 may be easily implemented without affecting 
the first stage. The second stage could even be used as an 
active filter to provide a sharp low pass characteristic. Use 
of a higher power operational amplifier for A 2 could 
provide sufficient output power for line or galvanometer 
driver applications. 
Performance of this amplifier approaches that of some 
commercial instrumentation amplifiers in many specifica-
tions. DC drift is below 10 MV/°C, input noise is very low, 
and common mode rejection ratio of greater than 70 db 
holds to beyond I KHz. If not intentionally limited, the 
bandwidth is at least 10 KHz. 
Notes: 
1. Applications requiring many amplifiers may be oper-
ated from common power supplies further reducing the 
system cost. 
2. The addition of a modified second stage would provide 
hi-pass, band-pass, or low-pass filter characteristics for 
special applications. Replacement of the output stage 
integrated circuit with a higher power type would 
extend its use to galvanometer driver, line driver, and 
other high power applications. 
3. Possible applications for this low-cost amplifier are: (1) 
signal conditioning for any type dc transducer, (2) 
input buffering of instruments; i.e., recorders, scopes, 
and computers, (3) line isolation, (4) impedance 
transformation, (5) elimination of common mode 
noise, (6) measuring signals from very high impedance 
sources such as crystal transducers, and (7) medical 
electronics (small signal pickups and amplification). 
4. No additional documentation is available. Specific 
technical questions, however, may be directed to: 
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: B74-10015 
Patent Status: 
NASA has decided-not to apply for a patent. 
Source: John C. Sturman
Lewis Research Center
(LEW-1 2222)
I.
. 
B74-l0015	 Category 01


Low cost instrumentation amplifier

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