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Two Amplifier System for Rejecting Common Mode Voltages

IP.com Disclosure Number: IPCOM000096861D
Original Publication Date: 1963-Dec-01
Included in the Prior Art Database: 2005-Mar-07
Document File: 3 page(s) / 53K

Publishing Venue

IBM

Related People

Peterson, EJ: AUTHOR

Abstract

The upper drawing is a generalized diagram of the system. The system includes at least one transducer located at remote station 10. The transducer at location 10 is connected to an analog to digital converter 12 via two lines 13 and 14. The voltage which indicates the state of the transducer at location 10 is a differential voltage appearing between lines 13 and 14. However, due to various spurious voltages, common mode potentials sometimes exist at the input to converter 12 between ground and both lines 13 and 14. These common mode signals generally tend to interfere with the analog to digital conversion of the differential voltages.

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Two Amplifier System for Rejecting Common Mode Voltages

The upper drawing is a generalized diagram of the system. The system includes at least one transducer located at remote station 10. The transducer at location 10 is connected to an analog to digital converter 12 via two lines 13 and
14. The voltage which indicates the state of the transducer at location 10 is a differential voltage appearing between lines 13 and 14. However, due to various spurious voltages, common mode potentials sometimes exist at the input to converter 12 between ground and both lines 13 and 14. These common mode signals generally tend to interfere with the analog to digital conversion of the differential voltages.

This circuit eliminates or compensates for common mode voltages. In general, the circuit consists of a high-gain operational amplifier 15 connected between line 14 and ground at the input of converter 12. The output of amplifier 15 is connected to the transducers at the remote location. If a voltage exists between line 14 and ground at the input of converter 12, amplifier 15 generates a voltage which changes the level of the apparatus at location 10, therefore eliminating the common mode voltage.

In the lower drawing, the circuit includes a strain gauge network 20 and two thermocouple networks 21 and 22. One of these transducers is selected by means of multiplex switches 23. Amplifier 15 detects if any potential exists between point A and the analog-to-digital converter system ground 25. If a voltage does exist, amplifier 15 generates a current which passes through resistors 26 and 27 and through resistance 28 to cancel the common mode voltage. In this manner, point A is maintained at virtual ground potential. Resistor 26 limits the amount of current which can flow. Its value is chosen...