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Automatic Feed Forward Gain Ranging Circuit

IP.com Disclosure Number: IPCOM000077453D
Original Publication Date: 1972-Aug-01
Included in the Prior Art Database: 2005-Feb-25
Document File: 3 page(s) / 41K

Publishing Venue

IBM

Related People

Affinito, FJ: AUTHOR [+2]

Abstract

Data acquisition systems with multiple low-level inputs frequently employ low-level multiplexing and a variable-gain amplifier to permit these inputs to share a common amplifier and subsequent signal processing facilities. Signal levels differ substantially, in the general case, among the input channels, necessitating that the amplifier gain be varied to the appropriate value for each channel to which it is switched. This imposes a serious limitation on system throughput speed, since practical amplifiers require a finite time to change gain and for the output to settle to a new value.

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Automatic Feed Forward Gain Ranging Circuit

Data acquisition systems with multiple low-level inputs frequently employ low-level multiplexing and a variable-gain amplifier to permit these inputs to share a common amplifier and subsequent signal processing facilities. Signal levels differ substantially, in the general case, among the input channels, necessitating that the amplifier gain be varied to the appropriate value for each channel to which it is switched. This imposes a serious limitation on system throughput speed, since practical amplifiers require a finite time to change gain and for the output to settle to a new value.

Gain values for each channel determined by a fixed program will not alleviate the problem if, as is often the case, the dynamic range of the data in a given channel exceeds the dynamic range of the amplifier at a fixed gain value.

The key feature of the feed forward gain ranging circuit is its ability to change gain to the required value in one and only one, gain change operation. All other known techniques for gain changing require more than one gain change to reach the required value. Simple feedback techniques require, in the worst case, N changes in gain to reach the correct value, where N equals the number of gain levels available. See a paper entitled "Automatic Gain Ranging Amplifier" by R.
L. Douglas presented at the ISA Aerospace Symposium in May 1970 (pages 384-389 of Proc.). In noncomputer controlled data acquisition systems, allowance for the worst case must be made in establishing the fixed value of time to be allocated for gain ranging to the required gain value.

Binary search techniques can reduce the number of gain change steps from N to log(2) N but only at the expense of a significant increase in hardware costs (e.g., vide: "An MSI Concept for a Binary Search Scanner", R. A. Bernay, Computer Design, June, 1972 (p. 65-73).

A further important advantage of the feed forward technique for gain ranging is that it permits computer-controlled data acquisition and control systems to operate with greater effectiveness and at lower cost, since memory space, programming and CPU time are not requ...