Nonlinear analog-to-digital converter measures the analog input level and continuously adjusts the digital readout scale sensitivity to effectively increase the accuracy. It is able to acquire more accurate low-level data

Munoz, R. M.

English

NASA Technical Reports Server

September 1965	 Brief 65-10277 
NASA TECH BRIEF H 
NASA Tech Briefs are issued by the Technology Utilization Division to summarize specific 
technical innovations derived from the space program. Copies are available to the public from 
the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia, 22151. 
Nonlinear Feedback Reduces Analog-to-Digital Converter Error 
Analog Input 
V in	 DigitalT__O_Comparator	 Scaler	 Output 
Output 
Number) Amplifier lifi 	
N (Normalized 
Summing 
Vr	 Digital 
Attenuator 
The problem: To design a simple, accurate analog-
to-digital converter, (ADC) that exhibits minimum 
error characteristics. The accuracy of conventional 
linear ADC's suffers at low operating levels due to 
their inherent quantization error, i.e., an inability to 
measure increments smaller then the least signifi-
cant digit. 
The solution: A nonlinear analog-to-digital con-
verter that measures the level of an analog input and 
continuously adjusts the scale sensitivity of the digital 
readout to attain an effective increase in accuracy. As 
the output level decreases the scale sensitivity increases 
to retain a more nearly constant accuracy as expressed 
in percentage of reading, thus permitting the acquisi-
tion of more accurate low-level data. 
How it's done: In order to obtain the nonlinearity 
necessary for scale adjustment, an accurate non-
linearity is placed in the conventional feedback path. 
This is done in the feedback elements consisting of the 
summing amplifier, reference voltage, and digital 
attenuator. These elements are capable of a high
degree of accuracy as compared to other possible 
nonlinear elements. 
A typical nonlinear analog-to-digital converter con- 
sists of six basic subsystems: an analog comparator, 
a clock and control gate, an up-down scaler, a digital 
attenuator, a summing amplifier, and a fixed voltage 
source. 
The analog voltage to be sampled is fed into the 
analog comparator, which produces an output if there 
is any difference between V 1 and V 1 (the feedback 
voltage). Any output from the comparator turns the 
AND gate on and allows the clock signal to be fed 
to the up-down scaler. The up-down scaler produces 
the digital output in the form of an output number. 
The nonlinear feedback loop consists of the digital 
attenuator, the summing amplifier, and the fixed volt-
age supply V. The feedback voltage V f is proportional 
to the product of the reference voltage Vr and the 
normalized digital output N, where N is the ratio of 
the output number to the number which represents 
full scale. The summing amplifier adds the fixed volt-
age V to the feedback voltage and passes the summed 
(continued overleaf) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19650000275 2020-03-11T17:19:31+00:00Z
voltages V r to the digital attenuator. The attenuator 
modifies V r by N, producing a feedback voltage which 
is a function of itself, a fixed voltage, and the 
normalized output number. 
Notes: 
I. The error characteristics for this nonlinear digital 
system are superior to those of linear systems. Error 
expressed in percent of reading is more nearly 
constant and extension of scale length is possible. 
2. The system produces attractive error profiles and 
can be implemented with only one component (a 
floating power supply or a summing amplifier) 
added to a conventional analog-to-digital con-
verter.
3. Variation of the fixed voltagq V, or the value of 
full scale, produces variations in the error profile. 
4. Inquiries concerning this invention may be directed 
to:
Technology Utilization Officer 
Ames Research Center 
Moffett Field 
Mountain View, California, 94035 
Reference: B65-10277 
Patent status: NASA encourages the immediate 
commercial use of this invention. Inquiries about 
obtaining rights for its commercial use may be made 
to NASA, Code AGP, Washington, D.C., 20546. 
Source: Robert M. Munoz 
(A RC-46) 
Brief 65-10277	 Category No. 01


Nonlinear feedback reduces analog-to-digital converter error

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