Circuit controls a voltage controlled oscillator from computer output binary data representing a rate at which the oscillator is to change. It operates with low level output devices such as integrated circuit registers and devices with somewhat variable output levels

Winkelstein, R. A.

English

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11III 
BIT 1 To Integrating 
Amplifier 
.
BIT1C
October 1967	 Brief 67-10357 
NASA TECH BRIEF im 
A	 11 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Digital-to-Analog Converter Operates from 'Low Level Inputs 
+50V 
±2.5V 
The problem: 
To control a voltage controlled oscillator from com-
puter output binary data representing a rate at which 
the oscillator is to change. The computer derived rate 
number is held in a buffer that is periodically read out 
into a digital-to-analog (D/A) converter. The buffer 
register uses integrated circuits not capable of driving 
conventional D/A converter circuits without prior 
amplification.
The solution: 
A circuit that operates with low level output devices 
such as integrated circuit registers and devices with 
somewhat variable output levels. Normal transition 
level is +2.5v so that if the flip-flop register output 
voltage at any binary order position is less positive 
than +2.5v, the "zero" state exists at the input to the 
D/A converter; if the register output voltage is more 
positive than +2.5v, the "one" state exists. 
(continued ovcrleaf) 
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 the United States 	 will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19670000357 2020-03-11T21:03:04+00:00Z
How it's done: 
A standard reference power supply of SO y is used 
for the reference source. By means of selected resistors 
in the input branches, the current in each succeeding 
branch from bit I to bit 11 is doubled as an initial 
calibration of the current input to the operational 
amplifier. 
As the illustration shows, the converter contains 
multiple binary scaled current sources (bits 1 through 
11) that are summed by the operational amplifier. 
Each of these sources has two steering diodes, D 1 and 
D2 . Diode D1 directs current flow to the operational 
amplifier and D directs current flow to the driving 
circuit. If the input voltage is less positive than +2.5v, 
current is steered by D2 away from the operational 
amplifier and into the driving circuit. If the input volt-
age is more positive than +2.5v, D 2 is back biased 
and D1 steers the current into the operational ampli-
fier where its binary value contributes to the output dc 
voltage. Amplifier gain is adjusted by R 1 from +5v to 
—5v over the complete range of code inputs. Bias is 
set for a constant +2.5v at the amplifier input by ex-
ternal means. The D/A converter is calibrated by 
adjusting R in each branch for monotonic operation. 
A major portion of drift and voltage uncertainty 
versus temperature variation is due to the variable 
voltage drop across the steering diodes leading to the 
operational amplifier. For example, in the most sig-
nificant bit position, a 50 m y change is equivalent to
one whole code increment change. This effect di-
minishes by a factor of two for each successive lower 
order bit position. To minimize this condition, the 
operational amplifier steering diodes in the four most 
significant bit positions (bits 8, 9, 10, and 11) are 
metal silicon hot carrier diodes with low barrier po-
tential to minimize drift due to temperature variation. 
The voltage drop temperature coefficient of these 
diodes approaches zero for currents in the vicinity of 
10 ma. At low currents the temperature coefficient is 
much less than that of the silicon junction diodes used 
in the other steering positions. 
Notes: 
1. The low level operational capability of this D/A 
converter makes it suitable for commercial com-
puter and control applications involving integrated 
circuitry. 
2. Inquiries concerning this innovation may be di-
rected to: 
Technology Utilization Officer 
Jet Propulsion Laboratory 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: B67-10357 
Patent status: 
No patent action is contemplated by NASA. 
Source: Robin A. Winkelstein 
(JPL-907) 
Brief 6710357	 Category 01


Digital-to-analog converter operates from low level inputs

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