A/D Converter - Low Cost

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					M       ORE and more digital tech-
        niques are finding applications
in formerly exclusive domains of analog
                                             er supply from which each amplifier (ground) state to the positive supply
                                             sinks a constant current independent of voltage. The resulting positive pulse re-
                                             the supply voltage.                        sets both integrator IClB and counter
electronics-tests and measurements,             Stage ICIA generates a train of IC2, causing the output lines of the
communications, and the recording and        pulses whose duration is determined by counter and the output of the integrator
reproduction of speech and music, to         the values of R5 and Cl. The frequency     (that is, the staircase waveform) to go to
name a few. One necessary stage in any       of the pulse train can be varied by ground potential. The process begins all
digital system that processes informa-       adjusting potentiometer RI. Pulses gen- over again as new pulses are generated
tion originating in analog form is the       erated by ICI A are applied to the nonin-  by IC1A and applied to the integrator
analog-to-digital or A/D converter. In       verting input of ICIB. This Norton and counter.
this article, we will present a low-cost     CDA is employed as an integrator which In operation, the amplitude of the
A/D converter that you can build using       generates a staircase waveform. The staircase waveform is continuously com-
readily available parts. The circuit can     staircase increases in amplitude as pared to the analog input signal. If the
be used to experiment with the conver-       pulses are received from ICIA. It is ap- input is a constant dc level, the staircase
sion of voltages, currents, and trans-       plied to the inverting input of compara- increases to a certain amplitude during
duced physical quantities from analog        tor ICI C.                                 each cycle until integrator IC1 B is reset
into digital form.                              The analog input signal is applied to   by ICI D. Similarly, IC2 will count up to
                                             the noninverting input of this compara- a certain binary number and then be
About the Circuit. The A/D con-              tor. As long as the staircase amplitude is reset. If the input waveform changes
verter circuit, as shown in the schematic,   less than that of the input signal, the    with time, the amplitude attained by the
employs a 12-bit CMOS counter and an         output of comparator ICI C remains at staircase and the magnitude of the bina-
LM3900 quad operational amplifier.           +V, the positive supply voltage. The ry count generated by IC2 just before
Each of the op amps in an LM3900 IC          staircase continues to increase in ampli- the reset pulse is applied will vary. Ac-
employs the concept of a “current mir-       tude until it just exceeds the input sig-  cordingly, the larger the input signal,
ror” to amplify differential signals.        nal’s amplitude, at which point the dif-   the greater the amplitude of the stair-
They are known as Norton current-dif-        ferential input current at ICI C becomes case and the count at the output lines of
ferencing amplifiers (CDAs) and are          negative. This causes the output of the IC2 at the instant before the reset pulse
shown schematically as containing cur-       comparator to go to ground potential,      causes the outputs of IClB and IC2 to
rent ‘sources to distinguish them from       and the resulting negative transition is   go to ground. The smaller the input sig-
conventional operational amplifiers.         capacitively coupled to the inverting in- nal, the lower the amplitude of the stair-
Among the advantages of Norton CDAs          put of comparator ICID.                    case and count of IC2 at the instant
are circuit simplicity, low cost, and the       The negative pulse momentarily tog- before the reset command takes effect.
requirement of only a single-ended pow-      gles the output of ICID from its normal The highest count attained by IC2 be-
AUGUST1990
A/D converter
fore the output lines are reset to zero       project. Also, use the minimum amount
describes the amplitude of the analog in-     of heat and solder consistent with the
put signal at the instant that comparator     formation of good connections. Before
ICI C changes states.                         applying any supply voltages, double
   Because Norton CDAs are employed,          check your wiring for errors that might
only a single-ended power supply is           cause damage to the ICs.
needed. National Semiconductor, the              To calibrate the circuit, connect its
manufacturer of the LM3900, states in         input to the positive supply voltage.
its data sheet that a supply delivering       Then monitor the output lines of IC2
from +4 to +36 volts can be used to           and adjust RI for the desired weighting
power the chip. The power supply rating       factor. This factor n will equal N, di-




of the CD4040 CMOS counter is + 1 to          vided by + V where + V is the positive
+ 15 volts. Accordingly, a supply fur-        supply voltage, N, is the highest count
nishing a voltage greater than or equal       attained by IC2 before it is reset, and n
to +4 volts and less than or equal to + 15    is the number of counts per volt.
volts can be used to power the entire cir-       This low-cost A/D converter can be
cuit. Current demand is modest, so eith-      used to gain hands-on experience with
er a battery or small, well-filtered, line-   one type of A/D conversion. It can also
powered supply is suitable.                   form the nucleus of some useful proj-
                                              ects. For instance, a latch, decoder, driv-
Construction. Printed-circuit, point-         er and display network can be added to
to-point wiring, or wrapped-wire assem-       provide a seven-segment readout of the
bly techniques can be used to reproduce       digital numbers generated by IC2. One
the analog-to-digital converter circuit.      interesting application would be a digi-
The use of IC sockets is recommended,         tal current meter that can be made by
and the standard precautions for the          adding, such a display network and by
handling of CMOS devices should be            eliminating RI 1. This can then in turn
employed with respect to IC2. Be sure to      be converted into a high-impedance
observe the polarities and pin basings of     (as much as 10 megohms) digital volt-
the semiconductors employed in this           meter.                                   0
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