Two Bit A/D Converter
This module for an electronic music synthesizer performs various logic functions selectable under voltage control. It also functions as an envelope generator and low frequency oscillator.
The inspiration for the design was drawn from two sources:
Juergen Haibles original Varilogic Module which can be accessed at:
http://www.synthfool.com/diy/hjvarlog.gif
And tomg's Boolean Logic Module which can be accessed at:
http://www.synthesist.xsystem.net/efm/maxx/page_029.html
The LM3900 is still in production at Texas Instruments. The datasheet is at:
http://www.ti.com/sc/docs/products/analog/lm3900.html
The purpose of the redesign was to reduce parts cost and put the logic modes under voltage control. It was first decided to make the positive and negative logic outputs always available simultaneously. The logic element then is a two input, two output block. The original Varilogic module supported the logic operations AND, OR and EXOR. So to put these under voltage control, a three state analog to digital converter is required, or really a "Two Bit" binary analog to digital converter with an absolutely minimum parts count and price. The LM3900 a single supply current differencing amplifier which is capable of acting as a current comparator with two summing node inputs each one a diode drop above ground. This allows us to use the most significant bit comparator to switch the threshold of the LSB comparator with only a diode and resistor.
Two Bit A/D Converter
The circuit is designed for a +15 volt single supply. It has been tested, the purpose of the 1N914 diode between pin 5 and pin 3 of the LM3900 is to prevent current leaving the pin 3 node when pin 5 is low. The circuit has the following truth table:
Mode In |
Bit 1 |
Bit 0 |
0 Volts |
0 |
0 |
~2.5 Volts |
0 |
1 |
~5 Volts |
1 |
0 |
~7.5 Volts |
1 |
1 |
The output buffers convert the 15 volt level into a unified 10 volt output for use with ARP standard synthesizers. Diodes may be needed in series with pins 4 and 5 if the output voltage needs to go to exactly 0 volts.
We still have two sections of the LM3900 left. One of them will be used to synthesize the wanted logic functions. The method used is similar to the McCulloch-Pitts Neuron where variable inhibitory levels are used to control the OR/AND function. If we assume a 10 volt logic signal into a 100K resistor the current is roughly 100 micro(u)amps. For an OR gate the threshold should be about 75% of 100 uamps or 75 uamps. Since 15 volts is our current supply the resistor value should be 150K ohms. We want to have some leeway in our currents and so we set our input to 110 uamps at 10 volt with a 91K resistor.
Varilogic II Core
The circuit has the following truth table:
B1 |
B0 |
Function of A, B |
0 |
0 |
Set/Reset Latch (Hysteric Comparator) |
0 |
1 |
OR Gate |
1 |
0 |
AND Gate |
1 |
1 |
EXOR Gate |
A CD4052 dual 4 way multiplexer is used to switch the currents into the pin 11 summing node. Pin 9 of the 4052 should connect directly to pin 5 of the 3900, also connect pin 10(4052) to pin 4 (3900). In Mode 1 the input current to pin 11 is set to 75 uamps by the 150K resistor to +15 volts. In Mode 2 the current of another 150K (75 uamps) resistor is added, the threshold increases to 150 uamps and the circuit becomes an AND gate. In Mode 3, the current is again set to 75 uamps but with the output of a permanently wired AND gate used to force the output low for the 1,1 state giving an EXOR truth table.
Slew Rate Limiter
Mode 0 needs additional explanation. The second section of the 4052 is used to feedback 150 uamps from the output to the non-inverting (+) input. This sets up a "memory" condition from the positive feedback. Also an approximately 25K resistor (shown as a 50K trimpot) is connected to the inverting input. This is enough current to overcome the combined currents of the 91K and 75K into the + input. If the A set input is pulsed to +10, the output will remain high until the B input is pulsed. If a slew rate limiter is connected from the Q output to the B input the circuit will either oscillate or act as a one-shot depending on the state of the A input. If the A input is pulsed high and returns low, the circuit will cycle once like a envelope generator. If the A input is held high the circuit will oscillate like an LFO. While not very precise, the circuit satisfies the design goals of minimum parts count and expense.
Grant Richter
January 4th, 2000