|M. Yamada's SAR ADC (Last Update: 2023 06 14)|
Analog to Digital Conversion
A large sector of the modern digital world is dedicated to interfacing with natural world analog data. Sensors (transducers) are devices that can measure natural phenomema such as sound, heat, pressure, distance, accelaration, flow, force, gas, smoke, methane, etc. and output continuous voltage levels. Microcontrollers possess the functionality to digitize (binary number) these voltage levels through the use of analog to digital conversion (ADC) circuitry.
Of the variety of different algorithms and processes ADCs engage, successive approximation is one of the most common.
Before the emergence of microcontrollers (μC) with built in ADC functionalit , there were microcprocessors (μP) that relied on external ICs dedicated to perform the ADC function. One such IC that employed the successive approximation algorithm was the ADC0804. This IC is no longer manufactured.
The photo to the right shows our implementation of the ADC0804 (click to enlarge). As the potentiometer is turned, the LEDs reflect the 8-bit digital approximation to the voltage level (0-5V → 0-255).
Although the ADC0804 is considered obsolete, successive approximation remains a fascinating concept and circuit to prototype.
G. Davidge (ACES '24) pointed us to an intriguing ADC project that he came across on the Hackaday web site: https://hackaday.io/project/181826-homemade-successive-approximation-register-adc
The ingenious circuit from Mitsuru Yamada undertakes the successive approximation algorithm to convert an analog voltage level between 0 and 5V into a 7-bit binary representation.
So intrigued were ACES with this circuit that J. Strain (ACES '23) elected to take on the project for his final Grade 12 ISP. He made significant headway.
J. Rogan (ACES '23) also accepted the project challenge once his final exams were over in early June '23.
Inspired by what these students were pursuing, Mr. D. took the project on in June '23 with some guidance from the students' experiences. This led to a successful result as shown on the scope image below, left. The prototype appears below, right, albeit with some less than impressive wiring.
The majority of the ICs in this project are in the 74HC family that require a 5V supply. Two exceptions are the LF398 sample and hold IC and the LM393 op amp comparator that benefit from access to +/-9V supply. To achieve this, a custom power supply was imagined and developed that includes a feature ACES might find useful in this and future projects. The onboard Wien Bridge Oscillator employs an LM741 op amp to produce a sinusoidal wave with a frequency in the 1400 Hz range. A summary of this project undertake in early June '23 can be found at, http://darcy.rsgc.on.ca/ACES/TEI4M/WeinBridgeOscillator/index.html
LF398 (Sample and Hold)