How Do ADCs Work? - The Learning Circuit

hello and welcome back to the learning circuit i'm karen and today we're going to learn about analog to digital converters [Music] let's start by making sure we have a good understanding of analog versus digital many of our electronic devices are digital circuits that use analog sensors to interact with the real world these analog sensors provide a signal that can register any value between 0 volts and a given maximum voltage like 5 volts or 9 volts digital signals are expressed in binary as we've learned before binary is a series of ones and zeros with each number placed counting up to one and then rounding over we can compare digital to a light switch turning the light either on or off whereas analog would be like a dimmer switch able to set the light level to any brightness between its maximum and off however digital signals are not limited to just on and off digital devices are often capable of counting to multiple places in binary every time the number increases and gains a new place it gains a bit you've probably heard bit before like with vintage 8-bit video games like atari or the original super mario brothers or like my shirt an 8-bit number would have 8 places or bits this can also be expressed to two to the eighth power which is two multiplied by itself eight times a decimal equivalent of two hundred and fifty-six looking at binary values expressed in ones and zeros as an exponent of two and as a decimal number we can see that when a binary number gains a bit the decimal number tells us how far it counts in order to gain that bit this decimal number is the resolution binary starts counting at 0 so while 8-bit has a resolution of 256 the highest 8-bit number is 255. analog-to-digital converters start as low as 8-bit and can be found as high as 32-bit but are most commonly 10 12 16 or 24 bit looking at the waveform of an analog versus a digital signal an analog signal would have a smooth continuous slope representing how the signal can be infinite values between 0 volts and the maximum voltage the waveform of a digital signal changes in steps with a finite number of steps equal to the adc's resolution divided evenly between 0 volts and the maximum voltage the signal change at each step goes low to high or high to low adcs can convert analog to digital signals in a variety of ways in the first type of adc we can see how a comparator is responsible for each step change previously we've learned that a comparator has two inputs if the positive input is greater the output is a high one and if the negative input is greater the output is a low zero a flash or direct conversion adc uses a number of comparators for parallel encoding here is a three bit adc having three output pins one for each bit since a three bit adc counts from zero to seven seven comparators are needed each comparator connects to the analog input signal as well as a voltage divider that splits the reference voltage the comparators process their signals simultaneously each outputting a 0 or 1 accordingly the comparators feed into a logic circuit which processes the data and outputs the 3-bit binary number the analog input wave is converted to a 3-bit digital wave this style of adc is great for how fast and efficient it can convert the signals due to its parallel processing however given the large number of comparators needed and since each bit requires its own output pin chips can quickly become large and expensive to manufacture to solve the problem of requiring an output pin for each bit rather than having numerous parallel outputs many adcs run the output signal through a shift register which then sends serial data out a single output pin the next style of adc is called a digital ramp or ramp compare with this style of adc a free running binary counter produces a sawtooth wave fed into a digital to analog converter the sawtooth signal gets compared to the analog input signal and when the signals match the corresponding binary number is output the adc's timer resets and the sawtooth starts over the sawtooth always starts from zero so the lower the signal the faster the signals match but the higher the signal this inconsistency and update frequency can be problematic in certain applications the next type improves upon the ramp model being slightly faster due to its special counter circuit known as a successive approximation register or sar this adc starts with its most significant bit comparing it to the analog signal if the signal is less than or greater than the counter adjusts the bit values up or down accordingly continuing to compare bits down to the least significant narrowing the range until the signals match then starting over again at zero the next type of adc tracks the previous output signal constantly comparing it to the analog input signal depending on if the input signal is less than or greater than it was previously the counter adjusts up or down this method is much faster since the counter never has to reset the downside to this type of adc is that it experiences a phenomenon informally known as bit bobble due to the fact that the output updates on every single clock pulse the signal will always change at least slightly up or down the next type of adc compares similarly to a ramp adc generating a sawtooth wave but instead of using a digital to analog converter this slope or integrating style adc uses an op-amp circuit called an integrator a slope adc uses the analog input voltage to charge up the internal capacitor and then measures the time it takes to discharge across the internal resistor however the rate of integration and the rate of count are independent of each other so over time they inevitably drift and the adc becomes less accurate a slight variation on this model is the dual slope adc its integrator circuit is driven positive and negative in alternating cycles to ramp down and then up rather than being reset to zero volts at the end of each cycle a few other types exist like a delta encoded or a sigma delta adc but are either too complex or rare enough to go into detail about how they work how the adc processes signals will affect its step recovery time an important factor to consider when selecting an adc step recovery is a measure of how quickly an adc changes its output to match a large sudden change in the analog input when choosing an adc you'll also want to pay attention to resolution i mentioned before that adcs most commonly are 10 12 16 or 24 bit note here the resolution differences between those chips the more bits the higher the resolution the more accurate the signal conversion think of it like video resolution in full resolution this video is 1920 by 1080 pixels or 1080p changing the video resolution down to 720p for the video to be clear it gets smaller but zoomed in you can see the quality looks degraded let's make it even more pronounced and change the resolution to 240p that looks horrible look at all those pixels to make the video look clear look how small i would have to get high resolution is definitely better hey let me out of here another factor to consider is sample rate the sample rate is the frequency at which the adc converts signals and outputs a new binary number sample frequency is measured in hertz and can be found in the datasheet sometimes labeled as clock frequency for applications with slow changing signals a low or slow sample frequency would be sufficient like measuring how much water is in a rain gauge however with signals that change rapidly like audio frequency signals that vary several thousand times per second the converter needs to be considerably faster the input signal frequency must not surpass what is called the nyquist frequency this is typically about half the sample rate if the input signal frequency is too great an effect called aliasing may occur as you can see this sample rate isn't fast enough to reflect an accurate analog to digital conversion ideally the digital and analog waves should look alike analog to digital converters are commonly used in processing audio such as in music recording as well as in many scientific instruments such as those that use radar or measure temperature pressure ph or light intensity how else can you think of using an adc or do you already know because you've used one in a project maybe you've used one you didn't even realize it keep a lookout for my next video where i show one way to use an adc i'll tell you a secret there's one in an arduino [Music] as always if you have any questions or comments about what we learned in this video you can find me maker karen on the element14 community on element14.com forward slash the learning circuit happy [Music] learning [Music] you

Channel: element14 presents

Views: 125,475

Keywords: electronics, hardware, hacking, mods, weekly, element14, maker, engineering, element14presents, Analog, Digital, Binary, Bits, Flash, Ramp, Parallel encoding, Comparator, Digital ramp, Ramp compare, SAR, Successive approximation register, Shift register, Tracking, Slope, Sawtooth, Waveform, Step recovery, Resolution, Delta-encoded, Sigma-delta, Sample rate, Sample frequency

Id: g4BvbAKNQ90

Length: 10min 12sec (612 seconds)

Published: Wed Jun 23 2021