#340 How good are the ADCs inside Arduinos, ESP8266, and ESP32? And extenal ADCs (ADS1115)

I often get questions about how to measure voltage with microcontrollers we will look at this topic at the quality of built-in and external analog to digital converters and I will show you some secrets so let's start pretty youtubers here is the guy with a Swiss accent with a new episode and fresh ideas around sensors and microcontrollers remember if you subscribe you will always sit in the first row voltages are analog and our controllers are digital this is why analog to digital converters short a DC's became so important they are the heart of digitalization the opposite by the way are digital to analog converters or DAC s they create an analog voltage out of a digital number in this video we will see how a DC's work look at how we can determine the quality of ADCs compare the a DC's of different microcontrollers with external chips learn how to tweak the calculations to get the best out of any ADC see how we can extend the range of ADCs discover some hidden things of microcontrollers and there a DC's look at different external boards and their usage in the pre-digital age transistors or even valves were very expensive and big so many different ADC converter principals were invented and used to save such components today with our silicon technology we do no more need to optimize for parts this is why a few exotic principals were left behind today we mainly use these two principals parallel comparator a DC's and successive approximation ADC s let's start with a parallel comparator a DC's they are fast expensive and not too accurate fast means they can convert signals up to gigahertz which is crazy if you ask me their main turf is software device and radios and other extremely fast processes if you are interested in what software-defined radio is you can watch video number 286 but how do these parallel competitor ABCs work they are quite simple and consists of only a few parts the first is a stable reference voltage this part by the way is vital for all a DC's then they divide the reference voltage into many voltage steps let's assume for simplicity we want to build a two bit ADC with a voltage range of 10 volts then we divide the 10 bolts into four steps of 2.5 volts we can do this with a simple voltage divider where we cut one resistor into like that we get voltages of 1.25 three point seven five six point two five and eight point seven five volts we now connect one input of a comparator to each of these voltages and connect the second input to the input voltage it is quite apparent what happens now a voltage of two volts for example is bigger than 1.25 volts so the first comparator triggers and all others not at eight volts three of four comparators trigger immediately when we apply the voltage if we add a few gates we get the two-bit signal at these pins of course all comparators have to read the signal at the same time otherwise we do not get accurate numbers for changing signals this is why we apply a clock to all comparators its most significant disadvantage is scalability for each additional bit of resolution we have to double the numbers of comparators a quick calculation for a 12 bit ADC shows 4096 comparators including the precision resistors and all the connections I would call this a nightmare for a chip designer and here we can discuss another important topic resolution and a see if we have a voltage of 1.3 volts we get a 1 if we apply 3.7 volts we also get a 1 we get such a staircase and see that this ADC has a resolution of 2.5 volts so it would not be suitable to monitor a lion battery for example we will need more pits to get more steps for this calculation I assumed that the resistors and the comparators are precise let's assume this resistor is a bit smaller than the others or this comparator does not switch at precisely 0 volts then this switching voltage would be smaller and we would get a 243 point 7 volts which would be wrong or not accurate you see there is a difference between resolution and accuracy resolution automatically increases with the number of resistors and comparators accuracy depends on their quality both are cost drivers for chips the more resolution or the more accuracy the more expensive not to forget speed which is also a significant cost driver the same thing applies if the reference voltage for example is only 9 volts the resolution would still be the same but the accuracy would be miserable do you think this is theoretical then look at the data sheet of the atmega328 used in Arduinos they use VCC as a reference and your USB power supply never delivers precisely 5 volts now we defined the voltage range the resolution and the accuracy of an ADC the next important thing is speed it is quite evident that for this ADC it mainly depends on the speed of the comparators plus the speed of the logic gates as said before parallel comparator a DC's go up to a few gigahertz an example is the 8090 208 which covers up to three gigahertz not bad also the price of $1700 for one chip is not bad you definitely pay attention to what you do if you work with one of those one remark if you read the data sheets of those fast ABCs you often see DP FS and not milli or microvolts this is because our f engineers mainly work with DB now we leave the expensive world of parallel comparator ADCs and continue with successive approximation ADCs their function in a way is very similar to what we saw before the input signal is compared to a reference signal what was done in parallel before is now done sequentially by only one comparator therefore we have to supply it with an ever changing reference signal and it takes longer let's assume an input voltage of 2.5 volts if our reference voltage starts at 0 volt and increases slowly at precisely 2.5 volts the comparator changes it output signal if we knew the reference voltage we would know the input voltage simple if we use a DAC to create the reference voltage we know of course which output voltage it creates because we apply the digital value at its input we just have to transfer this value to the output pins and ready is our ADC nearly as before we have to make sure that the value of the input signal is stable during the whole measurement otherwise the result would be wrong and as said before the measurement takes quite long this can be done by a sample and hold circuit which is mainly a capacitor and switch the switch is closed till the ADC wants to read a value and because the switch is closed the voltage of the capacitor tracks the input voltage as soon as the switch opens the voltage at the capacitor stays fixed at least if we make sure that we do not discharge it with our comparator another problem is quite apparent if we assume we have a 12 bit ADC for 10 volts and the input voltage is nine point nine volts the resulting digital value would be around 4050 our ADC would have to make 4049 wrong in tests until the comparator would switch from zero to one you can imagine this would be extremely slow fortunately we can do much better we can start with five volts the nine point nine volts are higher so the next value to test is 7.5 volts still not enough next eight point seven five and so on you see the principal instead of more than four thousand tests we will end with maybe ten tests and this is basically how these ABCs work just out of curiosity I show you how a typical DAC works it has a similar voltage resistant etwork as the parallel a DC's with two main differences the resistors are arranged in a r2r ladder and instead of a bunch of comparators we need a bunch of switches this network creates a voltage that is proportional to the binary value applied to the switches you find a link in the description if you want to do the calculations in the end an amplifier is added and ready is the DAC and of course also here a voltage reference but how about the resolution and the courtesy of a successive approximation ADC the same applies here the longer the ladder the more resolution of the DAC and with it also the ADC of course the accuracy is mainly determined by the accuracy of the DAC and the comparator most of the ADCs we use in our projects are of this type to save money we can even go a step forward and add a switch in front of the ADC like that we get four eight or even 16 inputs cool pay attention these chips only have one ADC and therefore can only convert one input at a time so their speed is divided by the number of analog inputs now we know the important stuff about a DC's let's apply this knowledge to the chips in our lab and start with the Arduino Uno the atmega328 has an 8 channel 10 bit successive approximation ADC so it is the type we saw before 1 ADC and 8 inputs it has a pin for a voltage reference but because this pin is not connected on the Arduino boards VCC is used as a reference which for sure is not optimal the Arduino has a built-in voltage reference but only for one point one volts with some tricks as shown in video number 10 you can increase its accuracy using this reference if we divide 5 volts by 1024 available steps we see that the resolution is 4.9 VD volt the smallest difference we can measure is around 5 millivolts it's a courtesy is around 2.2 LSP which is 2 point 2 times 4 point 9 equals 11 millivolt with an external voltage reference without it it can be much higher as we will later see its conversion time is 65 to 260 microseconds which is in line with what we learned slow and depending on how fast it hits the right value now we go on with the esp8266 it also has a 10-bit ADC without multiplexer so it has only one input and essential its initial range is only up to 1 volt which is vital to know fortunately many board manufacturers extend its range to 3.3 volts how is this done and how can we use it to extend the range even further they use a simple voltage divider like this one on the vemos d1 mini it reduces the voltage by a factor of 3 let's now assume we want to measure 24 volt our panels for sure we want the marching and set the maximum voltage to 30 volts we use a voltage divider like that at an input voltage of 30 volts we want an output voltage of 1 volt for the ADC pin this is the formula and if we enter the values we get 2.9 mega ohms minus the 220 K already there we have to add 2.7 mega and the remote should show the right value of course it will not display the proper value because the resistors are not precise and maybe we do not have the exact value this is why we use a mapping formula we first apply 30 volts and note the value measured by the ADC then we insert this value into the formula and get adequate values the same principle can be used for all other microcontrollers I probably would not try to measure about 50 volts with a simple method because it can become dangerous for humans and of course it only works for DC and not for AC voltages now we go on to the ESP 32 it also has its secrets it has two different 12 bit ADC s with a total of 18 channels their quality gives rise to discussions in my last video many people complained about the in accuracy of this ADC the datasheet has quite a lot to say which usually is not good because the manufacturer adds exceptions and prerequisites we will see if this is true when we lay to test it also keep in mind that the ESP s have quite a strong RF signal very close to the ADCs this can easily influence its readings this is why specifications are done with Wi-Fi and Bluetooth off if you encounter issues with the readings my first step would be to switch off all radios and test again another trick is used in the specifications the addition of a 100 nano farad capacitor this also reduces noise on the analog line but of course it also read measuring speed talking about speed the maximum is two million samples per second which translate in a conversion time of 0.5 microseconds which is much faster than the Arduino maybe this is partly responsible for the inaccuracy unfortunately this is not all in a small and casual post Igor mentioned the fact that all 10 pins of ABC 2 cannot be used if we use Wi-Fi so it only has 6 pins left GPIO 32 2 GPIO 39 fortunately some boards connect GPIO 36 and 39 to sensor VP and sensor VN so they should also work with Wi-Fi on Aliexpress we find modules with external ADCs the most common one is the ad s1 1 1 5 breakout port it has one 16 bit ADC with 4 input pins and it is connected via I square see it's sample rate is 860 samples per second maximum and it has a specialty a so called programmable gain amplifier short PGA it can be used to increase the resolution for smaller voltages we find a bunch of data about the courtesy in the datasheet and from the discussions before we can now understand most of it and see that this chip plays in a rather leek than the integrated a DC's of course you get other a DC's like the PCF 85 91 or the ad 7705 the PCF 85 91 is only 8 bits but it also has an 8-bit DAC so if you want to control something and measure the result this might be a suitable module the 80-77 of 5 also is 16-bit but only has two input channels and is connected by the fast spi bus and is primarily used for the digitalization of fast signals let's now check the different a DC's out I have here an exact voltage reference which can produce exactly two point five and five volts let's start with the Arduino Uno at 2.5 volts it displays 496 to 498 which is plus minus 1 LSB but is this correct we map the values with this formula and get mostly 2.5 or 2.5 1 volts not bad but now I increase the supply voltage of the Arduino to 5 point 2 volts which is still in the usb specifications now it shows only 2.4 volts at 4.8 volts supply voltage I chose 2 point 6 volts interesting but of course not good if I power the Arduino with 7 point 5 volts at the barrel check we get quite constant 2 point 5 1 volts so either you power your Arduino at precisely 5 volts or you watch video number 10 where I show how you can use the internal reference to stabilize these values next is the esp8266 it definitely is less stable and has glitches from 829 down to 821 but it is faster than the Arduino so I average across two values now we get fewer twitches if I average across 10 values we still have glitches even if I average across 100 samples we still get an unstable signal but at least we get 2 digits stable at 2 point 6 7 volts which is of course wrong you saw now one way to reduce noise in a signal by averaging and you saw how simple the average formula is we quickly can't correct the wrong voltage by adjusting this parameter to 3.09 now we get the desired 5 volts the chance all other measurements are also correct is high so let's check it with one bolt yes one volt is also okay of course not more precise but at least no gain error we corrected it using this method if I connect the AC rope in to ground it should read zero volts but it reads 0.01 volts this is the small offset error and we correct it here now it reads zero volts of course we should have corrected the offset before the gain error with averaging and the calibration formula we get at least the maximum out of a particular ADC as promised we want to extend the range to 30 volts we added 2 times 1 mega ohm resistors are sufficient and the factor is 31.8 now our we mostly one mini can measure 30 volts not bad let's look at the ESP 32 and do the measurements the raw data looks scary indeed but let's look at what we can do if we average across 100 values and adjust this factor to 3 point 6 1 we get a consistent 2.5 volts of course we have to adjust this number to the 12 bit of the ESP 32 average over 1,000 values creates a quite stable three-digit number but when we go to one world we see the non-linearity of the ESP 32 ADC it only shows 0.9 volts nearly 10% off so this ADC has a high resolution of 12 bits but because it is very inaccurate this resolution is useless now we use a real ADC chip the ad s1 1 1 5 connected to the ESP 32 we see it is much more stable and it has four bits more resolution impressive of course it is also much slower if I average across five values we hardly see the noise but we do not need averaging here it is good enough without only the fourth stitch it moves a little I removed the offset and adjusted its values to precisely the value of the 5 volts reference of course we have to adjust this number to the 16 bits of the ADC and the linearity at 2.5 volts it shows two point four nine nine four right on the reference overall this ADC is much better than all the built in a DC's tested before summarized we saw how the most used a DC's work and learned to determine their quality compared to a DC's of an Arduino Uno and to ESPs and saw that all of them could not be used for serious work because of their low quality we could apply some tweaking to improve the results we also extended the measuring range of a WHMIS d1 mini to 30 volts we discovered that the Arduino reacts on fluctuations of VCC and how to avoid it a tiny comment of Igor revealed that we could not use most of the ADC pins of the ESP 32 because they are used for Wi-Fi stabilization we tested an ad s1 1 1 5 external ADC board and saw that it has superior quality compared with all the internal ADCs and it became clear that we have to use external ADCs for serious work one last thing this is my nicest looking ADC in the lab it has eight parallel channels and is quite fast as always you find the relevant links in the description I hope this video was useful or at least interesting for you if true please consider supporting the channel to secure its future existence thank you bye

Channel: Andreas Spiess

Views: 122,587

Keywords: arduino, arduino project, beginners, diy, do-it-yourself, eevblog, electronics, esp32, esp32 datasheet, esp32 project, esp32 tutorial, esp32 weather station, esp8266, esp8266 datasheet, esp8266 project, greatscott, guide, hack, hobby, how to, iot, lorawan, nodemcu, project, simple, smart home, ttgo, wemos, wifi, ads1115, adc, How-to voltage, measure voltage, dac, PCF8591, AD7705, AD7606, arduino (brand), analog-to-digital converter (invention), analog-to-digital converter, digital-analog converter

Id: UAJMLTzrM9Q

Length: 24min 0sec (1440 seconds)

Published: Sun Jun 21 2020

Reddit Comments

[u/mager33]

Bottomline... Arduino is OK, but ESP8266 and especially ESP32 have problems, so AD1115 was recommended. This one is slow, any suggestion for an audio rate DAC?