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[TW#12287] ESP32 ADC accuracy #164

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nassks opened this issue Dec 14, 2016 · 181 comments
Open

[TW#12287] ESP32 ADC accuracy #164

nassks opened this issue Dec 14, 2016 · 181 comments
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Type: Bug bugs in IDF

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@nassks
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nassks commented Dec 14, 2016

Hi,

I'm testing the ESP32's ADC on the SparkFun ESP32 Thing.
Currently my measure seems very noisy, it constantly varies on the 5 first bits, and the zero value is around 90mV.
Is there a way to achieve better accuracy?

My test code :

adc1_config_width(ADC_WIDTH_12Bit);
adc1_config_channel_atten(ADC1_CHANNEL_7, ADC_ATTEN_0db);

while (true) {
        vTaskDelay(300 / portTICK_PERIOD_MS);
        printf("ADC value : %x\n", adc1_get_voltage(ADC1_CHANNEL_7));
}

Thank you in advance for your help.
@projectgus projectgus added the Type: Bug bugs in IDF label Dec 15, 2016
@projectgus
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The ADC has a non-linear response, and we are working on producing characterisation data which will be added to esp-idf to produce a linear output.

When you say "noisy", are you getting noisy readings on a constant voltage or is this noise caused by the non-linearity?

There's some more in this related Arduino bug: espressif/arduino-esp32#92

@projectgus projectgus changed the title ESP32 ADC precision ESP32 ADC accuracy Dec 15, 2016
@projectgus
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@CarlosGS : for issues, it is much better to start a new one than to post off-topic in an existing one.

For questions like this, the http://esp32.com forum may be an even better choice.

@michapr
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michapr commented Dec 27, 2016 •
edited

@projectgus : I think nassks means the "noise" within same values - additional I have seen that there is a "noise" between the inputs.
put 1 Volt (with capacititor) on inputs CH4 ..CH7 I get such results:
(using 0bd and 11db for the values)

ADC1 CH4 value: 3719 @ 0db, 1056 @ 11db
ADC1 CH5 value: 3889 @ 0db, 1052 @ 11db
ADC1 CH6 value: 3883 @ 0db, 1052 @ 11db
ADC1 CH7 value: 3891 @ 0db, 1052 @ 11db
-------
ADC1 CH4 value: 3867 @ 0db, 1056 @ 11db
ADC1 CH5 value: 3885 @ 0db, 1053 @ 11db
ADC1 CH6 value: 3887 @ 0db, 1040 @ 11db
ADC1 CH7 value: 3886 @ 0db, 1057 @ 11db
-------
ADC1 CH4 value: 3894 @ 0db, 1056 @ 11db
ADC1 CH5 value: 3855 @ 0db, 1061 @ 11db
ADC1 CH6 value: 3887 @ 0db, 1049 @ 11db
ADC1 CH7 value: 3875 @ 0db, 1053 @ 11db
-------

Difference is up to 5% - maybe it is normal?

But for 0V - I get a value "0" ... without "noise" - it is ok for me

Michael

@michapr
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michapr commented Dec 27, 2016 •
edited

Do not know why the crazy style came up before ;)

Code snippet for reference:

...
adc1_config_width(ADC_WIDTH_12Bit);
...
adc1_config_channel_atten(ADC1_CHANNEL_4,ADC_ATTEN_0db);
int val=adc1_get_voltage(ADC1_CHANNEL_4);
adc1_config_channel_atten(ADC1_CHANNEL_4,ADC_ATTEN_11db);
int val1=adc1_get_voltage(ADC1_CHANNEL_4);
printf("ADC1 CH4 value: %d @ 0db, %d @ 11db\n", val, val1);
....

Michael

@marcmerlin
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So, in case anyone finds this ADC bug, I've found another ADC issue apparently where it will return data between 0-50% input, and cap out at 4096 from 50 to 100% input, but only after I try to use RMT.
Details here: #462

@marcmerlin
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So, the lastest arduino-esp32 fixes the issue I just reported when RMT is turned on, analogread values would get damaged.

@projectgus projectgus mentioned this issue Apr 10, 2017
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@FayeY
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FayeY commented May 4, 2017 •
edited

Hi nassks, is this problem solved?

@igrr
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igrr commented May 4, 2017

@FayeY not yet, @Spritetm is working on it.

@FayeY FayeY changed the title ESP32 ADC accuracy [TW#12287] ESP32 ADC accuracy May 4, 2017
@dmody
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dmody commented May 6, 2017 •
edited

Just leaving my two cents on this...
I was hoping to use an inexpensive Voltage Reference IC (LM4040C30ILP) to help increase the accuracy of measured values. I too found the ADC to be non linear and have included my data below for anyone interested.
voltage, ADC
2 2222
3 3730
1 1033
1.5 1629
0 0
0.5 422
2.5 2840
2.8 3300
3.1 3980
3.007 3750

image

@dmody
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dmody commented May 7, 2017

took another crack at this, and assuming the ADC readings are related to the 3.3V regulator voltage (mine is at 3.275 V on a developer board). Here's what I found.
image

@negativekelvin
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@dmody interesting, have you tried to fit curves to the upper range and lower range separately?

@imotion-software
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In the future, are you planning to include linearization of adc1_get_voltage or it will be in charge to the user to correct non linear behavior?

Just to know, because if we implement linearization on user app and then adc1_get_voltage will change behavior we'll have to remove all those parts.

@dmody
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dmody commented May 11, 2017 via email

@markterrill
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really interested in this

@0xAl3xH
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0xAl3xH commented Jun 22, 2017

Are we going to fix this soon? I feel like many applications depend on this feature. Is there a workaround at least?

@Spritetm
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Yes, we are. It took us some time to build an automated testing rig to accurately and reliably grab the characteristics of a representative sample of ESP32s; that has rendered the first curves this week and we're working on converting these into some linearization code.

@0xAl3xH
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0xAl3xH commented Jun 30, 2017

@Spritetm any updates on this? Perhaps a timeline to when this might be fixed? Thanks!

@HubbyGitter HubbyGitter mentioned this issue Jul 10, 2017
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@MaxSNi
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MaxSNi commented Jul 28, 2017

@Spritetm @igrr I would love to hear an update on this - or if there is an effective workaround?

@dmody
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dmody commented Jul 28, 2017

@MaxSNi
The best workaround for me was to write a little function that returns the voltage based on the DAQ value using the formula I presented above. I can post the function if you like.

@MaxSNi
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MaxSNi commented Jul 28, 2017

@dmody I would love to see your function. I made a quick function from my own excel trend equation and while it does work better, its pretty crude...

@dmody
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dmody commented Jul 29, 2017 •
edited

@MaxSNi
//*************************************** Analog to Digital Conversion Function ****************************
float A2D(float SenVal)
{
if(SenVal<1) return(0);
else if (SenVal>4094) return(99);
else return(-0.000000000023926 * pow(SenVal,3) + 0.000000094746 * pow(SenVal,2) + 0.00074539 * SenVal + 0.14925);

}

@imotion-software
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Is there any update on this issue?
It sounds me really crazy that on an IoT chip there's no way to have a good ADC reading.

@markterrill
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Probably time to look at other options.
i realised a few months ago its simply not going to be ready anytime this calendar year for production use

@jan-bozelie
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we have a simmulair problem , i can live with a correction factor but we discoverd in testing 10 esp32 samples , the difference between samples can be >10 to 20% , making a calibration nessesery for each esp , making it unusable for serios work

@imotion-software
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imotion-software commented Aug 22, 2017 •
edited

Hi @jan-bozelie. I'm aware there should be huge differences betweens samples due to the internal reference voltage regulator, but at the moment ADCs suffer of two distinct issues:

  • Non linearities
  • Different end of scale due to internal voltage reference

I think we should split ADC issue in two steps. Once we have corrected the non linearities, the EOS issue could be easily faced using an external voltage reference or a calibration procedure.

This problem has been totally ignored by Espressif, and I think ADCs are really important for an IoT application.

@igrr ... can we have an official response from Espressif about the fact ADC issue can or cannot be solved? At the moment we have a peripheral which is absolutely useless.

@VincentBruinink
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VincentBruinink commented Dec 14, 2020 via email

@flodmotorgrodan
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flodmotorgrodan commented Dec 27, 2020 •
edited

The ADC has unusual linearity and offset drawbacks. When feed a linear ramp the lowest range is terribly non linear.

ESP32_ADC_vs_PWM_DAC_2

Also the ADC is "dead" with no reading below 100 LSB on a 4096 scale depending on amplification.

ADC_Sweep_2018-11-20_123554

It means the ADC inputs have to be offset with external components to start measuring from 0V. That is really bad.

We can live with the noise, and it is not that bad as you can see from the multisampled curve. But the ADC being detached from reality makes it a bad experience. The ESP32 is a fantastic device it is such a pity the ADC problem never gets corrected.

@pylorak
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pylorak commented Mar 13, 2021 •
edited

This is a comparison to other chips, no bug.

Actually, this probably is a bug. It becomes more clear to see it when you have some additional context. The ESP32 contains also an internal temperature sensor and even an analog comparator, but they were removed from all documentation and specifications because they turned out to be unusable months after release. Not not-working-unusable, but working-but-so-bad-that-they-were-unusable. The hardware should still be there in everyone's chips even today, just disabled/unsupported/unreachable through the IDF libraries because their functionality was officially discontinued.

Now, the following is just a hunch, but I assume there is a connection between the removed parts and the under-performing ADC: a general (design- or manufacturing-related) problem with the analog parts of the IC, which is why these components had to be removed and why the ADC performs so poorly. Back when the ESP32 V3 came out, I was actually most excited because I was hoping they would finally fix the analog issues: improve the ADC and maybe even re-enable the other integrated peripherals. Unfortunately this was not the case.

The ESP32 is a very cool and versatile part with a low price, and probably my biggest gripe with it is its ADC. You simply cannot fix some of its defects using multisampling, and per-device calibration is not an option for mass production. I've learned to use external ADCs in ESP32-based projects where I need more than just simple "room temperature measurements". One of my current favorites is the TLA2024. Cheap, small, accurate and easy-to-use.

@DavidSG2020
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I have been doing analog and digital electronics since the dawn of the microprocessor era., and I can say quite definitively that this is not a bug.

It is totally and utterly incompetent design papered over by marketing specmanship.

The guys who designed these parts are brilliant digitologists, but don't know squat about analog chip design.

IMHO the only kind of ADC for which "multisampling" can be justified as a means of hitting a headline spec is in a sigma-delta converter, where you can achieve 23 bit resolution with sufficient filtering. Been there, done that.

Headlining a 12-bit ADC then burying the truth in a table deep in a data sheet is simply deceptive. And that truth says to me "8 bits".

I am now a happily retired hobbyist, doing my first ESP32 design. Fortunately I don't need the ADC function except for a trivial reading of an LDR. I love the pure grunt of the ESP32. Well, atleast I am forewarned if my next project needs some serious A to D.

@srini1948
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srini1948 commented Mar 17, 2021 via email

@CarlosGS CarlosGS mentioned this issue Oct 18, 2021
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@CarlosGS
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I didn't realize that esp-idf now incorporates functionality that resolves the linearity issues (internally it uses a lookup table here). Espressif even provided a reasonable calibration solution by flashing chips since 2018 with an accurate Vref reading (link). The linearity correction works for all chips.

Not sure if there were any changes to the hardware regarding the low-end zero value of ~70mV. That could still be a problem depending on the application.
But at least the calibrated API provides a layer of abstraction that enables the commercial use of the embedded ADC. Thanks!

@dmody
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dmody commented Oct 21, 2021 via email

@eos1d3
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eos1d3 commented Jan 21, 2022 •
edited

For what I read from ESP32, ESP32-S2, ESP32-C3 and ESP32-S3 IDF programming guide, I find there is a table which is exactly the same for all these chips. There is no update for C3 and S3.

https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/peripherals/adc.html#_CPPv425adc2_config_channel_atten14adc2_channel_t11adc_atten_t

+----------+-------------+-----------------+
|          | attenuation | suggested range |
|    SoC   |     (dB)    |      (mV)       |
+==========+=============+=================+
|          |       0     |    100 ~  950   |
|          +-------------+-----------------+
|          |       2.5   |    100 ~ 1250   |
|   ESP32  +-------------+-----------------+
|          |       6     |    150 ~ 1750   |
|          +-------------+-----------------+
|          |      11     |    150 ~ 2450   |
+----------+-------------+-----------------+
|          |       0     |      0 ~  750   |
|          +-------------+-----------------+
|          |       2.5   |      0 ~ 1050   |
| ESP32-S2 +-------------+-----------------+
|          |       6     |      0 ~ 1300   |
|          +-------------+-----------------+
|          |      11     |      0 ~ 2500   |
+----------+-------------+-----------------+

I think only original ESP32 has problem measuring voltage low below 100-150mV. It is obvious an original ESP32 default fault. I assume they fix this for later chips.

The ADC is nosier than other 8 bit or 32 bit MCUs. But calibration is not unique for ESP32. STM32 also calibration function before the use of ADC. But they do not use lookup table and raw value can be used directly. I think STM32 has hardware to offset the ADC error.

I would like to know how S2, C3 and S3 perform. But the use of lookup table indicates the linearity issues are still there.

@Tobbera
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Tobbera commented Jan 26, 2022

@MaxSNi //*************************************** Analog to Digital Conversion Function **************************** float A2D(float SenVal) { if(SenVal<1) return(0); else if (SenVal>4094) return(99); else return(-0.000000000023926 * pow(SenVal,3) + 0.000000094746 * pow(SenVal,2) + 0.00074539 * SenVal + 0.14925);

}

Thank you for this one. This is the only off-the-shelf solution that I have found to improve the accuracy without doing manual measurements and building LUT´s etc.

@jan-bozelie
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jan-bozelie commented Jan 29, 2022 via email

@melihkucukcalik
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ESP32-S3 ADC noise test

I made some tests with ESP32-S3 Devkit-C. First I insert Devkit to breadboard , and measure a fixed voltage around 1.85V which is divided by resistors from 3V3 and filtered by a capacitor. The results was so bad, there is %5 noise on ADC values.

Then I solder a capacitor (10nf) directly to ground and ADC port pin on Devkit. This reduce the noise so much. I understand that making tests on breadboard mislead tests.

Then I made one last test. instead of using Devkit on board USB-UART connector , I connect an external UART-USB converter and supply Devkit with battery. UART converter supply was separated. This also improves the signal quality. So keep in mind that your computer USB port also can have noise on 5V output.

Cap Filter on breadboard:
image

Cap Filter soldered to Devkit:
image

Cap removed while running:
image

@eos1d3
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eos1d3 commented Apr 22, 2022

@marcmerlin Did you have oversampling with the tests? And how many of sampling for each point?

From my case, I do not think USB supply from PC will make any difference. For example, I have tested STM8S and also STM32G0 ADC using the USB power supply from the same PC, the noise is just 1 or 2 from 10 bit (1023) reading without any capacitor. Using the same USB from the PC, ESP32, C3 and S3 are totally unacceptable.

I recently changed the PSU of my PC, and find the old PSU will have noise around 2 or 3 from 1023. The difference is so small.

And I also notice when WiFi is turned on, the noise is even much larger for ESP chips.

You finding is interesting, I think I will try S3 module only to test again. The ESP boards never gave me good results.

@melihkucukcalik
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First I want to say that Wifi is closed. This morning I rebuild my test setup with more better looking for photo. You can find that setup photo at down. I also made few more test. And again results are good. And there is no over sampling. Max value readed is 1759 min value is 1755. Difference is so small 5. With battery it seems that difference is down to 4 (rarely 5 I saw). This tests didn't show that if there is linearity errors on ESP32-S3. But it shows that it has acceptable noise level. If you take care while drawing PCB you can use it where ever you want.

The mainwhile code is just like following:

vTaskDelay(50 / portTICK_PERIOD_MS);

adc_val =  adc1_get_raw(ADC1_CHANNEL_3);

sprintf ( test_str, " %d \n \r",adc_val);
uart_write_bytes(UART_TEST_PORT_NUM, (const char*)test_str, strlen(test_str));

USB powered with Caps added to pins:
image

Test Setup:
image

@MitchBradley
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MitchBradley commented Apr 22, 2022 •
edited

The most serious ESP32 ADC problem was not the random noise or the nonlinearity, but rather the fact that the value would make big jumps every few seconds. Random noise picked up in external circuitry can be reliably reduced with good layout and good analog design practices including power supply filtering. Random noise due to internal factors can be reduced by mathematical filtering such as IIR polynomial filters or FIR filters like averaging. Nonlinearity can be reduced with lookup tables or math functions that invert the nonlinearity. But big jumps that occur at long time intervals compared to the sample rate are very hard to handle. It is unclear what was causing the big jumps. Some issues have identified data transmission problems where bits are getting lost or added due to problems in interconnects like I2S or RMT or UART channels. In other cases, it seems that the ADC is affected by the WiFi radio - which is unpredictable due to its interaction with outside WiFi devices. Still another possibility is internal chip voltage and current shifts related to cache refills and other FLASH accesses - again difficult to impossible to predict. We can hope that S3 corrects some of these big-jump problems with better isolation between the ADC and the many other functional elements on the chip, but the limited experiments above, which mainly focus on short-term random noise behavior, do not tell us much about the long-term behavior that limited the ESP32's ADC general utility.

@chmorgan
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chmorgan commented Apr 22, 2022 via email

@srini1948
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It would have been ideal for my project but for the ADC. I am not sure why they dont fix it.

@MitchBradley
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If you knew how much it costs to do a major revision to a chip design, and how difficult it is to integrate analog and digital functions on a large chip with many different sections including a WiFi radio, they you would know why they don't just "fix" it.

@eos1d3
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eos1d3 commented Apr 22, 2022 •
edited

The accuracy is one issue. But the way of handling continuous ADC DMA reading is another HUGE issue while Espressif seems did not know and understand what is required to make it useful.

Current IDF implementation of continuous ADC DMA reading involves ISR at end of each conversation. They use very large ring buffer to store EVERY instance of ADC values so that buffer is crazily large. And for each read operation by application, the buffer is full of duplicates of the same channels and with old values which are mostly useless for general applications. And this repeats again for each ADC channels. So it may use many KB for just ADC reading and this is crazy.

To solve the noise issue, Espressif suggest to use oversampling. But with this, the CPU will be mostly used for calculating the average values of ADC readings.

Currently the way of handling ADC in IDF is mostly suitable for audio recording or oscilloscope type applications as they save every reading from ADC. So there is no way to get real time ADC and the buffer is full of old and useless readings. From my test, if application does not handle it correctly, it may take more than 60 seconds to get recent values.

I did contact them via company account for this issue and they gave good response. It seems they understood and I was told than they will have 'real time' version of functions added in future IDF.

Compared with STM32, their ADC is just perfect. All ADC oversampling and conversion is done by ADC hardware with zero CPU time. And it is absolutely clean with no noise issue at all. And it uses just one 16 bit integer for each channel reading.

I will try again for S3, but the initial test with huge CPU and memory usage did not impress me. I believe oversampling via software is not the solution for ESP ADC noise.

PS: In ESP32, ESP32-S2, ESP32-C3 & S3, the ADC modules are very different. It seems they tried to improve ADC for each generation. And the original ESP32 is the worst for ADC.

PPS: @MitchBradley, which ESP32 revision for "big jumps every few seconds"? I tried latest ESP32 V3 and do not have this issue.

@rsaxvc
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rsaxvc commented May 1, 2022

ADC can also be run through the ULP to support multiple separate ring buffers as needed. However, it's quite complicated compared to other microcontrollers.

@Guangyang13
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Random noise due to internal factors can be reduced by mathematical filtering such as IIR polynomial filters or FIR filters like averaging.

Hi, can I know how to design and implement the IIR filter to reduce the noise? Is it got any example or source that I
can go through? Thank you very much

@MitchBradley
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The simplest IIR filter looks like this in C. There are many more complex ones.

#define factor  0.1
float last = 0.0;
float next(float sample) {  // sample is the value from the ADC converted to a float
   last =  last * (1.0 - factor) + sample * factor;
   return last;
}

You can make the filtering "better" by reducing the value of factor, at the expense of it taking longer for new data to affect the output value. Conversely, larger factor values respond faster with worse filtering. Filtering is a big topic and there is a lot of literature on the internet about it. The floating point calculations can be done with integer arithmetic if you know how - but simply replacing "float" with "int" will not work.

That is all I am willing to say. Good luck.

@eos1d3 eos1d3 mentioned this issue Oct 31, 2022
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@italex33
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italex33 commented Dec 10, 2022 •
edited

This is a VERY long thread to discuss the Analog inability of the ESP32. All kinds of USELESS things are listed here. Why bother too much with the ESP32 when you can use, for example, the MKR1010? It is way much better and VERY accurate. The ESP32 in my opinion is a work in progress. The ESP32 ADCs are basically useless!

@eos1d3
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eos1d3 commented Dec 10, 2022 •
edited

It is because many use ESP32 chips or modules for productions, not just prototyping. The ESP32xx modules are just around $2.

It is true that ADC of all Espressif products are completely useless. But the way they handle ADC API is another big problem.

I even asked them in Espressif Developer Conference 2022 and none of them have courage to answer.

@DavidSG2020
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DavidSG2020 commented Dec 10, 2022 •
edited

This is a VERY long thread to discuss the Analog inability of the ESP32. All kinds of USELESS things are listed here. Why bother too much with the ESP32 when you can use, for example, the MKR1010? It is way much better and VERY accurate. The ESP32 in my opinion is a work in progress. The ESP32 ADCs are basically useless!

Sure. 8x the price. A fraction of the memory. A fifth of the clock rate. A no-brainer, really. If I need a decent ADC I'll just add an external chip

1670656534916

My hobby would be unaffordable at the price.

Obviously the ADC in the ESP is rubbish. I am grateful to threads like this for letting me know before I have a faild design because of it. Matter of fact, my current project will use the ESP32 ADC, but I only need a very rough reading to autotune a detector circuit. If something comes up where I need north of 8 bits I'll now know I'll have to use an external chip. Or, if speed is not an issue, a PWM output and a 10 cent comparator.

@italex33
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My hobby would be unaffordable at the price.

Obviously the ADC in the ESP is rubbish. I am grateful to threads like this for letting me know before I have afield design because of it. Matter of fact, my current project will use the ESP32 ADC, but I only need a very rough reading to autotune a detector circuit. If something comes up where I need north of 8 bits I'll now know I'll have to use an external chip. Or, if speed is not an issue, a PWM output and a 10 cent comparator.

If money is an issue, and no ADCs are needed, the ESP32 is a good choice, I have a couple projects where the ADCs are not needed, my garage opener uses digital pins only, also my thermostat, but my Personal Weather Station where I have 4 analog devices is a different story, in this case, the ESP32 is useless. I added the ADS1115 but this makes the PWS more complex, more expensive, and more coding.

@michapr
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michapr commented Dec 10, 2022 •
edited

I added the ADS1115 but this makes the PWS more complex, more expensive, and more coding

I have used the ADS1115 in many projects with ESP32 last years and cannot confirm that solution is really more complex (of course depending on the project...) - and it is not more expensive than other solutions.

Anyway - everyone has their favorite processor and their favorite solutions - and that's a good thing given the variety of solution approaches ... :)

@krupis
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krupis commented Jan 18, 2023 •
edited

I came here because I have ESP32 ADC issues myself. From my experience, I can never achieve accuracy greater than 30mV on the ESP32 boards no matter what I do. For some applications its good enough but for others totally unacceptable.

I am very surprised that they did not improve this on their recent flagship ESP32-S3. The ADC is just as bad on this board as on the old ESP32.

I still have my hopes that they manage to improve the ADC on the device that they announced not so long ago:
https://www.espressif.com/en/news/ESP32-P4

I am not a hardware or analog specialist but it makes me wonder what really is the reason behind their horrible ADC's? Why cant they just embed some analog IC on their SoC and solve all ADC problems?

@italex33
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I came here because I have ESP32 ADC issues myself. From my experience, I can never achieve accuracy greater than 30mV on the ESP32 boards no matter what I do. For some applications its good enough but for others totally unacceptable.

The ESP32 analog inputs are UNUSABLE to say the least, I have no idea how you can use these inputs with such inaccuracy. I am using one with 4 analog inputs, but I am using the ADS1115.

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