Raspberry pi PICO ADC reading [closed] - raspberry-pi-pico

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Why not get zero when taking ADC readings on Raspberry Pi Pico? Even though I ground the ADC pin, the analog reading always fluctuates between 10 to 20. How can analog reading be reduced to zero?

Answer 1

Question
How to make Rpi Pico do ADC with an output range starting from zero, and hopefully no fluctuation of 10 to 20?
Answer
I googled and found How2Electronics has a newbie friendly tutorial on Pico ADC, with a short demo code (References 1, 2. Appendices B, C).
The demo code does not seem to have any problem as reported by the OP. So I decided to try the demo code in my Pico setup, to see if I can repeat the OP's situation.
I read the Pico datasheet which says that Pico ADC has an internal offset of about 30mV, and that its offset can be reduced by using an external voltage of 3.0V (Appendix D).
Now the OP says his ADC results of 0V (signal grounded) always fluctuates between 10 and 20. So let me calculate if this 10~20 offset is within the Pico datasheet's 20~30mV internal/intrinsic zero offset. Since Pico ADC resolution is 12 bit, and if 3V3 power rail is used as analog reference, then the intrinsic is 30mV / 3V * 4096 ~= 30/3000 * 4096 ~= 5 < 10. In other words, the OP's zero offset seems twice the spec.
I have not verified my always dodgy calculation. Perhaps I can verify it by working backwards: What is the offset in mV, if offset value is 20/4096. Suppose analog reference is 3V3, then offset in mV = 3V3 * (20/4096) = 3300mV * 20 / 4096 = 16mV, looks OK.
Anyway, perhaps I can repeat the OP's test and compare with his zero offsets.
I modified/expanded How2Electronics's demo program, to do the following:
(a) Do ADC with the three ADC pins, ADC0, ADC1, ADC2, (b) Find max, min, avg values and print out results (Appdenix F).
I then use the program to find the outputs of ADC0, 1, 2 pins connected to (a) Analog Ground, (b) Enabled 3V3. The ADC results are summarized below:
--- Sample Output ---
%Run -c $EDITOR_CONTENT
Name = testPicoAdcV01()
Function = test Pico ADC pins GP26, 27, 28
Date = 2022apr25hkt1111
ADC Results = [368, 368, 336] (ADC0, 1, 2 connected Aanlog Gound)
Max = 368
Min = 336
Avg = 357
ADC Results = [65391, 65519, 65519] (ADC0, 1, 2 connected 3V3)
Max = 65519
Min = 65391
Avg = 65476
The ADC output for (a) 3V3 input is avg 65476, and (b) Analog ground is avg is 357, or
ADC zero offset of 357/65476 = 5.45% ~= 5%, or 3V3 = 3300mV * 5% ~= 165mV.
The Pico ADC error spec of 5% or 150mV (not yet verified) of measurement range of 3V3 does not seem reasonably good. So I am thinking of using an external 3V0 voltage reference for the Pico Analog Reference pin, hoping that accuracy could be improved. I will be using the voltage reference IC TL431 to check if it is good (Appendix H, Ref 5).
Using a 2.5V external reference, the zero offset has improved to 0.4%, but still not very good. I would prefer to use AD7705, with two real differential input channels, built in PGA, and assembled modules with on board LM285-2.5 voltage references.
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References
(1) How to use ADC in Raspberry Pi Pico - How2Electronics, 2021apr21
(2) Raspberry Pi Pico Complete Guide [Pinout + Features + ADC (08:56) + I2C + OLED + Internal Temperature Sensor + DHT11 - How2Electronics
(3) Rpi Pico Datasheet (4.3. Using the ADC) - Rpi
(4) Raspberry Pi Pico 3.3V_EN pin control voltage inquiry - EE.SE, Asked 2021jun22, Viewed 1k times
(5) TL431 / TL432 Precision Programmable Reference IC - TI
(6) LM385B-2.5 2.5V Micropower Voltage References - TI
(7) Zonri ADS1256 24-Bit Sampling Module, ADC Module, Single/Differential Input - AliEXpress US$26
(8) AD7705 SPI 2 fully differential input channel 16-bit Σ-Δ ADC, PGA Datasheet - Analog Devices
(9) AD7705/TM7705 16-Bit ADC Module, Input Gain, Programmable SPI Interface, (LM285-2.5 external voltage reference)- AliExpress US$1.2
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Appendices
Appendix A - Rpi Pico ADC Pinout
Appendix B - Rpi Pico wiring for testing ADC program v0.1
------------------------------------------------------------------------------------
Pin name Pin # Connected to
-------------------------------------------------------------------------------------
Rpi 3V3 Output 36 -
Rpi 3V3 Enable 37 -
Analog Gnd 33 Rpi Pico Ground
Analog Ref 35 Rpi 3V3
ADC0 26 Rpi Ground
ADC1 27 Rpi 3V3
ADC2 28 2V5
ADC3 - Not available, connected to Pico internal temperature sensor
-------------------------------------------------------------------------------------
Appendix C - How2Eloectronics's ADC Demo Code
Appendix D - How to improve ADC Performance
Rpi Pico Datasheet (4.3. Using the ADC) - Rpi
4.3. Using the ADC
The RP2040 ADC does not have an on-board reference and therefore uses its own power supply as a reference.
On Pico the ADC_AVDD pin (the ADC supply) is generated from the SMPS 3.3V by using an R-C filter (201 ohms into 2.2μF). This is a simple solution but does have the following drawbacks:
We are relying on the 3.3V SMPS output accuracy which isn’t great
We can only do so much filtering and therefore ADC_AVDD will be somewhat noisy
The ADC draws current (about 150μA if the temperature sense diode is disabled, but it varies from chip to chip) and therefore there will be an inherent offset of about 150μA200 = ~30mV*. There is a small difference in current draw when the ADC is sampling (about +20μA) so that offset will also vary with sampling as well as operating temperature.
Changing the resistance between the ADC_VREF and 3V3 pin can reduce the offset at the expense of more noise - which may be OK especially if the use case can support averaging over multiple samples.
Driving high the SMPS mode pin (GPIO23), to force the power supply into PWM mode, can greatly reduce the inherent ripple of the SMPS at light load, and therefore the ripple on the ADC supply. This does reduce the power efficiency of the board at light load, so the low-power PFM mode can be re-enabled between infrequent ADC measurements by driving GPIO23 low once more. See Section 4.4.
The ADC offset can be reduced by tying a second channel of the ADC to ground, and using this zero-measurement as an approximation to the offset.
For much improved ADC performance, an external 3.0V shunt reference, such as LM4040, can be connected from the ADC_VREF pin to ground.
Note that if doing this the ADC range is limited to 0-3.0V signals (rather than 0-3.3V), and the shunt reference will draw continuous current through the 200R filter resistor (3.3V-3.0V)/200 = ~1.5mA.
Note that the 1R resistor on Pico (R9) is designed to (maybe) help with shunt references that would otherwise become unstable when directly connected to 2.2μF. It also makes sure there is a little filtering even in the case that 3.3V and ADC_VREF are shorted together (which is a valid thing to do if you don’t care about noise and want to reduce the inherent offset).
Finally, R7 is a physically large 1608 metric (0603) package resistor, so can be relatively easily removed if a user wants to isolate ADC_VREF and do their own thing with the ADC voltage, for example powering it from an entirely separate voltage (e.g. 2.5V). Note that the ADC on RP2040 has only been qualified at 3.0/3.3V but should work down to about 2V.
Appendix E - Pico 3V3 Enable pin to enable 3V3 Power
Raspberry Pi Pico 3.3V_EN pin control voltage inquiry - Asked 2021jun22, Viewed 1k times
Appendix F - Testing Pico's 3 ADC Pins V0.1
programName = 'testPicoAdcV01()'
programFunction = 'test Pico ADC pins GP26, 27, 28'
programDate = '2022apr25hkt1111'
programAuthor = 'tlfong01'
systemInfo = 'Chinese Windows 10, Thonny IDE 3.3.13, Python 3.7.9, Rpi Pico'
import machine
import utime
# *** Configuration ***
adcPinNum0 = 26
adcPinNum1 = 27
adcPinNum2 = 28
adcPin0 = machine.ADC(adcPinNum0)
adcPin1 = machine.ADC(adcPinNum1)
adcPin2 = machine.ADC(adcPinNum2)
adcPinDict = \
{
'0' : {
'AdcPinNum': 0,
'AdcPin' : adcPin0,
},
'1' : {
'AdcPinNum': 1,
'AdcPin' : adcPin1,
},
'2' : {
'AdcPinNum': 2,
'AdcPin' : adcPin2,
},
}
# *** Adc Functions ***
def getAdcResults(adcPinNum):
adcPin = adcPinDict[str(adcPinNum)]['AdcPin']
adcResults = adcPin.read_u16()
return adcResults
# *** Sample Test ***
#adcResults = getAdcResults(adcPinNum = 0)
#print(adcResults)
def getAdcResultsList(adcPinNumList):
adcResultsList = [0] * len(adcPinNumList)
for adcPinNum in adcPinNumList:
adcResults = getAdcResults(adcPinNum)
adcResultsList[adcPinNum] = adcResults
return adcResultsList
# *** Sample Tests ***
#adcResultsList = getAdcResultsList([0, 1, 2])
#print(adcResultsList)
def printAdcResultsList(adcResultsList):
print('ADC Results =', adcResultsList)
print('Max =', max(adcResultsList))
print('Min =', min(adcResultsList))
print('Avg =', (sum(adcResultsList)) / len(adcResultsList))
return
# *** Sample Tests ***
#adcResultsList = getAdcResultsList([0, 1, 2])
#printAdcResultsList(adcResultsList)
def testPicoAdcV01():
print('Name =', programName)
print('Function =', programFunction)
print('Date =', programDate)
adcResultsList = getAdcResultsList([0, 1, 2])
printAdcResultsList(adcResultsList)
return
# *** Main ***
testPicoAdcV01()
# *** End of Program ***
# *** Sample Output ***
'''
>>> %Run -c $EDITOR_CONTENT
Name = testPicoAdcV01()
Function = test Pico ADC pins GP26, 27, 28
Date = 2022apr25hkt1102
ADC Results = [208, 20645, 17828]
Max = 20645
Min = 208
Avg = 12893.67
>>>
'''
# *** End of sample output ***
Appendix G - ADC results with ADC0, 1, 2 connected to (a) Analog Ground, (b) Enabled 3V3 Output
Complete program listing with sample output
# *** Sample Output ***
'''
>>> %Run -c $EDITOR_CONTENT
Name = testPicoAdcV01()
Function = test Pico ADC pins GP26, 27, 28
Date = 2022apr25hkt1111
ADC Results = [368, 368, 336] (ADC0, 1, 2 connected Aanlog Gound)
Max = 368
Min = 336
Avg = 357
ADC Results = [65391, 65519, 65519] (ADC0, 1, 2 connected 3V3)
Max = 65519
Min = 65391
Avg = 65476
>>>
'''
Appendix H - Rpi Pico ADC External Analog Voltage Reference Using TL431
Appendix I - Rpi Pico ADC External Analog Voltage Reference Using TI LM385B25 2V5 Volgtage References
LM385B-2.5 Micropower Voltage References - TI
Appendix J - Zonri ADS1256 24-bit ADC Module to calibrate Rpi Pico ADC Pins
Appendix K - AD7705 16-bit ADC to Calibrate Rpi Pico ADC
(8) AD7705 SPI 2 fully differential input channel 16-bit Σ-Δ ADC Datasheet - Analog Devices
(9) AD7705 16-Bit ADC Module, Input Gain, Programmable SPI Interface, TM7705- AliExpress US$1.2
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