STM32H743 ADC values fluctuating after voltage divider

[Jonah9Veh]

I have a +30V -30V input to be fed to STM32H7 ADC 16-bit

I tried using a 2 resistor potential divider using 33K 3.3K ; 1K 100E, 1M 100K, resistor combinations but the output is not correctly read by the ADC. (For 3V input, ADC reads from 2.2V to 3.2V randomly fluctuating) Not constant

But if I bypass the divider circuit and feed an input less than 3.3V to the ADC pin directly, it reads perfectly (For 3V input, it reads 2.9999).

Should sampling time affect the resistor values? 

[unsigned_char_array]

Random fluctuations should not happen. Check if the power supply of the MCU and the ADC (VDDA) and the reference voltage are stable. What did you measure?
What happens if you supply a stable voltage from a low impedance source? Such as a 1.2V battery or a bench power supply? Same fluctuations?
Did you read the datasheet of the MCU? The ADC is not designed for high impedance sources since its sample and hold circuit loads the signal. At least decouple the output of the voltage divider with a capacitor. But preferably use a buffer.
Did you calibrate the ADC?

[Jonah9Veh]

I tried with an input from GPIO pin of a Nucleo board. Direct input to ADC pin works fine but if checked after the divider it starts getting erratic.

Made an opamp buffer after divider circuit using general purpose LM358, but still fluctuating. Will try with a precision opamp. 

How about this divider circuit? Will also try a buffer after this.

[unsigned_char_array]

The divider circuit is not the problem. If the voltage is stable its equivalent circuit is an ideal voltage source with a series resistance. It will be loaded by the ADC so the reading won't be correct, but it should not fluctuate. You already tried a buffer with a unity gain stable OPAMP and that didn't change the result.

Could you please answer my other questions?

[TDK]

The ADC will never read a constant value, you will always have some noise, at 16 bits at least. But 1 V of noise is excessive.

You can see general tips on improving accuracy here:

How to optimize the ADC accuracy in the STM32 MCUs - Application note

> Direct input to ADC pin works fine but if checked after the divider it starts getting erratic.

Suggests something else at play here.

How is this set up? On a breadboard with questionable connections? Pictures would help.

[Jonah9Veh]

@TDK @unsigned_char_array 

• VCC and VDDA are from the same source and is stable as observed. Couldn't check it with any different source.
• No calibration done. Can it be done before the ADC reads something stable? How to calibrate it? Please help with that.

An existing PCB is used with wires taken out from those ADC pins and connected to the circuit on a stripboard.

VIN_A = 10V from a calibrator source.

• Multimeter reads a constant 2.172V output after the divider and buffer. Input to ADC is 2.172V.

This is the read ADC output count with voltage. This trend remains almost the same with or without the buffer.

Count	Voltage
43172	2.1739161
43172	2.1739161
42967	2.16359329
42967	2.16359329
43194	2.17502403
43194	2.17502403
43053	2.16792393
43053	2.16792393
42881	2.1592629
42881	2.1592629
43195	2.17507434
43195	2.17507434
43082	2.16938424
43082	2.16938424
43101	2.17034101
43101	2.17034101
43201	2.17537642
43201	2.17537642
43118	2.17119694
43118	2.17119694
43077	2.16913247
43077	2.16913247
43110	2.17079425
43110	2.17079425
43112	2.17089486
43112	2.17089486
43199	2.1752758
43199	2.1752758
43214	2.17603111
43214	2.17603111
43083	2.16943455
43083	2.16943455
43033	2.16691685
43033	2.16691685
43046	2.16757154
43046	2.16757154
43038	2.16716862
43038	2.16716862
43188	2.17472172
43188	2.17472172
43145	2.17255664
43145	2.17255664
43172	2.1739161
43172	2.1739161
43085	2.1695354
43085	2.1695354
43008	2.165658
43008	2.165658
43167	2.17366433
43167	2.17366433
42969	2.16369414
42969	2.16369414
43129	2.17175102
43129	2.17175102
42978	2.16414738
42978	2.16414738
42971	2.16379476
42971	2.16379476
43145	2.17255664
43145	2.17255664
43231	2.17688704
43231	2.17688704
43007	2.16560769
43007	2.16560769
43097	2.17013955
43097	2.17013955
43097	2.17013955
43097	2.17013955
43123	2.17144871
43123	2.17144871
43200	2.17532611
43200	2.17532611
43119	2.17124748
43119	2.17124748
43142	2.17240548
43142	2.17240548
43034	2.16696739
43034	2.16696739
43014	2.16596031
43014	2.16596031
43121	2.17134809
43121	2.17134809
43026	2.16656446
43026	2.16656446
43161	2.17336226
43161	2.17336226
43123	2.17144871
43123	2.17144871
43123	2.17144871
43123	2.17144871
42975	2.16399622
42975	2.16399622
43109	2.17074394
43109	2.17074394
43070	2.16878009
43070	2.16878009
43166	2.17361403
43166	2.17361403
43085	2.1695354

[MasterT]

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@Jonah9Veh wrote:

But if I bypass the divider circuit and feed an input less than 3.3V to the ADC pin directly, it reads perfectly (For 3V input, it reads 2.9999).

ADC can output stable results only if its saturated...

Internal ADC are always noisy, especially when Reference voltage has no Vref- differential part.

Just do oversampling & averaging

[unsigned_char_array]

I noticed a few things.
First off all I see a lot of duplicate values. Are you sure each data point is one sample or are you saving the values faster then they are sampled?

plot of counts

The histogram (bin size 16 counts) doesn't look particularly like Gaussian (noise).

histogram of counts (bin size 16 counts)

Perhaps there aren't enough samples to conclude this. But in my experience you should see noise levels of 1 or maybe 2 counts. This spread of 350 counts is way too large!

FFT doesn't show any particular frequency stand out, but there are not enough samples to be sure of this.

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@Jonah9Veh wrote:

@TDK @unsigned_char_array 

• No calibration done. Can it be done before the ADC reads something stable? How to calibrate it? Please help with that.

---

If you use LL you need to call LL_ADC_StartCalibration().

Example:

	LL_ADC_Disable(ADC1);
	LL_ADC_SetChannelPreSelection(ADC1, LL_ADC_CHANNEL_16);
	LL_ADC_StartCalibration(ADC1, LL_ADC_CALIB_OFFSET_LINEARITY, LL_ADC_SINGLE_ENDED);

	/* Wait for calibration completion */
	while (LL_ADC_IsCalibrationOnGoing(ADC1) != 0UL)
	{
		;
	}
	LL_ADC_Enable(ADC1);

	while ((ADC1->ISR & ADC_ISR_ADRDY) != ADC_ISR_ADRDY);// On this STM32 series, after ADC enable, a delay for ADC internal analog stabilization is required before performing a ADC conversion start.
	LL_ADC_REG_StartConversion(ADC1);

For HAL you need to call HAL_ADCEx_Calibration_Start().

@Jonah9Veh wrote:

@TDK @unsigned_char_array 

An existing PCB is used with wires taken out from those ADC pins and connected to the circuit on a stripboard.

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How long are the wires?

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@Jonah9Veh wrote:

@TDK @unsigned_char_array 

• Multimeter reads a constant 2.172V output after the divider and buffer. Input to ADC is 2.172V.

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A multimeter on voltage mode reads a DC voltage. It doesn't measure noise or ripple. You need an oscilloscope for that.

[unsigned_char_array]

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@MasterT wrote:

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@Jonah9Veh wrote:

But if I bypass the divider circuit and feed an input less than 3.3V to the ADC pin directly, it reads perfectly (For 3V input, it reads 2.9999).

ADC can output stable results only if its saturated...

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Exactly.

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@MasterT wrote:

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Internal ADC are always noisy, especially when Reference voltage has no Vref- differential part.

Just do oversampling & averaging

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Not that noisy! 350 counts between max and min. That's a lot.
I disagree with averaging something that noisy. The source of the noise needs to be fixed first. Then the remaining noise (1-2 counts) can be averaged to reduce noise and optionally increase resolution.

[mƎALLEm]

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@TDK wrote:

You can see general tips on improving accuracy here:

How to optimize the ADC accuracy in the STM32 MCUs - Application note.

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+ AN5354 " Getting started with the STM32H7 Series MCU 16-bit ADC"