External influences with ADC

Question

I use atmega328p. I'm trying to read data from a variable resistor using an ADC. The microcontroller also has LEDs connected. If I ignite them, the data from the ADC increases noticeably.

Initializing the ADC:

/***** ADC *****/
ADCSRA |= (1<<ADPS2) | (1<<ADPS1) | (1<<ADPS0); //128
// 16MHz/128 = 125kHz
ADCSRA |= (1<<ADIE); //enable interrupt
ADCSRA |= (1<<ADEN); //enable adc
//ADMUX |= (1<<REFS0) | (1<<REFS1); //1.1В
ADMUX |= (1<<REFS0); //AVcc with capasitor as AREF pin

ADCSRA |= (1<<ADSC); //new start
ADMUX |= (1<<MUX0) | (1<<MUX2); //ADC5

How can I solve this problem?

I apologize for the scheme, but I do not have the right program at hand.

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Extra info due to answers:

• In the normal state, the ADC value is 2. If I light the LED, the value is increased to 20. I changed the 220 Ω resistor to 1 kΩ, now value increases from 2 to 7. Voltage comes from USB 3.0.
• In an attempt to make the PSU stronger I tried to solder the second USB connector and energize in parallel. Nothing changed.

Answer 1

Some things to try:

Increase the value of the bypass caps on Aref, Vcc, and AVcc (p20) of the 328P.

Insure that the supply and ground path of the variable resistor are not directly inline with the supply or ground path of the LEDs.

Reduce the load on the 328P by driving the LED's with a transistor or additional gate.

For further critical noise reduction the Atmega328/P data book contains additional improvement methods, see the ADC chapter.
http://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-42735-8-bit-AVR-Microcontroller-ATmega328-328P_Datasheet.pdf

Answer 2

I guess the problem here is either some coupling with the current-carrying line inside the MCU or a drop in Vref voltage that makes the analog signal appear larger in comparison.

Either problem would be solved by driving the LED through a transistor. Alternatively, you could try reversing the LED and letting the MCU sink current to turn it on, by outputting "0". This would still incur the same current through the MCU, but through another path.

Another possible solution is measuring relative to the internal 1.1 voltage reference instead of Vref, as the 1.1 reference is less likely to be affected. But then you need to use a voltage divider to scale down the maximum signal voltage.

Answer 3

Have you connected AVCC to 5 volt supply? I assume you have connected.

You have updated ADMUX after starting conversion. First set the proper value of ADMUX. Then set ADSC to 1.

Another important factor is data type. If you need to store the ADC value in a variable or do any mathematics , then do it inside ISR(ADC_vect){ }. All the data types must be - volatile int / volatile float etc (i.e. need to have volatile keyword) . Otherwise you will get junk value. Then you have to set ADSC to 1 again to start new conversion.