ADC conversion process (sample and hold) in Embedded C - Step-by-Step Execution

Concept Flow - ADC conversion process (sample and hold)

Start ADC Conversion

↓

Sample Input Voltage

↓

Hold Sampled Voltage

↓

Convert Analog to Digital

↓

Store Digital Value

↓

End

The ADC process starts by sampling the input voltage, holding it steady, converting it to a digital value, and then storing the result.

Execution Sample

Embedded C

void ADC_Convert() {
  float sample = SampleInput();
  HoldVoltage(sample);
  int digitalValue = ConvertToDigital(sample);
  StoreValue(digitalValue);
}

This code samples an analog input, holds it, converts it to digital, and stores the result.

Execution Table

Step	Action	Input/Variable	Result/Output
1	Start ADC Conversion	-	Begin process
2	Sample Input Voltage	Analog input voltage	sample = 2.5V
3	Hold Sampled Voltage	sample = 2.5V	Voltage held steady at 2.5V
4	Convert Analog to Digital	sample = 2.5V	digitalValue = 512 (10-bit ADC)
5	Store Digital Value	digitalValue = 512	Value stored in ADC register
6	End	-	Conversion complete

💡 Conversion ends after storing the digital value.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
sample	undefined	2.5V	2.5V	2.5V	2.5V
digitalValue	undefined	undefined	undefined	512	512

Key Moments - 2 Insights

Why do we need to hold the voltage after sampling?

Holding the voltage keeps the sampled value steady during conversion, as shown in step 3 of the execution table.

What does the digitalValue represent after conversion?

digitalValue is the number representing the analog voltage in digital form, as seen in step 4 and 5.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'sample' after step 2?

A undefined

B 512

C 2.5V

D 0V

💡 Hint

Check the 'sample' variable in the variable_tracker after step 2.

At which step is the analog voltage converted to a digital value?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the 'Action' column in the execution_table for conversion.

If the input voltage changes during holding, what would happen to the digitalValue?

A digitalValue would be accurate regardless

B digitalValue would be incorrect because voltage is not steady

C digitalValue would be zero

D Conversion would stop immediately

💡 Hint

Refer to the importance of holding voltage steady in step 3.

Concept Snapshot

ADC Conversion Process (Sample and Hold):
1. Start conversion
2. Sample input voltage
3. Hold voltage steady
4. Convert analog to digital
5. Store digital value
Holding ensures stable conversion.

Full Transcript

The ADC conversion process begins by starting the conversion. The system samples the analog input voltage, for example 2.5 volts. Then it holds this sampled voltage steady to prevent changes during conversion. Next, the analog voltage is converted into a digital number, such as 512 for a 10-bit ADC. Finally, the digital value is stored in a register, completing the conversion process. Holding the voltage is important to get an accurate digital value. If the voltage changes during holding, the digital result would be incorrect.

Practice

1.

What is the main purpose of the sample and hold stage in an ADC conversion process?

easy

A. To amplify the analog signal before conversion

B. To convert the analog signal directly to digital bits

C. To capture and hold the analog voltage steady during conversion

D. To reset the ADC hardware before starting conversion

Solution

1. Step 1: Understand the role of sample and hold

   The sample and hold stage captures the analog voltage at a specific moment and holds it steady.

2. Step 2: Differentiate from other ADC stages

   Conversion to digital bits happens after holding; amplification or reset are separate steps.

3. Final Answer:

   To capture and hold the analog voltage steady during conversion -> Option C

4. Quick Check:

   Sample and hold = capture and hold voltage [OK]

Hint: Sample and hold freezes input voltage for accurate conversion [OK]

Common Mistakes:

• Confusing sample and hold with conversion step
• Thinking sample and hold amplifies signal
• Assuming sample and hold resets ADC

2.

Which of the following is the correct way to start ADC sampling in embedded C?

ADC_Start();

easy

A. ADC_Start(); // Starts sampling and conversion immediately

B. ADC_Start; // Missing parentheses causes error

C. ADC_Start[]; // Incorrect syntax for function call

D. Start_ADC(); // Wrong function name

Solution

1. Step 1: Identify correct function call syntax

   Functions in C require parentheses even if no arguments are passed.

2. Step 2: Check each option's syntax

   ADC_Start(); // Starts sampling and conversion immediately uses correct syntax with parentheses; others have syntax errors or wrong names.

3. Final Answer:

   ADC_Start(); // Starts sampling and conversion immediately -> Option A

4. Quick Check:

   Function call needs parentheses () [OK]

Hint: Function calls always need parentheses () in C [OK]

Common Mistakes:

• Omitting parentheses when calling functions
• Using brackets [] instead of parentheses
• Using incorrect function names

3.

Consider the following embedded C code snippet for ADC conversion:

int adc_value = 0;
ADC_Start();
while(!ADC_ConversionComplete()) {}
adc_value = ADC_Read();
printf("%d", adc_value);

What will be the output if the analog input voltage corresponds to digital value 512?

medium

A. 0

B. 512

C. 1023

D. Conversion error

Solution

1. Step 1: Analyze the ADC process in code

   ADC_Start() begins sampling; the while loop waits until conversion is done; ADC_Read() reads the digital value.

2. Step 2: Understand the analog input mapping

   If analog input corresponds to digital 512, ADC_Read() returns 512, which is printed.

3. Final Answer:

   512 -> Option B

4. Quick Check:

   ADC_Read() returns converted digital value [OK]

Hint: Wait for conversion complete before reading ADC value [OK]

Common Mistakes:

• Reading ADC value before conversion completes
• Assuming ADC_Read() returns zero by default
• Confusing max ADC value with current reading

4.

Identify the error in this ADC conversion code snippet:

ADC_Start();
if(ADC_ConversionComplete()) {
    int value = ADC_Read();
    printf("%d", value);
}

What is the problem?

medium

A. Missing semicolon after ADC_Start()

B. No error, code is correct

C. Incorrect function name ADC_Read()

D. Using if instead of while to wait for conversion

Solution

1. Step 1: Check how conversion completion is handled

   The code uses if to check conversion complete, which may be false if conversion not done yet.

2. Step 2: Explain why while loop is needed

   Using while waits until conversion finishes; if may skip reading if conversion incomplete.

3. Final Answer:

   Using if instead of while to wait for conversion -> Option D

4. Quick Check:

   Wait loop needed to ensure conversion done [OK]

Hint: Use while loop to wait for ADC conversion completion [OK]

Common Mistakes:

• Using if instead of while to wait
• Assuming conversion is instant
• Ignoring need to wait before reading

5.

You want to read an analog sensor value using ADC with sample and hold. Which sequence of steps is correct in embedded C?

1. Start sampling
2. Wait for sample and hold to stabilize
3. Start conversion
4. Wait for conversion complete
5. Read digital value

Which option correctly orders these steps?

hard

A. 1, 2, 3, 4, 5

B. 3, 1, 2, 5, 4

C. 1, 3, 2, 4, 5

D. 2, 1, 3, 5, 4

Solution

1. Step 1: Understand sample and hold timing

   First, start sampling to capture the analog signal, then wait for it to stabilize (hold).

2. Step 2: Start conversion after hold stabilizes, then wait for conversion to finish, and finally read value

   This ensures accurate digital reading of the stable analog input.

3. Final Answer:

   1, 2, 3, 4, 5 -> Option A

4. Quick Check:

   Sample, hold, convert, wait, read = 1,2,3,4,5 [OK]

Hint: Sample and hold before conversion, then wait and read [OK]

Common Mistakes:

• Starting conversion before sample stabilizes
• Reading value before conversion completes
• Mixing order of steps