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Analog-to-digital converters are among the most widely used devices for data acquisition. Digital computers use binary (discrete) values, but in the physical world everything is analog (continuous). Temperature, pressure (wind or liquid), humidity, and velocity are a few examples of physical quantities that we deal with every day.

A physical quantity is converted to electrical (voltage, current) signals using a device called a transducer. Transducers used to generate electrical outputs are also referred to as sensors. Sensors for temperature, velocity, pressure, light, and many other natural physical quantities produce an output that is voltage (or current).

Therefore, we need an analog-to-digital (ADC) converter to translate the analog signals to digital numbers so that the micro controller can read and process the numbers.

Figure 11.1: Microcontroller Connection to Sensor via ADC

Figure 11.2: An 8-bit ADC Block Diagram

## Major characteristics of the ADC

### Resolution

The ADC has n-bit resolution, where n can be 8, 10, 12, 16, or even 24 bits. Higher-resolution ADCs provide a smaller step size, where step size is the smallest change that can be discerned by an ADC. Some widely used resolutions for ADCs are shown in Table 11.1. Although the resolution of an ADC chip is decided at the time of its design and cannot be changed, we can control the step size with the help of what is called Vref.

n-bit Number of steps Step size
8 256 5V/256 = 19.53 mV
10 1024 5V/1024 = 4.88 mV
12 4096 5V/4096 = 1.2 mV
16 65,536 5V/65,536 = 0.076 mV
Note: Vref = 5V
Table 11.1: Resolution versus Step Size for ADC (Vref = 5V)

### Vref

Vref is an input voltage used for the reference voltage. The voltage connected to this pin, along with the resolution of the ADC chip, determine the step size. For an 8-bit ADC, the step size is Vref / 256 because it is an 8-bit ADC, and 2 to the power of 8 gives us 256 steps. See Table 11.1.

• If the analog input range needs to be 0 to 4 volts, Vref is connected to 4 volts. That gives 4 V / 256 = 15.62 mV for the step size of an 8-bit ADC.
• If we need a step size of 10 mV for an 8-bit ADC, then Vref = 2.56 V, because 2.56 V / 256 = 10 mV.
• For the 10-bit ADC, if the Vref = 5V, then the step size is 4.88 mV as shown in Table 11.1. Tables 11.2 and 11.3 show the relationship between the Vref and step size for the 8- and 10-bit ADCs, respectively.
• In some applications, we need the differential reference voltage where Vref =  Vref (+) – Vref (–). Often the Vref (–) pin is connected to ground and the Vref (+) pin is used as the Vref.
Vref (V) Vin Range (V) Step Size (mV)
5.00 0 to 5 5 / 256 = 19.53
4.00 0 to 4 4 / 256 = 15.62
3.00 0 to 3 3 / 256 = 11.71
2.56 0 to 2.56 2.56 / 256 = 10
2.00 0 to 2 2 / 256 = 7.81
1.28 0 to 1.28 1.28 / 256 = 5
1.00 0 to 1 1 / 256 = 3.90
Note: In an 8-bit ADC, step size is Vref/256
Table 11.2: Vref Relation to Vin Range for an 8-bit ADC

Vref (V) Vin Range (V) Step Size (mV)
5.00 0 to 5 5 / 1024 = 4.88
4.96 0 to 4.096 4.096 / 1024 = 4
3.00 0 to 3 3 / 1024 = 2.93
2.56 0 to 2.56 2.56 / 1024 = 2.5
2.00 0 to 2 2 / 1024 = 2
1.28 0 to 1.28 1.28 / 1024 = 1.25
1.024 0 to 1.024 1.024 / 1024 = 1
Note: Vref Relation to Vin Range for an 10-bit ADC
Table 11.3: Vref Relation to Vin Range for an 10-bit ADC

### Conversion time

Conversion time is defined as the time it takes the ADC to convert the analog input to a digital number. The conversion time is dictated by the clock source connected to the ADC in addition to the method used for data conversion and technology used in the fabrication of the ADC.

### Digital data output

In an 8-bit ADC we have an 8-bit digital data output of D0–D7, while in the 10-bit ADC the data output is D0–D9. To calculate the output voltage, we use the following formula:

```DOUT = VIN / Step size
```

where Dout = digital data output (in decimal), Vin = analog input voltage, and step size (resolution) is the smallest change, which is Vref/256 for an 8-bit ADC.