Analog-to-digital converter (ADC) operation


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What is an ADC? Why are ADCs important?

An analog to digital convertor (ADC) is a device or circuit used to convert analog value into digital value.

It is important because, most of the sensors are devices that measure analog quantities, and normally give an analog output. The output from most sensors is converted into a digital signal using an ADC.

Operation of an ADC:

Analog signals are constantly changing so that for a converter to make a measurement, a sample-and-hold technique is used to capture the voltage level at a specific instant in time:

The N-channel field effect transistor (FET) in the sample-and-hold circuit has a low impedance when turned “ON” and a very high impedance when turned “OFF”. The voltage across capacitor C follows the input analog voltage when the FET is “ON” and holds the dc level of the analog voltage when the FET is turned “OFF”. During the “OFF” period the ADC measures the dc level of the analog voltage and converts it into a digital signal. As the sampling frequency of the ADC is much higher than the frequency of the analog signal, the varying amplitude of the analog signal can be represented in a digital format during each sample period and stored in memory.

Types of ADCs:

Flash converters:

Flash converters which are very fast and expensive with limited accuracy, that is 6-bit output with a conversion time of 33 ns. The device can sample an analog voltage 30 million times per second.

The comparators give a “0” output when the analog voltage is less than its reference voltage, and a “1” output when the analog voltage is higher than its reference input voltage. These outputs are then encoded to give a 3-bit digital word. The devices are very fast, but expensive and with limited applications. A number of flash converters are commercially available, including the 8-bit flash converter manufactured by Maxim (MAX 104), which gives a resolution of±0.39% with an output sample rate of 1 GSPS (109 samples per second).

Successive approximation:

Successive approximation is a high-speed, medium-cost technique with good accuracy, that is, the most expensive device can convert an analog voltage to 12 bits in 20 μs, and a less expensive device can convert an analog signal to 8 bits in 30 μs.

The logic in the successive approximation method sets and compares each bit, starting with the MSB (b1). Starting with Vx at 0, the first step is to set b1 to 1, making Vx = VR/2, and then to compare it to the input voltage. If the input is larger, b1 remains at 1, and b2 is set to 1, making Vx (3VR/4). This new value is again compared to the input voltage, and so on. If Vx is greater than the input, b1 is reset to 0, and b2 is set to 1, and then compared to the input. This process is repeated to the LSB of the DAC.

Ramp-up or Stepped-up:

A Ramp-up or Stepped-up ADC is a low-speed device that compares the analog voltage to a ramp voltage generated by an integrator or a resistor network. The voltage is ramped up, or stepped up, until the voltage from the DAC is within the resolution of the converter. When it equals the input voltage, the steps are counted, and the digital word count represents the analog input voltage