We live in an analog World with digital ways of communication, convenience and data processing. So, there is an underlying task of converting data from one domain to the other i.e., from analog to digital or vice-versa all the time. We call these data converters as Analog to Digital Converters (ADC) or Digital to Analog Converters (DAC). In this guide, let us focus on the analog to digital converters, see how a simple ADC works and what are the different types of ADC Circuit Designs.
This is not an in-depth guide on Analog to Digital Converters and different types of ADCs but rather a brief introductory discussion on some important types of ADC Circuits. Analog to Digital Conversion is a huge and fascinating subject on its own and if you are interested, we can make a series of guides on individual ADC Circuit that we mentioned here.
Importance of Analog to Digital Conversion
What is an Analog to Digital Converter (ADC)? It is a device or circuit that convert analog data (a continuous signal in time with varying magnitude) into digital data (discrete data with fixed magnitudes).
The main benefit of digital data is that you can easily store it and also the chance of data corruption due to any analog signals is very less. Hence, most of computing has switched to digital domain.
Digital data has also the advantage of being accurate (this depends on the ADC process) and easy to manipulate at crazy high speeds. But our World is analog. Real World physical quantities like temperature, pressure, light, gas, etc. are continuous analog signals. So, we have to use data converters such as ADCs to convert these analog signals into digital values so that our digital devices can easily process, store, analyze and compute the data.
When ever we want to measure, analyze and process any analog data, we usually convert it into digital data using one of the methods mentioned below, process the digital data and convert the digital data back to analog data using Digital to Analog Converters (DAC), if necessary.
Anatomy of a Typical ADC
It is clear that ADCs are an integral part of modern signal processing. Before we proceed with different types of ADC Circuit Designs, let us quickly have a look at how a typical ADC process works.
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The first step of ADC is to sense the physical quantities such as temperature, pressure, humidity (or any analog quantity) and convert it into an electrical signal (either current or voltage). For this, we use Sensors which are devices that detect changes in physical quantities and produce and equivalent electrical signal (usually voltage).
Next, we have to condition the output of the sensor. This is because, the output of the sensor (usually voltage) is often in millivolts (very low magnitude). Using signal conditioners such as amplifier, filters etc., we convert this small electrical signal into a relatively large signal (usually amplified by a factor of 100 or 1000).
Now comes the heart of the analog to digital conversion: the Sample and Hold Circuit. As the analog signal is continuously changing, we need a mechanism to hold the signal while we process it. The sample and hold circuit will take a sample of the analog signal and holds it for a duration set by the timing circuit (clock circuit).
Depending on the type of ADC Circuit we use, the data from the sample and hold block is converted into a digital code.
An important point to note about ADCs in general is that there is always a compromise between the speed of conversion and sampling rate due to the continuous nature of the analog signal.
Types of ADC Circuit Designs
There are several types of Analog to Digital Converter Designs. Some of the popular ADC Circuit Design are:
- Ramp or Counter ADC
- Successive Approximation Register (SAR) ADC
- Dual Slope or Integrating ADC
- Flash ADC
- Sigma Delta ADC
We will briefly see about these ADC Circuits one-by-one.
Ramp or Counter ADC
The least expensive and also the slowest of all ADC Circuit designs is the Counter ADC. It is also known as Tracking ADC. The main components of this ADC Circuit are: a Voltage Comparator, a Counter and a DAC (Digital to Analog Converter). That’s right. This ADC has a DAC in its design.
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We feed the main analog signal to one input of the voltage comparator. Coming to the other input of the comparator, it is the analog output of the DAC. The input to the DAC is actually the binary output of the counter.
When the conversion process begins, the control circuit will reset the counter to 0 and gradually increases the count based on the clock signal. DAC will generate appropriate analog signal for the counter data and as soon as the output of DAC exceeds the main analog input voltage, the control circuit stops the counter.
The value of the counter at which the DAC exceeds the main analog input is the final digital data.
Successive Approximation Register (SAR) ADC
The Successive Approximation Register (SAR) type ADC is an extremely popular implementation for a long time. If you take a look at the block diagram of SAR ADC, it is similar to the Counter ADC except that in place of the main Counter, we have a Register and Latch Circuit.
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As the name suggests, the SAR ADC successively compare the analog input to the output of the DAC. The input to the DAC is a binary-weighted code from the successive approximation registers.
Initially, the MSB of the DAC’s input is set to 1 while all other bits are made 0s. we now compare the output of the DAC corresponding to this input (this output of DAC is actually half of its maximum range) with the input analog voltage.
If the analog input voltage is greater than the output of the DAC, the output of the comparator becomes HIGH (i.e., 1). As a result, the control logic will keep the MSB as 1 but also makes the bit next to MSB as 1.
This process is essentially adding a quarter of the output range to the previous half range. Now, the output of the DAC is 3/4th of its range. The comparison process continues as long as the input voltage is greater than the output of the DAC.
If in case the output of DAC is greater than the analog input voltage, then the current bit is set to 0 and the next significant bit is set to 1. This comparison process continues until the result is a close value to the analog input.
Dual Slope or Integrating ADC
Next, we have a high-speed and high-resolution converter in the form of Dual Slope ADC. It is also known as Integrating ADC as it uses an Integrator circuit in the ADC Design.
The integrator has two switchable inputs: the main analog input voltage and a reference voltage. As the integrator generates two different ramp signals (corresponding to the two inputs), this type of ADC is known as Dual Slope ADC.
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First, the integrator is set with the main analog input and the output of the integrator charges to this value. After certain time (specific clock cycles), the input of the integrator is set to the reference voltage (which is having an opposite polarity than the input analog signal) and the output of the integrator charges down to this value.
During both these ramps, the counter starts counting and it stops when the output of the integrator becomes 0. The value of the counter is the digital code corresponding to the input analog voltage.
Flash ADC
The fastest and most expensive of all analog to digital converters is the Flash ADC Circuit. It has one comparator per voltage step so a 4-bit Flash ADC will have 24 = 16 comparators.
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One input of all the comparators is connected to the main analog input signal. The other input of the comparators is a point in a string of resistors (that essentially form voltage dividers).
As a result, each comparator as you move up the string trips at a higher voltage than the previous one. The outputs of all the comparators are connected to a priority encoder whose output is the digital code.
Sigma Delta ADC
The next analog to digital converter is the Sigma Delta (Σ-Δ) ADC. It is also a type of high-resolution ADC. This ADC uses oversampling as its main feature for sampling the input analog signal.
In Sigma Delta ADC, we apply the analog input signal to one input of a differential amplifier. The other input of the differential amplifier is the output of a DAC.
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Output of the differential amplifier passes through an integrator and then to a comparator. The output of the comparator is the input to the DAC and also to the main filtering circuit.
Conclusion
ADC Circuits are very important for data acquisition and front-end designs of all digital systems such as oscilloscopes. This is an introductory guide on analog to digital conversion, how a typical ADC works and what are the different types of ADC Circuit Designs. We saw the block level implementations of all these ADC Types and also their working.
