General principle of operation of ADCs

Let's consider the main range of issues that can be attributed to the principle of the operation of analog-to-digital converters (ADCs) of different types. A sequential account, bitwise balancing - what is hidden behind these words? What is the principle of the ADC microcontroller? These, as well as a number of other issues, we will consider in the framework of the article. We will devote the first three parts to the general theory, and from the fourth subheading we will study the principle of their work. You can meet the terms of ADC and DAC in different literature. The principle of operation of these devices is slightly different, so do not confuse them. So, the article will consider the conversion of signals from analog form to digital, while the DAC works the other way around.

Definition

Before considering the principle of the ADC, let's find out what kind of device it is. Analog-to-digital converters are devices that convert a physical quantity into a corresponding numerical representation. As an initial parameter, almost anything can act: current, voltage, capacitance, resistance, shaft rotation angle, pulse frequency, and so on. But in order to have certainty, we will only work with one transformation. This is the "voltage-code". The choice of this format is not accidental. After all, the ADC (the principle of operation of this device) and its features depend to a large extent on what measurement concept is used. By this is meant the process of comparing a certain value with a previously established standard.

Characteristics of the ADC

The main ones are the bit depth and frequency of the conversion. The first is expressed in bits, and the second is expressed in counts per second. Modern analog-to-digital converters can have a 24 bit capacity or conversion speed, which reaches GSPS units. Note that the ADC can simultaneously provide you with only one of its characteristics. The greater their performance, the more difficult it is to work with the device, and it itself is more expensive. But the benefit can be obtained by the necessary digits, sacrificing the speed of the device.

Types of ADCs

The principle of operation varies among different groups of devices. We will consider the following types:

1. With a direct transformation.
2. With successive approximation.
3. With parallel transformation.
4. Analog-digital converter with charge balancing (delta-sigma).
5. Integrating ADCs.

There are many other conveyor and combined types that have their own special characteristics with different architectures. But those samples that will be considered within the framework of the article are of interest because they play an indicative role in their niche of devices of this specificity. Therefore, let's study the principle of the ADC, as well as its dependence on the physical device.

Direct analog-to-digital converters

They became very popular in the 60-70s of the last century. In the form of integrated circuits are produced from the 80's. These are very simple, even primitive devices that can not boast of significant indicators. Their bit depth is usually 6-8 bits, and the speed rarely exceeds 1 GSPS.

The principle of operation of the ADC of this type is as follows: an input signal is input simultaneously to the plus inputs of the comparators. The negative terminals are supplied with a voltage of a certain value. And then the device determines its operating mode. This is done due to the reference voltage. Let's say that we have a device where there are 8 comparators. When the ½ reference voltage is applied, only 4 of them will be included. Priority encoder will generate a binary code, which is fixed by the output register. Concerning merits and demerits, it can be said that such a principle of operation allows the creation of high-speed devices. But to get the necessary bit capacity you have to sweat a lot.

The general formula for the number of comparators looks like this: 2 ^ N. Under N, you need to set the number of digits. The example considered earlier can be used again: 2 ^ 3 = 8. A total of 8 comparators are needed to obtain the third digit. This is the principle of the ADC, which were created first. Not very convenient, so in the subsequent there were other architectures.

Analog to Digital Converters

Here we use the "weighting" algorithm. In short, devices using this technique are called simply ADCs for sequential counting. The principle of operation is as follows: the device measures the value of the input signal, and then it is compared with the numbers that are generated by a certain technique:

1. Half of the possible reference voltage is set.
2. If the signal overcomes the limit of the value from point # 1, then it is compared with the number that lies in the middle between the remaining value. So, in our case this will be ¾ of the reference voltage. If the reference signal does not reach this index, then the comparison will be performed with the other part of the interval in the same way. In this example, it is ¼ of the reference voltage.
3. Step 2 must be repeated N times, which will give us a H-bit result. This is due to the H number of comparisons.

This principle of operation allows you to obtain devices with a relatively high conversion speed, which are ADCs of successive approximation. The principle of operation, as you can see, is simple, and these devices are excellent for different cases.

Parallel analog-to-digital converters

They work like serial devices. The formula for calculating is (2 ^ H) -1. For the case considered earlier, we need (2 ^ 3) -1 comparators. For operation, a certain array of these devices is used, each of which can compare the input and individual reference voltages. Parallel analog-to-digital converters are quite fast devices. But the principle of building these devices is such that to maintain their performance requires considerable power. Therefore, it is not advisable to use them with battery power.

Analog-to-digital converter with bitwise balancing

It operates according to a similar scheme as the previous device. Therefore, in order to explain the operation of the ADC of bitwise balancing, the principle of work for beginners will be considered literally on the fingers. At the heart of these devices is the phenomenon of dichotomy. In other words, a consistent comparison of the measured quantity with a certain part of the maximum value is carried out. Values in ½, 1/8, 1/16 and so on can be taken. Therefore, the analog-to-digital converter can perform the entire process for H iterations (consecutive steps). And H is equal to the ADC bit width (look at the previously given formulas). Thus, we have a significant gain in time, if the speed of the technique is particularly important. Despite the considerable speed, these devices are also characterized by a low static error.

Analog-digital converters with charge balancing (delta-sigma)

This is the most interesting type of device, not least due to its operating principle. It consists in the fact that the input voltage is compared with what was accumulated by the integrator. Pulses with negative or positive polarity are fed to the input (all depends on the result of the previous operation). Thus, it can be said that such an analog-to-digital converter is a simple tracking system. But this is only as an example for comparison, so you can understand what a delta-sigma ADC is. The principle of operation is system, but for the efficient operation of this analog-to-digital converter is not enough. The end result is an endless stream of units and zeros that goes through the digital LPF. Of these, a certain bit sequence is formed. A distinction is made between ADC converters of the first and second orders.

Integrated analog-to-digital converters

This is the last special case that will be considered within the framework of the article. Next, we will describe the operation of these devices, but at a general level. This ADC is an analog-to-digital converter with push-pull integration. You can meet a similar device in a digital multimeter. And this is not surprising, because they provide high accuracy and at the same time suppress noise well.

Now let's focus on his principle of work. It consists in that the input signal charges the capacitor for a fixed time. Typically, this period is the unit of the network frequency that powers the device (50 Hz or 60 Hz). It can also be a multiple. Thus, high-frequency noise is suppressed. At the same time, the effect of the unstable voltage of the grid source of electricity generation on the accuracy of the result is leveled.

When the charging time of the analog-to-digital converter ends, the capacitor begins to discharge at a certain fixed rate. The internal counter of the device counts the number of clock pulses that are generated during this process. Thus, the longer the time interval, the more significant the indicators.

ADCs of push-pull integration have high accuracy and resolving power. Due to this, as well as a relatively simple structure of construction, they are performed as chips. The main drawback of this principle of work is the dependence on the indicator of the network. Remember that its capabilities are tied to the duration of the power supply frequency period.

Here's how the double-integration ADC works. The principle of operation of this device, although it is quite complex, but it provides quality indicators. In some cases, this is simply necessary.

Choose an APC with the principle of work we need

Let's say we face a certain task. How to choose a device so that it can satisfy all our requests? First, let's talk about resolution and accuracy. Very often they are confused, although in practice they are very weakly dependent on one from the second. Remember that a 12-bit analog-to-digital converter can have less accuracy than an 8-bit one. In this case, the resolution is a measure of how many segments can be extracted from the input range of the measured signal. So, 8-bit ADCs have 2 ⁸ = 256 such units.

Accuracy is the total deviation of the resulting conversion from the ideal value, which should be for a given input voltage. That is, the first parameter characterizes the potential capabilities of the ADC, and the second shows what we have in practice. Therefore, we can approach a simpler type (for example, direct analog-to-digital converters), which will satisfy the needs due to high accuracy.

In order to have an idea of what is needed, first we need to calculate the physical parameters and construct a mathematical formula for the interaction. Important in them are static and dynamic errors, because when using different components and principles of constructing a device, they will have different effects on its characteristics. More detailed information can be found in the technical documentation that the manufacturer of each particular device offers.

Example

Let's look at the ADC SC9711. The principle of operation of this device is complicated due to its size and capabilities. By the way, speaking about the latter, it must be noted that they are really diverse. So, for example, the frequency of possible work ranges from 10 Hz to 10 MHz. In other words, it can do 10 million counts per second! And the device itself is not something integral, but has a modular structure of construction. But it is used, as a rule, in a complex technique, where it is necessary to work with a large number of signals.

Conclusion

As you can see, ADCs basically have different principles of work. This allows us to select devices that satisfy the requests that arise, and at the same time allow us to use the available means wisely.