Let's look at the main range of issues that can be attributed to the principle of operation of analog-to-digital converters (ADCs) of various types. Sequential counting, bitwise balancing - what is hidden behind these words? What is the principle of operation of the ADC microcontroller? These, as well as a number of other questions, 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 ADC and DAC in various 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 to digital form, while the DAC works the other way around.
Definition
Before considering the principle of operation 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. Almost anything can act as an initial parameter - current, voltage, capacitance,resistance, shaft angle, pulse frequency and so on. But to be certain, we will work with only one transformation. This is "voltage-code". The choice of this format of work is not accidental. After all, the ADC (the principle of operation of this device) and its features largely depend on which concept of measurement is used. This is understood as the process of comparing a certain value with a previously established standard.
ADC Specifications
The main ones are bit depth and conversion frequency. The former is expressed in bits and the latter in counts per second. Modern analog-to-digital converters can be 24 bits wide or up to GSPS units. Please note that the ADC can only provide you with one characteristic at a time. The higher their performance, the more difficult it is to work with the device, and it itself costs more. But the benefit is that you can get the necessary bit depth indicators by sacrificing the speed of the device.
ADC types
The principle of operation varies for different groups of devices. We will look at the following types:
- With direct conversion.
- With successive approximation.
- With parallel conversion.
- A/D converter with charge balancing (delta-sigma).
- Integrating ADCs.
There are many other pipeline and combination types that have their own special characteristics with different architecture. But thosethe samples that will be considered within the framework of the article are of interest due to the fact that 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 A/D Converters
They became very popular in the 60s and 70s of the last century. In the form of integrated circuits, they have been produced since the 80s. These are very simple, even primitive devices that cannot boast of significant performance. Their bit depth is usually 6-8 bits, and the speed rarely exceeds 1 GSPS.
The principle of operation of this type of ADC is as follows: the positive inputs of the comparators simultaneously receive an input signal. A voltage of a certain magnitude is applied to the negative terminals. And then the device determines its mode of operation. This is done with reference voltage. Let's say we have a device with 8 comparators. When applying ½ reference voltage, only 4 of them will be turned on. The priority encoder will generate a binary code, which will be fixed by the output register. Regarding the advantages and disadvantages, we can say that this principle of operation allows you to create high-speed devices. But to get the required bit depth, you have to sweat a lot.
The general formula for the number of comparators looks like this: 2^N. Under N you need to put the number of digits. The example considered earlier can be used again: 2^3=8. In total, to obtain the third category, it is necessary8 comparators. This is the principle of operation of ADCs, which were created first. Not very convenient, so other architectures later appeared.
Analog-to-digital successive approximation converters
Here the "weighting" algorithm is used. In short, devices that work according to this technique are simply called serial counting ADCs. The principle of operation is as follows: the device measures the value of the input signal, and then it is compared with numbers that are generated according to a certain method:
- Sets half of the possible reference voltage.
- If the signal has overcome the value limit from point 1, then it is compared with the number that lies in the middle between the remaining value. So, in our case it will be ¾ of the reference voltage. If the reference signal does not reach this indicator, then the comparison will be carried out with the other part of the interval according to the same principle. In this example, this is ¼ of the reference voltage.
- Step 2 needs to be repeated N times, which will give us N bits of the result. This is due to doing H number of comparisons.
This principle of operation makes it possible to obtain devices with a relatively high conversion rate, which are successive approximation ADCs. The principle of operation, as you can see, is simple, and these devices are great for various occasions.
Parallel analog-to-digital converters
They work like serial devices. The calculation formula is (2 ^ H) -1. ForIn the previous case, 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 voltage. Parallel analog-to-digital converters are fairly fast devices. But the principle of construction of these devices is such that significant power is required to support their performance. Therefore, it is not practical to use them on battery power.
Bitwise Balanced A/D Converter
It operates in a similar way as the previous device. Therefore, in order to explain the functioning of a bit-by-bit balancing ADC, the principle of operation 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 value 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 complete the entire process in N iterations (consecutive steps). Moreover, H is equal to the bit depth of the ADC (look at the previously given formulas). Thus, we have a significant gain in time, if the speed of the technique is especially important. Despite their considerable speed, these devices also have low static accuracy.
A/D converters with charge balancing (delta-sigma)
This is the most interesting type of device, not leastthanks to its principle of operation. It lies in the fact that the input voltage is compared with what has been accumulated by the integrator. Pulses with negative or positive polarity are fed to the input (it all depends on the result of the previous operation). Thus, we can say that such an analog-to-digital converter is a simple servo system. But this is just an example for comparison, so you can understand what a delta-sigma ADC is. The principle of operation is systemic, but for the effective functioning of this analog-to-digital converter is not enough. The end result is a never-ending stream of 1s and 0s through the digital low-pass filter. A certain bit sequence is formed from them. A distinction is made between first and second order ADC converters.
Integrating analog-to-digital converters
This is the last special case that will be considered in the article. Next, we will describe the principle of operation of these devices, but at a general level. This ADC is a push-pull analog-to-digital converter. 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 interference well.
Now let's focus on how it works. It lies in the fact that the input signal charges the capacitor for a fixed time. As a rule, this period is a unit of the frequency of the network that powers the device (50 Hz or 60 Hz). It can also be multiple. Thus, the high frequencies are suppressed.interference. At the same time, the influence of the unstable voltage of the network source of electricity generation on the accuracy of the result is leveled.
When the analog-to-digital converter charge time ends, the capacitor starts to discharge at a certain fixed rate. The device's internal counter counts the number of clock pulses that are generated during this process. Thus, the longer the time period, the more significant the indicators.
ADC push-pull integration have high accuracy and resolution. Due to this, as well as a relatively simple construction structure, they are implemented as microcircuits. The main disadvantage of this principle of operation is the dependence on the network indicator. Remember that its capabilities are tied to the frequency period of the power supply.
This is how a double integration ADC works. The principle of operation of this device, although it is quite complicated, but it provides quality indicators. In some cases, this is simply necessary.
Choose the APC with the principle of operation we need
Let's say we have a certain task ahead of us. Which device to choose 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 depend very little on one another. Be aware that a 12-bit A/D converter may be less accurate than an 8-bit A/D converter. In thatIn this case, resolution is a measure of how many segments can be extracted from the input range of the measured signal. So, 8-bit ADCs have 28=256 such units.
Accuracy is the total deviation of the obtained conversion result from the ideal value, which should be at a given input voltage. That is, the first parameter characterizes the potential capabilities that the ADC has, and the second shows what we have in practice. Therefore, a simpler type (such as direct analog-to-digital converters) may be suitable for us, which will satisfy the needs due to high accuracy.
To have an idea of what is needed, first you need to calculate the physical parameters and build a mathematical formula for interaction. Important in them are static and dynamic errors, because when using various components and principles of building a device, they will affect its characteristics in different ways. More detailed information can be found in the technical documentation offered by the manufacturer of each specific device.
Example
Let's take a look at the SC9711 ADC. The principle of operation of this device is complicated due to its size and capabilities. By the way, speaking of the latter, it should be noted that they are truly diverse. So, for example, the frequency of possible operation ranges from 10 Hz to 10 MHz. In other words, it can take 10 million samples per second! And the device itself is not something solid, buthas a modular construction structure. But it is used, as a rule, in complex technology, where it is necessary to work with a large number of signals.
Conclusion
As you can see, ADCs basically have different principles of operation. This allows us to select devices that will satisfy the needs that arise, while also allowing us to manage our available funds wisely.
