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The role of Voltage Follower in ADC Acquisition Circuit

When designing analog-to-digital conversion (ADC) circuits, voltage followers are almost indispensable. This circuit element plays an important role in ensuring signal quality and accuracy. This article will explore why a voltage follower is added to the front stage of many high-precision ADC acquisition circuits, as well as the specific benefits and technical principles of doing so. By deeply understanding the characteristics of high input impedance and low output impedance of voltage followers, we can better understand its key role in electronic measurement and signal processing systems.

The role of voltage follower in classic circuits

Catalog

Voltage Follower Overview

A voltage follower, also known as a buffer amplifier, is a basic operational amplifier configuration where the output is connected directly to the inverting input, and the input signal is fed into the non-inverting input. This setup provides high input impedance, low output impedance, and a unity gain, making it ideal for interfacing between different circuits without altering the signal voltage. It effectively serves as a buffer, ensuring that the signal integrity is maintained while preventing the loading of the signal source, which is crucial in applications requiring impedance matching and signal isolation.

Basic circuit of voltage follower

Basic circuit of voltage follower

Analog-to-Digital Converter (ADC) Overview

An Analog-to-Digital Converter (ADC) is an essential electronic device that transforms continuous analog signals into discrete digital numbers, allowing digital systems to process real-world analog information. The conversion process involves sampling the input signal at set intervals and then quantizing it to a fixed number of levels determined by the ADC's resolution. This digital output represents the analog input in binary form, facilitating the integration of analog inputs with digital processing systems.

Why do many ADC acquisition circuits add a voltage follower in the front stage?

Everyone knows that voltage followers have the advantages of high input impedance and low output impedance. When the input impedance is large, the follower is equivalent to disconnecting the circuit from the previous stage, just like the principle of a resettable fuse, disconnecting the power circuit through high impedance. The output impedance of the voltage follower is very low, which is equivalent to short-circuiting the subsequent circuit. The input voltage value of the subsequent circuit is equal to the voltage value at the output end of the voltage follower.

The application of voltage followers in ADC acquisition circuits is mainly to solve the problems of input impedance and output impedance, ensuring accurate signal transmission and protection of subsequent circuits. The following are several key functions of voltage followers:

  • High input impedance: The high input impedance of the voltage follower ensures that it has minimal interaction with the previous circuit and almost no current is drawn from the signal source, thereby avoiding the loading effect, that is, the performance of the previous circuit will not be affected by the connection of the ADC.

  • Low output impedance: The low output impedance of the voltage follower helps to effectively drive the input of the ADC, ensure the stability of the signal during transmission and reduce signal attenuation. This is critical to maintaining signal integrity and reducing distortion.

  • Isolation and protection function: The voltage follower can also isolate potential interference between the previous circuit and the ADC, such as switching noise, to protect the signal source from these interferences.

Voltage follower voltage diagram

Figure 1: Voltage follower voltage diagram

The voltage values at the input and output of the voltage follower are basically the same, and the gain is 1.

In the ADC acquisition circuit, if the accuracy requirement is not high, the voltage value after voltage division is transmitted to the voltage follower through 2 resistors. Some circuit designers directly connect the voltage value after voltage division to the pin of the CPU's built-in ADC or the acquisition pin of the ADC chip. In actual projects, the error between the voltage value collected and the theoretical voltage value is large. In software design, the collected value is compensated by the program, and the compensated voltage value is the same as the actual voltage value.

ADC acquisition circuit diagram

Figure 2: ADC acquisition circuit diagram

The reason why the collected voltage value is inconsistent with the actual voltage value is that the main ADC collection end also has an impedance. After the external voltage divider resistor is connected in parallel with the ADC end resistance value, the entire voltage divider resistance value changes, so the voltage value collected by the ADC is different from the theoretical value.

For example: the project needs to monitor a power supply voltage of 5V, using the ADC with a single-chip microcomputer, and the working voltage of the single-chip microcomputer is 3.3V. Therefore, it is necessary to divide the 5V voltage, with a 20K upper voltage divider resistor and a 10K lower voltage divider resistor. After the voltage is divided, the voltage value transmitted to the ADC is 1.67V. The actual ADC end also has a resistance value. Assuming that this resistance value is 10K, after the ADC end resistance value and the 10K voltage divider resistor are connected in parallel, the impedance becomes 5K, and the actual voltage value collected by the ADC is 1V.

Summary

In practical applications, if a voltage follower is not used and the divided voltage is directly connected to the ADC, the impedance of the ADC input terminal itself will be connected in parallel with the external voltage divider resistor, which will change the overall impedance value, resulting in a deviation between the actual collected voltage value and the theoretical value. Using a voltage follower can effectively avoid this situation and ensure that the collected voltage value is more accurate and stable. The voltage follower also acts as an isolation to protect the subsequent circuits. Therefore, many ADC acquisition circuits will add a voltage follower in the front stage.

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FAQ

1. Does a voltage follower increase current?

No, a voltage follower does not increase current. It is designed to have a high input impedance and a low output impedance, which means it does not draw significant current from the input source nor does it boost the current in the output.

2. How accurate is a voltage follower?

A voltage follower is highly accurate in terms of voltage replication from input to output. It ideally has a unity gain (gain of 1), meaning the output voltage is the same as the input voltage, with very minimal voltage drop across the follower.

3. How can a voltage follower circuit prevent signal loading?

A voltage follower prevents signal loading by presenting a high input impedance to the source circuit and a low output impedance to the load. This configuration minimizes the current drawn from the source, thereby not affecting the source voltage, and allows the follower to drive the load effectively without significant voltage drop.

4. What is the function of a voltage buffer?

The function of a voltage buffer (another term for a voltage follower) is to isolate the input from the output while transferring the voltage from the input to the output without attenuation. It protects the input source from load effects and provides a stable voltage to the load regardless of variations in the load's impedance.

5. Why use a voltage follower?

Using a voltage follower can reduce the loading effect of the previous stage of the circuit, provide a low impedance path for the next stage, and ensure that the voltage level remains stable even if the load changes. It also protects sensitive components from noise and interference in subsequent stages, maintaining signal integrity at different stages of the electronic system, especially in analog signal processing.

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