Xiaozhi Science Popularization丨Why do instruments generally have set values and readback values?

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Xiaozhi Science Popularization丨Why do instruments generally have set values and readback values? [Copy link]

Why do instruments generally have set values and readback values? For example, we know that the power supply needs to output a certain voltage, so we set it, and the test voltage result will be displayed on the main interface of the instrument. So how is the set value of the instrument converted into an actual electrical signal to control the instrument's operation, and how do we get the readback value? In this article, Xiaozhi will explain this question to you, hoping to be helpful to you.

In fact, the set value and readback value on the instrument are related to what we often call analog-to-digital conversion (ADC) and digital-to-analog conversion (DAC). Before that, let's first understand what analog signals and digital signals are.

丨Analog signals and digital signals

Analog signals can be understood as physical quantities in the real world, but these physical quantities are electrical signals. For example, radio signals, etc., are characterized by time continuity and amplitude continuity. Its advantages are intuitiveness and easy implementation. However, it also has some disadvantages, such as being easily interfered by various noises. If you want to process analog signals, you need to design various circuits, which will be troublesome.

Digital signals are signals that are processed by sampling analog signals. They are characterized by time discreteness and amplitude discreteness. The advantages of digital signals are to make up for the shortcomings of analog signals. For example, digital signals have strong anti-interference ability, are easy to store, and are convenient for various operations.

Since computers have now occupied almost every field, the signal processing in these fields must be converted into signals that computers can understand, and then handed over to computers for processing. These signals that computers can understand are digital signals. So a device is needed to convert analog signals into digital signals, which is called an analog-to-digital converter (ADC). By analogy, since ADC converts analog quantities into digital quantities, there is also a device that can convert digital quantities into analog quantities, which is called a digital-to-analog converter (DAC).

The functions of ADC and DAC are shown in the figure (ADC and DAC are sometimes abbreviated as A/D and D/A)

丨Introduction to ADC and DAC Principles

1. Analog-to-Digital Conversion ADC

1. Four basic parts

① Sampling: The instantaneous value obtained by measuring the continuously changing analog signal at regular intervals.

②Hold: After sampling, the signal is held for a period of time so that the ADC has sufficient time to perform ADC conversion. Generally, the higher the sampling pulse frequency and the denser the sampling, the more sampling values there are, and the closer the output signal of the sample-and-hold circuit is to the waveform of the input signal. Requirements for sampling frequency (satisfying the sampling theorem): sampling frequency Fs >= 2*the highest frequency Fmax in the input analog signal spectrum.

③Quantization: Convert the sampled voltage into an integer multiple of a minimum unit voltage.

④Encoding: Use binary code to represent the quantization level after quantization.

The finer the quantization level, the smaller the quantization error, the more bits of binary code used, and the more complex the circuit.

2. Classification

① Integral type: Convert the input voltage into a pulse width signal or pulse frequency, and use a timer/counter to obtain a digital value.

Advantages: simple circuit and high resolution.

Disadvantages: Conversion accuracy depends on integration time and conversion rate is low.

② Successive approximation type: It is composed of a comparator and a DAC converter through a successive approximation logic. It compares the input voltage with the output of the built-in DAC converter for each bit sequentially starting from the highest bit, and outputs a digital value after n comparisons.

This type of ADC can be viewed as using a fast approximation-fast sorting method to make the DAC output value close to the analog value to implement the ADC.

Advantages: high speed, low power consumption, cost-effective at low resolution (12 bits).

Disadvantages: average conversion rate and medium circuit scale.

③Σ-Δ modulation type: It uses an integrator, comparator, 1-bit DAC converter and digital filter to convert the input voltage into a pulse width signal, which is then processed using a digital filter to obtain a digital value.

Advantages: High-resolution measurements can be easily achieved.

Disadvantages: low conversion rate and large circuit scale.

3. Main parameters

① Resolution: The change in input analog voltage required for the output digital quantity to change by one adjacent value. It is generally expressed in the number of binary bits. The resolution of n means one-nth of 2 of the full scale Fs.

②Quantization error: The error caused by quantizing the analog quantity with the limited number of bits of the ADC. To accurately represent the analog quantity, the number of bits of the ADC needs to be very large or even infinite, so ADC devices have quantization errors. The maximum deviation between the step-shaped conversion characteristic curve of an ADC with limited resolution and the conversion characteristic curve of an ADC with infinite resolution is the quantization error.

③Conversion rate: the number of conversions per second.

④Conversion range: The maximum voltage that the ADC can measure is generally equal to the reference voltage. Exceeding this voltage may damage the ADC. When the signal is small, you can consider lowering the reference voltage to improve the resolution. After changing the reference voltage, the corresponding conversion value will also change. The reference voltage needs to be taken into account when calculating the actual voltage. Therefore, the reference voltage must be very stable and free of high-order harmonics.

⑤Offset error: When the ADC input signal is 0, the ADC conversion output signal is not 0.

⑥ Full-scale error: The difference between the input signal corresponding to the ADC full-scale output and the ideal input signal value.

⑦Linearity: The maximum deviation between the actual ADC transfer function and the ideal straight line.

2. Digital-to-Analog Conversion DAC

1. Basic structure

①Digital register: stores input digital quantity and control signal.
②Analog switch and conversion network: the conversion network is generally composed of a column of resistors, and the analog switch and the corresponding resistor are weighted by bit.
③Reference voltage source: used to determine the conversion coefficient.
④Summing amplifier: adds the analog quantity from the conversion network.

2. Classification

① Voltage output type: The voltage is output directly from the resistor network, and an amplifier is usually added at the output end to reduce the output impedance.

② Voltage output type without an amplifier at the output end: reduces the delay at the output end and is often used in high-speed situations.

③Current output type: Generally, current output is rarely used directly, and most of them will use an external current-voltage conversion circuit to obtain voltage output.

According to how the current-voltage conversion is performed, it can be divided into two categories:

Connect a load resistor to the output pin
Disadvantages: High output impedance, must be used within the specified output current range, otherwise the DAC may be damaged.

· External operational amplifier
Disadvantages: The delay of the external operational amplifier and wires makes the response slower.

The current output type is rarely used, and the voltage output type is generally used.

3. Main parameters

① Resolution: The ratio of the minimum output voltage (that is, the voltage when the input digital quantity is 1) to the maximum output voltage (that is, the voltage when the input digital quantity is the maximum (each bit is 1)). It is usually expressed by the number of bits of the input digital quantity.

②Conversion range: The maximum voltage that the DAC can output, generally related to the reference voltage or its multiples.

③ Establishment time: the delay time from input digital quantity to output analog quantity.

④Conversion accuracy: similar to the conversion accuracy of ADC.

Conclusion

The current development of embedded systems is centered around core processors such as microcontrollers and FPGAs, both of which are digital chips that process digital signals.

Embedded systems are usually designed to solve a specific problem, such as a temperature gun, which needs to convert the physical signal of human body temperature into a digital signal in the form of voltage and then hand it over to the core control device for processing. Therefore, ADC is needed to convert this physical signal into an electrical digital signal. To be more precise, a sensor is needed to convert the physical signal of temperature into an analog signal in the form of voltage, and then ADC is needed to convert the analog signal into a digital signal.

The role of DAC is similar. When the core processor needs to drive the actual physical device, it needs to convert the digital signal into an analog signal. Because the actual physical device can only understand analog signals, DAC is responsible for completing the conversion from digital to analog.

The test instrument also works on a similar principle. For example, the DC power supply converts the voltage setting value (digital signal) of the main interface into an actual voltage analog output through a series of DAC core control processes. The voltage readback value is converted into a digital signal through ADC sampling and displayed on the main interface.

*Some of the information and pictures in this article are from the Internet. The copyright belongs to the original author. If there is any infringement, please contact me to delete it. Thank you!

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