Voltmeter and Ammeter

I. Design Purpose
  ADC (Analog-to-Digital Converter) is a skill every electronics engineer needs to master. This training camp aims to introduce ADC knowledge and provide hands-on practice. It covers how to implement ADC signal conversion, measurement and data acquisition, control system input, and communication and signal processing. Digital voltmeters and ammeters combine ADC technology with circuit measurement principles, accurately converting analog voltage and current signals into digital displays for easy reading and analysis by electronics engineers. This device not only improves the accuracy and efficiency of circuit measurements but also helps engineers better understand circuit behavior, serving as a powerful tool for electronic design and troubleshooting, significantly supporting the work of electronics engineers. In product applications, digital voltmeters ensure the accuracy and safety of circuit design while providing strong support for product quality control and subsequent maintenance.
This project leverages

a core board plus expansion board design concept, employing plug-in components to simplify learning and deepen exploration.
The core board uses the domestic Wuhan Xinyuan Semiconductor CW32 as the main controller, while also being compatible with other similar development boards; however, the CW32 offers significant advantages.
The project is highly comprehensive and practical, and the completed design can be used as a desktop instrument.
The project offers abundant learning materials, including circuit design tutorials, PCB design, code programming, and training for engineers' debugging skills.

II. Hardware Design and PCB Analysis
2. MCU Selection Analysis
To reduce the learning cost, this project uses the LCSC CW32F030C8Tx development board (core board) as the main controller, but this does not mean we will cover this section less. From the perspective of training engineers, the correct selection of the main controller is crucial, as it relates to the overall advantages of the project. Regarding the voltmeter and ammeter, the author conducted some debugging and testing using STM32/CW32 and some other 32-bit microcontrollers. Here, we only compare it with the STM32F103C8T6 as a reference for learning device selection, mainly to provide ideas and improve understanding.
Key advantages of the CW32 in this project

: Wide operating temperature range: -40~105℃;
Wide operating voltage range: 1.65V~5.5V (STM32 only supports 3.3V systems)
; Superior interference immunity: HBM ESD 8KV; All ESD reliability meets the highest international standard (STM32 ESD 2KV)
; Project focus - Better ADC: 12-bit high-speed ADC, achieving ±1.0LSB INL 11.3ENOB; Multiple Vref reference voltages... (STM32 only supports VDD=Vref);
Stable and reliable eFLASH technology.

A detailed explanation of these advantages will be provided in the chapters on ADC sampling and related extensions.
The main characteristics of the CW32 ADC: This project requires a focus on the 4 reference voltage sources. (Content from the "CW32x030 User Manual")
3. Voltage Sampling Circuit:
The voltage divider resistors in this project are designed to be 220K+10K, therefore the voltage division ratio is 22:1 (ADC_IN11).
The voltage divider resistor selection

is designed to measure the maximum voltage. For safety reasons, this project uses 30V (the actual maximum display value can be 99.9V or 100V).
The ADC reference voltage is 1.5V in this project, and this reference voltage can be configured through the program.
To reduce the power consumption of the sampling circuit, the low-side resistor (R7) is usually chosen as 10K based on experience.

Then, the high-side resistance of the voltage divider resistor can be calculated using the above parameters.

The required voltage division ratio is calculated, i.e., the ADC reference voltage. The input voltage is designed; using known parameters, 1.5V/30V = 0.05 can be calculated.
The high-side resistance is calculated as the low-side resistance/voltage division ratio; using known parameters, 10K/0.05 = 200K can be calculated.
A standard resistor is selected: a resistor slightly higher than the calculated value of 200K is chosen. We usually choose E24 series resistors; therefore, in this project, 220K, which is greater than 200K and closest to the calculated value, is selected.

If, in actual use, the voltage to be measured is lower than 2/3 of the module's design voltage (66V), the voltage divider resistor can be replaced and the program modified to improve measurement accuracy. The following example illustrates this:

Assuming the measured voltage is no higher than 24V and other parameters remain unchanged,
calculations show 1.5V/24V = 0.0625, 10K/0.0625 = 160K. 160K is a standard E24 resistor and can be directly selected, or a higher value 180K can be chosen with some redundancy.

If, in actual use, the voltage to be measured is higher than the module's 99V design voltage, a different resistor can be selected. To achieve a wider voltage measurement range, you can choose to replace the voltage divider resistor or modify the reference voltage. The following example illustrates this:

Assuming the measured voltage is 160V, we can choose to increase the voltage reference to expand the range.
Given that the voltage division ratio of the selected resistor is 0.0145, we can calculate 160V * 0.0145 = 2.32V using the formula. Therefore, we can choose a 2.5V voltage reference to increase the range (increasing the range will reduce accuracy).

Considering the potential fluctuations in the measured power supply, a 10nF filter capacitor is connected in parallel with the low-side voltage divider resistor to improve measurement stability.
4. Current Sampling Circuit:
This project uses a low-side current sampling circuit for current detection. When learning the common ground between the low-side of the sampling circuit and the development board's meter interface, please do not solder R0!
The sampling current designed for this project is 3A, and the selected sampling resistor (R0) is 100mΩ. The selection of the sampling resistor mainly needs to consider the following aspects:

the maximum value of the pre-designed measurement current;
the voltage difference caused by the 3A current sensing resistor in this project;
and the power dissipation of the current sensing resistor, which is generally not recommended to exceed 0.5V. A suitable package should be selected based on this parameter. Considering the power dissipation (temperature) issue under high current, a 1W packaged metal wire-wound resistor was selected
. The voltage amplification factor across the current sensing resistor is also important. Since no operational amplifier is used to build the amplification circuit in this project, the factor is 1. The current

sensing resistor value can then be calculated using the above parameters.

Amplification circuits are used, so a larger sampling resistor is needed to obtain a higher measured voltage for measurement.
Considering that a larger resistor will result in a larger voltage drop and higher power consumption, a larger resistor cannot be selected indiscriminately.
This project uses a 1W package resistor, corresponding to a power rise of 1W.

Based on the above data, a 100mΩ current sensing resistor was selected for this project. According to the formula, 3A * 100mΩ = 300mV, 900mW can be calculated.
To cope with different usage environments, especially high current scenarios, the R0 resistor can be replaced with constantan wire or a shunt. The replacement can be selected according to the actual usage scenario. For safety and educational purposes, this project will not discuss the range beyond 3A in detail, but the principle is the same.
 
 5. Digital Tube Display
This project uses a digital tube as the display unit.
This project uses two 0.28-inch three-digit common-cathode LED displays as the display device. Compared to a display screen, LED displays offer better visibility in complex environments. The brightness of the LED displays can be increased by using smaller current-limiting resistors, depending on the specific needs of the application environment. Furthermore, LED displays have better mechanical properties and are not as easily damaged by external forces as display screens. They are widely used in industrial applications where stability and reliability are crucial. From a development board learning perspective, this makes it easier to learn electronic measurement principles and related development in a targeted manner.
In this project, actual testing showed that the current-limiting resistors (R1~R6) for the LED displays were configured to 300Ω. The corresponding brightness for both red and blue LED displays was good and the brightness was soft and not glaring.
Strictly speaking, the current-limiting resistors should be added to the segments; adding them to the digits would affect the display effect. Our actual design places them in the digits to save a few resistors, but the impact on the display is not significant. Therefore, we add them to the digits for convenience.
The
driving principle of LED displays mainly involves controlling the switching state of each segment of the LED display to display numbers, letters, or symbols. The following is a detailed explanation of the driving principle:

Basic Structure of a Digital Tube:

A digital tube typically consists of seven or eight LED segments (eight segments in this project). Each segment represents a part of the digital tube and can display numbers 0-9, letters AF, etc.
Digital tubes come in two types: common cathode and common anode. The difference lies in whether the common terminal COM (the end connecting all LEDs) is connected to the negative or positive terminal of the power supply.


Driving Methods:

Segment Selection: The desired number or character is displayed by controlling the on/off state of each segment of the digital tube. Each segment corresponds to a control signal; when the control signal is on, the segment lights up, and vice versa. (a, b, c, d, e, f, g, dp)
Bit Selection: The digital tube to be displayed is selected by controlling the bit lines of the digital tube. Bit line control sets the bit line of the digital tube to be displayed to a high level, and the bit lines of other digital tubes to a low level. By continuously switching the state of the bit lines, the display switching between multiple digital tubes can be achieved.


Driving Circuit:

The driving circuit for a digital tube can be implemented using hardware circuits, such as integrated circuits like digital signal processors (DSPs), microcontrollers (MCUs), or shift registers, to generate control signals suitable for the LEDs.
These control signals can be in the form of pulse width modulation (PWM) signals, serial data signals, etc. By controlling the frequency, width, and amplitude of these signals, the brightness of the digital tube can be controlled, thereby displaying the desired numbers or letters.


Software Control:

In addition to hardware driving circuits, the driving of digital tubes can also be implemented through software control. By programming to generate control signals suitable for the digital tubes, more flexible and complex display effects can be achieved, such as scrolling or alternating display of numbers.


Driving Common Cathode and Common Anode Digital Tubes:

For common cathode digital tubes, the common cathode pin is connected to the negative terminal of the power supply, and the control pin is connected to the output pin of the control chip. When a certain number needs to be displayed, the control chip outputs the corresponding encoded signal to the control pin, causing the corresponding LED segment to light up.
For common anode digital tubes, the working principle is similar to that of common cathode digital tubes, except that the common anode pin is connected to the positive terminal of the power supply, and the control pin is connected to the output pin of the control chip.


Encoded Display:

In order for the digital tube to display the corresponding numbers or characters, the segment data port must output the corresponding character encoding. For example, to display the number "0", the character code for a common anode seven-segment display is 11000000B (i.e., C0H), while the character code for a common cathode seven-segment display is 00111111B (i.e., 3FH). The specific code depends on the actual seven-segment display.


Dynamic and Static Display:

Seven-segment displays can use either static or dynamic display methods. In static display, each of the eight segments of each seven-segment display is connected to an 8-bit I/O port address. As long as the I/O port outputs a segment code, the corresponding character is displayed and remains unchanged. Dynamic display, on the other hand, lights up each segment of the seven-segment display one by one, achieving simultaneous visual display through rapid switching.



In summary, the driving principle of seven-segment displays is to control the switching state of each segment of the seven-segment display to display numbers, letters, or symbols, and to achieve display switching between multiple seven-segment displays through segment selection and digit selection. Furthermore, the driving of seven-segment displays can be implemented through hardware circuits or software control, and common cathode or common anode seven-segment displays can be selected as needed.
This project actually uses dynamic scanning display to drive the seven-segment display.
Let's calculate the current required for the digital tubes.
This project actually uses dynamic scanning to drive the digital tubes, so at any given time, only a maximum of 8 segments of the digital tubes (or LEDs) can be lit, or in other words, only one segment can be lit. According to the design, the required driving current is approximately 11mA (IO port high-level voltage 3.3V ÷ 300Ω).
At this point, it's important to ensure that the selected MCU has sufficient current-sinking/source-sinking capability.
Analysis of the datasheet shows that the CW32 is fine. (Some chips are not suitable.)
6. LED Indicator:
 
Since chip I/O often has a greater current-sinking capability than current-source capability, LED1 is designed to be active low (lit).
To reduce the current consumption of the LEDs, some LED brightness is sacrificed, the component parameter types are reduced, and the LED current-limiting resistor is chosen as 10K.
There's nothing particularly noteworthy about LEDs in this project.
7. Button Circuit Design
 8. TL431 Circuit Design for Voltage Measurement and Calibration
This project adds an extra TL431 circuit to provide a 2.5V reference voltage. This can be used to provide an external voltage reference for the chip to calibrate the AD converter. From a product design perspective, due to the inherent ADC performance advantages of the CW32, this circuit is not necessary. This circuit is designed on the development board to learn the relevant application principles.
The TL431 is a relatively "old" device, a classic, and widely used one, still found in many electronic products.
Many beginners may be encountering this device for the first time, so we will briefly explain the principles of this product to help everyone better apply the TL431.
TI defines it as a "Precision Programmable Reference" in its name. On the first page of the references, we can focus on several key characteristics.
Precision: Precision indicates that its output voltage is very accurate. I used a ±0.5% accuracy TL431, which measured 2.495V on the board at room temperature. Compared to common Zener diodes, the accuracy is vastly different. In the application circuit diagram, the TL431 is represented by a Zener diode symbol.
Adjustable output voltage: The adjustable output voltage is between Vref and 36V. In our project, we use the output Vref voltage, which is approximately 2.5V. Therefore, we use 2.5V in the description, which is approximately equal to Vref.
Sinking current capability: This refers to how much current the output voltage pin can provide, which is greatly affected by the resistance value (R13) in the application circuit. It should not be less than 1mA. If there is no need for sinking current, do not design the current too high, as this will cause unnecessary power consumption.
In this project, I used GPIO close to the voltage and CW32, and tried to optimize the trace length.