CW32 Voltage and Current Meter

The power supply circuit design
uses an LDO as the power source. Considering that most voltmeter products are used in industrial scenarios with 24V or 36V power supplies, the SE8550K2 with a maximum input voltage of up to 40V was selected. The main reason for not using a DC-DC step-down circuit to handle the large voltage difference is to avoid introducing DC-DC ripple interference during the design process; a secondary reason is to reduce project costs.
The voltage sampling circuit
uses a 220K+10K voltage divider resistor, resulting in a voltage division ratio of 22:1 (ADC_IN11). The current
sampling
circuit
uses a low-side current sampling circuit for current detection. The low-side of the sampling circuit shares a common ground with the meter interface on the development board.
The designed sampling current is 3A, and the selected sampling resistor (R0) is 100mΩ.
The selection of the sampling resistor should mainly 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 generally, it is not recommended to exceed the power consumption of the current sensing resistor by more than 0.5V.
Appropriate resistors should be selected based on this parameter. For the packaging, considering the power consumption (temperature) issues under high current, this project chose a 1W packaged metal wire-wound resistor
. The voltage amplification factor across the current sensing resistor is 1, as no operational amplifier was used in this project.

The digital tube driver
uses two 0.28-inch three-digit common-cathode digital tubes as display devices. Compared to displays, digital tubes have better visibility in complex environments. The brightness of the digital tubes can be increased by using smaller current-limiting resistors depending on the actual usage environment. Furthermore, digital tubes have better mechanical properties and are not easily damaged by external forces like displays. They are widely used in industrial applications requiring stability and reliability. From a development board learning perspective, it is easier to learn electronic measurement principles and related development in a targeted manner. An additional
TL431 circuit
is added to provide a 2.5V reference voltage, which can be used to provide an external voltage reference for calibrating 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 related application principles.
DRC Design Considerations:
Power supply traces should be as wide as possible, approximately 20-60 mil.
Ordinary signal traces: around 10 mil
. ADC signal traces: 10 mil or 8 mil. Excessive width may affect signal integrity when the traces are too long.
Since high-speed circuit design is not involved, the 3W rule is not emphasized here.
Standardize component pin spacing: Use imperial units, make good use of the grid and mesh in EDA software, and arrange component positions and traces reasonably.