Hangjing Company has launched an RVDT sensor featuring a three‑channel true redundant design.

In modern electronic devices, operational amplifiers are an indispensable component. Particularly in the design of domestically produced op‑amp circuits, selecting appropriate components and optimizing the circuit architecture have become focal points for many engineers. Today, we will delve into the key principles and techniques of designing domestic op‑amp circuits, helping you elevate your circuit‑design expertise. 1. Understand the Basic Characteristics of Operational Amplifiers First and foremost, when designing a domestic op‑amp circuit, it is essential to grasp the amplifier’s fundamental characteristics. The core performance of an op‑amp hinges on parameters such as gain, input impedance, and output impedance. Gain determines the signal‑amplification capability; input impedance affects the source‑load interaction; and output impedance influences the signal‑transmission efficiency. Only by thoroughly understanding these basic traits can we proceed with more effective circuit design. 2. Choose the Right Domestic Operational Amplifier While the market offers a wide array of domestically manufactured op‑amps, not every model is suitable for your specific project. When making your selection, consider several critical factors: bandwidth, power consumption, operating temperature range, and cost. For instance, if your application involves high‑frequency signal processing, bandwidth becomes the primary consideration. Conversely, for projects with stringent low‑power requirements, prioritize low‑power consumption.

I. Design Background

1.1 The Necessity of Redundant Design and High-Reliability Redundant Architectures

In strategic sectors critical to national development and people’s livelihoods, the stable operation of equipment is not only a core technical metric but also a linchpin for fortifying the security perimeter. Whether in the construction of critical national infrastructure or in mission‑critical systems operating under extreme conditions, high reliability stands as an essential requirement—serving as the foundational pillar that ensures mission success, safeguards safety thresholds, and delivers cost‑effectiveness across the entire lifecycle.

Reliability refers to a product’s ability to perform its intended functions under specified conditions and within a defined time frame. In conventional design, system reliability can be enhanced through measures such as selecting high‑quality components, simplifying the structural design, and strengthening testing and validation. However, when confronted with extremely demanding mission requirements and highly complex environments, redundant design emerges as a more efficient and straightforward solution.

At the heart of redundant design is the provision of backup units for critical system components. When the primary channel or component fails, the backup unit automatically and instantaneously takes over, ensuring uninterrupted operation and undiminished performance. This approach offers multiple advantages, including high fault tolerance, a high success rate, an extended service life, and low operational risk.

The two‑redundancy architecture employs a dual‑channel parallel operating mode: in the event of a single‑channel failure, the remaining healthy channel ensures continued normal system operation. The three‑redundancy architecture offers even higher reliability; when a single channel fails, the processor can maintain stable system performance through a “majority‑rules” logic. Consequently, in systems with stringent safety and reliability requirements, a three‑redundancy design is the more efficient and dependable technical choice.

Figure 1: Error Detection in Dual-Redundancy Architecture

Figure 2: Error Detection in a Triple-Redundant Architecture

1.2 High-Reliability RVDT Sensor

Sensors are the “sensory nerves” of modern technological systems, serving as the pivotal bridge between the physical world and digital platforms. They perform the critical functions of information sensing and data acquisition, forming the cornerstone for the practical implementation of automation and intelligent technologies. From automatic screen rotation on smartphones and precision operations by industrial robots to smart home automation, urban‑level smart management, and even the precise diagnostics of medical devices and the comprehensive control systems of new‑energy vehicles, all rely on sensors to deliver accurate, reliable data.

According to their measurement principles, sensors can be classified into various types, including photoelectric, capacitive, piezoelectric, inductive, and Hall‑effect sensors.

An RVDT is a rotary variable differential transformer, a type of electromechanical sensor that falls under the category of non-contact inductive sensors based on the principle of electromagnetic induction. It is used for measuring angular displacement and boasts high precision, high reliability, long service life, and high resolution, playing a vital role in industrial automation, aerospace, the automotive industry, and other fields.

Figure 3 Peripheral circuitry required for the operation of a electromechanical sensor

Inductive sensors operate based on electromagnetic coupling between coils; movement of the ferromagnetic core alters the coupling strength, thereby modulating the output signal. The amplitude of the output signal exhibits a linear relationship with the core’s displacement. Inductive angular and linear displacement sensors enable non-contact conversion of physical quantities into electrical signals, offering both long service life and high precision, and thus hold great potential for a wide range of applications.

However, this type of sensor requires an external excitation unit and a demodulation unit, resulting in a large number of peripheral components and a complex structure. When employed for redundancy design, it not only occupies significant space and poses high design challenges but also makes it difficult to control costs. Moreover, its electromechanical architecture and magnetic mutual‑inductance coupling make it susceptible to strong external electromagnetic interference, leading to errors in the estimation results.

1.3 Layout of the Aerospace Crystal Technology Field

Based on the aforementioned application challenges, Hangjing has continuously deepened its expertise in sensing and signal conditioning, establishing a comprehensive technology and product portfolio. Starting with the high‑temperature‑tolerant fluxgate sensor HJ11867, the company has successfully achieved full domestic substitution for the imported AD598 and AD698 components, and has launched the HJG598 and HJG698 LVDT/RVDT excitation‑demodulation modules, which are now being delivered in volume. Extending its offerings, Hangjing has developed the HJMAG803–HJMAG805 series for measuring weak magnetic signals, along with a high‑performance Hall‑effect magnetic‑sensing product line, thereby building an end‑to‑end, highly reliable sensor solution that spans fluxgate sensors, excitation‑demodulation, weak‑magnetic‑field detection, and Hall‑effect sensing.

Figure 4 Schematic diagram of a triple-redundant Hall‑effect RVDT sensor (power supply only required)

Figure 5: Overview of the Quartz Crystal Sensor Field

At this stage, the company is proudly introducing the HJR402 RVDT angular displacement sensor, which is based on the differential Hall‑effect magnetic sensing principle. By detecting the sinusoidal and cosinusoidal signals generated by an internal magnet and performing high‑precision sampling and angle calculation, the sensor outputs a voltage signal that exhibits an ideally linear relationship with the angular position. It offers high‑accuracy output across the full measurement range, outstanding anti‑interference performance, and enables the linear conversion of angular displacement into an electrical signal.

The product features convenient lead‑out wiring, requires no external components, and employs a three‑channel true redundancy architecture. Each of the three channels independently acquires signals and performs mutual validation, perfectly meeting the high‑reliability design requirements of safety‑critical systems. It is currently a premium, comprehensive sensor solution in the industry.

II. Introduction to the HJR402 Sensor

2.1 Physical Structure of the Sensor

Figure 6: Electrical schematic of the HJR402 triple-redundant Hall-effect sensor

The electrical structure of the HJR402 sensor consists of a permanent magnet and a three‑channel Hall‑effect element processor. All internal components are made from non‑magnetic materials, which effectively enhances measurement accuracy; the housing is constructed from aluminum alloy and features a sealed mounting design. A photograph of the actual device is shown below:

The aluminum alloy housing assembly includes a pivot bearing, an index dial, mounting holes, and lead wires. The device is easy to install and simple to use, requiring no external components. The sensor package dimensions are shown below.

The lead wires comply with the ARF-250-1/1×0.2mm² aviation‑grade flexible cable standard, with a wire length of 450 ± 20 mm (customization available). The specific functions of the lead terminals are listed in the table below:

Table 1: Lead Terminal Functions

Lead-out line	Symbol	Functionality	Lead-out line	Symbol	Functionality
Red line	VCC1	Power Supply 1	White line	VO1	Output 1
Yellow line	VCC2	Power Supply 2	Green Line	VO2	Output 2
Orange Line	VCC3	Power Supply 3	Purple Line	VO3	Output 3
Black line	Ground	Earth

2.2 Electrical Parameters of the Sensor

Table 2: Electrical Characteristics Parameters Table

Unless otherwise specified, VCC = +5 V, TA = +25 °C.

Parameter Name	Symbol	Test conditions	Standard value			Unit
			Minimum value	Typical value	Maximum value
Power supply voltage	VCC			5		V
Measurement range*	Range		0		90	°
			-45		45
Quiescent current	IS	RL=∞	20	25		mA
Output short-circuit current	IS	RL=0			15	mA
Channel error*	ε_ch				1.5	%
Nonlinearity error	ε_NL	Measurement range within 60°	-0.5		0.5	%
		Measurement range within 90°	-1		1	%
Output temperature drift	TC_out	-55℃≤TA≤+125℃		30	50	ppm/°C
Programmable voltage range	VO		0		100	%VCC
Permanent magnet strength*	BHMAX	Radially polarized magnet	3000	4000		Gs

Note: *Design Assurance

2.3 Description of Typical Characteristics

1. This sensor employs a triple-redundant design, with the three sensor channels operating and outputting independently without mutual interference. If any one channel fails, the normal output of the other two channels remains unaffected, making it a true redundant‑architecture sensor.

2. This sensor allows flexible configuration of the output voltage range and angular measurement span according to the user’s specific requirements. In the low‑linearity error mode with a tolerance of ±1%, the maximum angular span can be set to 90°; within a 60° measurement range, the device exhibits superior linearity, with errors kept below 0.5%. When the shaft rotation exceeds the calibrated range, the output remains at a stable high or low level.

3. This product is equipped with a permanent magnet whose magnetic field strength is no less than 3,000 Gs; it features a high intrinsic operating magnetic field and strong resistance to external electromagnetic interference. Experimental verification demonstrates that, under complex and intense electromagnetic conditions, the maximum deviation of its output voltage can be kept within 1.5%, and its anti‑interference performance is approximately three times better than that of conventional electromechanical RVDT sensors.

4. This product is easy to use and requires no external auxiliary components. It features high output accuracy, robust reliability, excellent consistency across multiple channels, and superior resistance to electromagnetic interference and noise, enabling precise conversion of angular displacement into corresponding electrical signals.

5. Since the output signals VO1, VO2, and VO3 are proportional to the supply voltage VCC, it is recommended to power them using separate reference supplies—VCC1, VCC2, and VCC3—and to strictly avoid directly applying a 5 V digital supply.

III. Summary

By analyzing the operating principle of the HJR402 sensor and comparing it with the structure and operating principles of conventional electromechanical sensors, the following comparative table has been compiled.

Table 3: Comparison of HJR402 Sensor with Conventional Electromechanical Sensors

Comparison Items	HJR402 sensor	Electromechanical sensor	Comparison results
Sensor Principle	Hall-effect sensor	Inductive sensor	Different principles
Sensor type	RVDT sensor	RVDT sensor	Type consistency
Measurement accuracy	High precision, high linearity	High precision, high linearity	Consistent performance
Service life	long	long	Consistent performance
Reliability	Three-way true redundancy design	Single-channel output	HJR402 Better
Fault diagnosis capability	Equipped with self-diagnosis capabilities	Fault-free self-identification function	HJR402 Better
Volume and Application Complexity	Simple to use and compact in size	Complex application and relatively large size	HJR402 Better
Anti-interference capability	Strong anti-interference capability	The anti-interference capability is average.	HJR402 Better
Response speed	Fast response speed	Relatively slow response speed	HJR402 Better
Power consumption level	Low power consumption	High power consumption	HJR402 Better

As shown in the table above, the HJR402 triple-redundant sensor exhibits significantly superior overall performance compared to conventional electromechanical sensors, enabling it to fully replace traditional electromechanical RVDT sensors in high-end applications requiring high precision, high reliability, and operation in complex electromagnetic environments.