ROHM Expands its General-Purpose Analog IC BusinessFocusing on noise reduction to capture market share

Yoshii:　

The widespread adoption of IoT is accelerating the transition to a sensor-driven society. Nowadays, various types of sensors that detect everything from pressure, light, and gas to temperature, weight, and acceleration are being utilized across a wide range of applications. However, these sensors cannot operate independently. The signals they produce are too weak for MCUs to process directly. Therefore, op amp ICs are needed to amplify these signals. As our reliance on sensors grows, so does the need for op amp ICs.

Yoshii:　

Until about 10 years ago, our offerings were limited to what were commonly referred to as general-purpose op amp ICs. These products are characterized by low speed and accuracy.

However, a few years ago, we shifted our strategy to strengthen our op amp and comparator IC business. This involved increasing the number of developers and advancing our technological capabilities. Since then, we have been working on expanding our lineup of high-performance products, including models with high slew rates and low input offset voltage (Fig. 1).

Op amp ICs are general-purpose products that can be replaced with equivalent performance alternatives on a circuit board, making it challenging to distinguish them from competing products. However, without clear differentiation, competing with European and US semiconductor manufacturers will be difficult. In response, ROHM is focusing on noise as a point of differentiation. Our approach is built on two core principles: resistance to noise and minimal noise generation. These pillars guide the development of our products.

Yoshii:　

In simple terms, we focused on EMC measures. EMC, or Electromagnetic Compatibility, refers to the ability of an electronic or electrical device to operate normally without emitting electromagnetic interference that disrupts other devices or being affected by interference from its surroundings. EMI consists of 2 elements: EMI (Electromagnetic Interference) and EMS (Electromagnetic Susceptibility) (Fig. 2).

EMI indicates the amount of noise emitted by a device, which can negatively impact the operation of other electronic devices. Op amp ICs, however, inherently do not emit significant noise. Therefore, we focused on the noise present in the output signal. The output signal of an op amp IC always includes fluctuations, which are inevitably amplified along with the signal. If these fluctuations are significant, it can become difficult to accurately read the signals detected by the sensor. To address this, we worked on suppressing these fluctuations, resulting in what’s commonly referred to as a low noise op amp IC.

EMS, on the other hand, refers to a device’s susceptibility to electromagnetic waves. If this characteristic is low, the output signal of the op amp IC may fluctuate due to the effects of electromagnetic waves, making it difficult to accurately read the sensor signals. Op amp ICs with high EMS provide strong resistance to EMI, which is why we refer to them as high EMI immunity op amp ICs.

Yoshii:　

Our low-noise op amp ICs, such as the LMR180xxx, TLRx37xxx, and BD728xxx deliver industry-leading low-noise performance. The input-equivalent noise voltage, which corresponds to fluctuations in the output signal, has been minimized to just 2.9nV/√Hz (@1kHz) (Fig. 3).

Yoshii:　

As a vertically integrated semiconductor manufacturer, we handle everything from design to manufacturing, allowing us to fully leverage our strengths to reduce noise.

There are two factors that contribute to noise in op amp ICs (Fig. 4). The first is noise originating from the manufacturing process. This type of noise, known as flicker noise, is dependent on the ‘cleanliness’ of the manufacturing process. A clean atomic arrangement prevents flicker noise. However, crystal defects disrupt electron flow, generating noise. To address this, it is necessary to refine the manufacturing process to eliminate impurities that can cause crystal defects.

The second factor is thermal noise, which can be addressed through innovative circuit design. Specifically, by adjusting the size of transistors and lowering the resistance values.

Making these improvements is no easy task, however. As a vertically integrated semiconductor manufacturer, we benefit from close collaboration between the development and design departments, enabling greater flexibility. What’s more, to further strengthen this collaboration, we established a technology development division a few years ago. This division works on both device development and circuit development under the same chain of command. By working seamlessly together, both teams have been able to significantly accelerate product development.

Yoshii:　

Although we have successfully developed industry-leading low-noise op amp ICs, we believe that simply expanding the lineup based on this feature alone would not be compelling enough. To enhance their appeal, we wanted to combine the best-in-class low noise performance with another standout feature. This led to the development of the industry's smallest op amp IC (Fig. 5). Utilizing WLCSP (Wafer Level Chip Scale Package) technology allowed us to achieve a compact footprint of 0.88mm x 0.58mm while maintaining a noise level as low as 12nV/√Hz (@1kHz).

Yoshii:　

Our high EMI tolerance op amp ICs are marketed under the EMARMOUR™ brand, symbolizing protective armor against EMI. We were the first in the industry to introduce high EMI resistance op amp ICs, and they continue to deliver outstanding performance. You could say that we’ve pioneered a new product category.

Perhaps the biggest feature is that the output voltage remains completely stable even when exposed to external noise (Fig. 6). While some competitors have followed our lead by commercializing op amp ICs that claim high EMI resistance, their output fluctuates significantly under noisy conditions.

Yoshii:　

There are no reported issues in the market. In reality, such problems frequently occur in the testing environments of electronic device manufacturers. Noise testing is typically conducted during the final stages of device development. If issues are identified at this point, it becomes necessary to revise the design and implement additional noise countermeasures, entailing rework. This not only wastes time and resources, but also increases design and development costs.

Furthermore, it leads to an increase in the number of components and associated costs, as the most common noise suppression countermeasure involves inserting a CR filter that combines MLCCs (Multilayer Ceramic Capacitors) and resistors. Another significant challenge is the limited number of locations where noise testing can be performed. As noise-related standards become increasingly stringent, the number of mandatory test items continues to grow. Ideally, we aim to pass noise testing on the first attempt.

These problems are more likely to occur in areas where small signals are amplified, but by adopting our high EMI immunity op amp ICs that are unaffected by external noise, such issues do not arise. This equates to less time and costs spent on design and development, fewer components, and a reduced burden on noise testing.

Yoshii:　

The biggest difference lies on the basis for claiming high EMI immunity. Competitor products are evaluated for EMI resistance primarily through the DPI (Direct Power Injection) test, where noise is directly applied to the IC terminals. However, EMI immunity in DPI testing can be easily improved by inserting a CR filter, making evaluation based solely on the DPI test insufficient.

In contrast, we conduct not only the DPI test, but also radio wave radiation tests, Bulk Current Injection (BCI) testing, and proximity immunity evaluations, confirming that the output remains stable under all these conditions (Fig. 7). Competitor products exhibit significant output voltage fluctuations when undergoing such tests. Furthermore, even in DPI testing, competitor products can achieve relatively high noise immunity in higher frequency ranges, such as 400MHz or 900MHz, but often struggle with poor immunity to noise at lower frequencies (Fig. 8).

Yoshii:　

EMI was improved by reviewing three key areas: circuit design, layout, and processes/elements. For the circuit, we enhanced EMI resistance by integrating high-frequency impedance adjustment (RF-IA: Radio Frequency Impedance Adjuster) circuits at multiple points in the op amp design. These circuits are not merely composed of capacitors (C) and resistors (R), but also leverage parasitic components to achieve the desired performance.

For the layout, comprehensive shielding was applied to signal lines highly sensitive to noise. This approach prevents noise from propagating to the surrounding signal lines. I believe that our process and element technologies will be quite difficult for competitors to replicate. Transistors with large parasitic capacitance are more effective at absorbing noise. By increasing the size of the transistor and deepening the junction, more noise can be absorbed. In contrast, competitors typically manufacture amp ICs using processes optimized for MCUs and other large-scale LSIs. When prioritizing miniaturization and cost, parasitic capacitance naturally decreases, resulting in greater susceptibility to noise.

Yoshii:　

Yes, it is possible. We have already commercialized the BD5294EYFV-C op amp IC that incorporates both low-noise and high EMI immunity technologies to ensure the IC neither generates nor is affected by noise. In terms of noise reduction, input equivalent noise voltages of 20nV/√Hz at 1kHz and 50nV/√Hz at 10Hz have been achieved. And as for resistance to external noise, we believe it demonstrates outstanding noise immunity.