**Foreword**
With the rapid advancement of digital technology and integrated circuit (IC) manufacturing, the demand for high-performance Digital-to-Analog Converters (DACs) has grown significantly. In fields such as mobile phone development, wireless communication, and other digital applications, DACs play a crucial role. To meet market demands, DACs must offer high update rates, stability, and low power consumption. As IC fabrication processes have advanced into the nanoscale, improvements in speed and power efficiency have become more achievable. However, achieving high performance in DACs still heavily relies on the stability of the bandgap reference voltage, which serves as a critical analog reference. Due to its low temperature coefficient, the bandgap reference is widely used in mixed-signal circuits. Any deviation in the reference voltage directly affects the output of the DAC, making it essential that the reference source meets or exceeds the DAC’s accuracy requirements. Therefore, designing a low-power, low-temperature-coefficient, and high-power-supply-rejection reference is vital. Techniques such as operating MOS transistors in subthreshold mode and using full-MOS structures are being explored to reduce power consumption while maintaining stability. Additionally, improving the power supply rejection ratio (PSRR) through techniques like self-bias current mirrors and optimized startup circuits helps enhance the overall performance of the reference. Despite these efforts, there is still room for improvement in circuit design and simulation results.
After evaluating various methods and considering practical application needs for DACs, this paper employs a 40 nm CMOS process. Based on traditional principles, a stable PTAT current is generated using negative feedback and proportional resistance design. The layout was simplified by replacing self-bias current mirrors with resistors, reducing mismatch and area. This approach makes the circuit more practical and efficient.
**1 Basic Design Principle of Bandgap Reference**
The traditional bandgap reference uses the characteristics of operational amplifiers and bipolar junction transistors (BJTs). By combining the negative temperature coefficient of VBE(Q1) with the positive temperature coefficient of ΔVBE, the reference voltage can be adjusted to achieve zero temperature coefficient. Using a 40 nm CMOS process, a high-stability bandgap reference circuit is designed. Figure 1 shows the actual circuit configuration.
**1.1 Reference Core Circuit Diagram Analysis**
As shown in Figure 1, the core circuit operates under specific conditions when the supply voltage and enable signal are active. The voltage drop across R1 is calculated based on the BJT's VBE and the resistance values. By carefully selecting R1 and R2, the sum of the two components can be balanced to achieve a temperature-independent reference voltage.
**1.2 Startup Circuit Analysis**
The startup circuit ensures proper operation of the bandgap reference. When the enable signal (EN) is low, the op amp and PTAT module are inactive. When EN is high, the circuit activates, allowing the reference voltage to stabilize. This circuit improves reliability and reduces power consumption during idle states.
**1.3 OPAMP Actual Circuit Diagram Analysis**
The OPAMP in Figure 1 operates in deep negative feedback, ensuring that the input voltages are equal. This feature is crucial for generating a stable bandgap voltage. The two-stage amplifier includes biasing networks and switching transistors that control the operational state. Simulations show that the differential gain meets the required specifications.
**2 Simulation Results and Layout**
**2.1 Layout and Post-Simulation Results Analysis**
Using a 40 nm CMOS process, simulations were conducted with Cadence and Spectre software. The circuit successfully operated with a supply voltage of 2.5 V, an output voltage of 1.184 V, and a startup time of 0.5 μs. The simulated temperature drift coefficient was 8.7×10â»âµ/°C, indicating good thermal stability. The PSRR at low frequencies reached -85 dB, showing strong noise suppression capabilities.
**2.2 Comparison and Analysis with Other Literature Parameters**
Comparing the results with existing literature, this design demonstrates good PSRR but still faces challenges in temperature stability. Further improvements in balancing the positive and negative temperature coefficients are needed. The layout was optimized to reduce parasitic effects and improve matching between components.
**3 Conclusion**
This paper presents a high-precision, fast-start CMOS bandgap reference suitable for integration into high-speed DACs. With a reference voltage of 1.184 V, a startup time of 0.5 μs, and a PSRR of -85 dB, the design meets key performance metrics. The layout optimization reduces parasitic effects, making the circuit more reliable and efficient. Overall, this bandgap reference offers a solid foundation for future high-resolution and high-speed DAC applications.
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