Bandgap voltage reference source design based on 14-bit Pipeline ADC - Power Circuit - Circuit Diagram

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At present, reference voltage sources are widely utilized in various integrated circuits, including high-precision comparators, A/D and D/A converters, and dynamic random access memories. As a critical unit module within integrated circuits, the reference voltage source plays a vital role.

The accuracy of the reference voltage, its temperature stability, and noise immunity directly impact the performance of the chip and the entire system. In particular, in D/A and A/D data conversion systems, the performance of the reference source is closely linked to the quantization accuracy of the quantizer. With the increasing precision requirements of D/A and A/D converters, designing an accurate and stable reference source has become essential. Therefore, creating a high-performance reference source holds significant importance.

1 Analysis of Circuit Design and Principles

1.1 Analysis of Traditional Bandgap Reference

In the conventional bandgap voltage reference structure, a stable output Vref with a constant temperature is achieved by combining a voltage VBE with a negative temperature coefficient and a voltage VT with a positive temperature coefficient in a linear manner. Figure 1 illustrates a conventional bandgap reference. However, in practical applications, compensating for high-order voltage components not adequately addressed in Vref remains a crucial part of the design. The higher temperature coefficient primarily arises from the temperature characteristics of the bipolar transistor.

After completing the design, I obtained:

According to the aforementioned formula, the first-order coefficient term can be easily eliminated by adjusting the circuit under most processes. However, due to the insufficient offset of the process parameter r and the coefficient δ introduced by the resistor, the high-order voltage component persists. Specifically, the C₂ term cannot be eliminated, resulting in a temperature coefficient that cannot be sufficiently low.

1.2 Improved High-Order Compensation Bandgap Reference Source

To achieve a bandgap reference source with a sufficiently low temperature coefficient, higher-order temperature coefficients need further compensation. The compensation method is illustrated in the circuit structure shown in Figure 2. Based on the traditional circuit, a compensation circuit structure is added: since the gain of operational amplifier A₃ is large, it forces the terminal voltages of Q₂ and R₄ to be equal, then I₄=V_BE/R₄, and the current mirror ensures that the current flowing through transistor Q₃ matches this value.

This generates a difference term Tln(T) between the V_BEs of Q₂ and Q₃. This difference term is introduced into I_R1 through the op amp gm₁ and gm₂ to correct the high-order terms in V_BE. In Figure 2, the inputs V₁, V₂, and V₂, V₃ of the four-input op amp are connected to each other, ensuring the same gain A1 and identical parameters, i.e., the same output impedance.

For transistors Q₁ and Q₂, being identical means their terminal voltages depend solely on the current flowing through their collectors. By setting B₁ and B₂ controlled by the resistance value, temperature coefficient, and tube V_BE voltage, and adjusting gm₁ and gm₂, the high-order term is corrected. Adjusting R₄ eliminates the first-order term. Repeated optimization yields a good temperature coefficient.

1.3 Overall Circuit Analysis

The proposed circuit structure is shown in Figure 3. The system comprises four modules: power-saving and bias circuits, op amps, reference voltage output modules, and high-order curvature compensation. The working principle of the reference core structure and the high-order curvature compensation circuit is highlighted in the improved bandgap reference previously analyzed. The power control switch VC1 on the left side of Figure 3 works as follows: when VC1 is low (0), M6 is on, M4 is off, M7's gate potential is high, and M7 is off, resulting in no current in the M7 branch. Current mirrors M10 and M11 replicate this current, causing the differential amplifier's tail current to be zero, rendering the differential amplifier inactive, and placing the entire circuit in a power-saving state. When VC1 is high (3.3V), M6 turns off and M4 turns on, enabling the bias circuit composed of M1 to M6 to provide a suitable bias voltage for M7’s gate. The bias of the Cascode structure (M8, M9, M10, M11) is achieved via voltage self-biasing. Similarly, M10 and M11 replicate the M7 branch current, and M12 and M13 provide a bias voltage for the tail current source. The bias circuit provides all the bias voltages required for the stage-folding cascode op amp circuit. In practical circuits, to ensure matching, the tube length in the bias circuit should match that of the corresponding tube in the op amp.

The op amp is one of the key components in the bandgap voltage reference circuit. The loop gain and circuit offset determine the accuracy and stability of the reference source output. To enhance circuit stability and simplify the design, a single-stage operational amplifier with high gain is employed instead of a two-stage compensated operational amplifier. High-gain single-stage op amps include both telescopic and folding types. Since the op amp is connected to the feedback loop, the telescopic op amp is not used due to its limited output swing. Instead, a folding op amp is used here.

2 Simulation Results Analysis

The circuit shown in Figure 3 was simulated in a 0.35 μm BSIM 3v3 CMOS process using Cadence Spectre software, yielding the following simulation results.

2.1 Reference Output vs. Power Supply Voltage

Figure 4 shows the reference output versus supply voltage (0 to 3.3V). The simulation results indicate that the minimum supply voltage for the bandgap reference structure under normal operating conditions can reach 1.6V, with an output reference voltage V_ref=(1.17443 ± 0.00043V) across the temperature range of -40 to +100°C. The temperature coefficient of the output voltage of the bandgap reference is r_TC=2.077 ppm/°C. At 25°C and 3.3V, the power consumption is less than 110μW (total circuit power consumption is 109.89μW). At 25°C and 1.6V, the power consumption is less than 9μW (total circuit power consumption is 8.453μW).

Simulating the supply voltage rejection ratio (PSRR) for this bandgap voltage reference source yields -65dB at 100Hz at room temperature and without filter capacitors at a 3.3V supply voltage. Adding a filter capacitor to the output of the reference can improve the PSRR, reducing noise interference and minimizing the reference voltage transient overshoot during circuit startup.