Using a switching power supply to power a high-speed analog-to-digital converter

Traditionally, ADC manufacturers have recommended using linear regulators to provide clean power to the converter . Linear regulators can suppress low-frequency noise that is often present in system power supplies . In addition, a combination of ferrite beads and decoupling capacitors can be used to reduce high-frequency noise. Although effective, this approach limits efficiency, especially in systems where the linear regulator must step down from a power rail that is several volts higher than its output voltage. Low-dropout regulators (LDOs) are typically 30% to 50% efficient, while DC/DC regulators can reach 90% efficiency. Figure 1 shows the typical efficiency of a step-down switching regulator such as the ADP2114 from Analog Devices.

Figure 1 Typical efficiency of the ADP2114 switching regulator

Although DC/DC converters are much more efficient than LDOs, they often suffer from significant performance degradation when directly powering high-speed analog- to-digital converters due to excessive noise. This noise comes from at least two sources: noise that enters the converter directly through the power supply ripple, and noise caused by magnetic coupling effects. Power supply ripple appears as different tones (or spurs) in the output spectrum of the ADC, or causes an overall increase in the noise floor. The ADC's susceptibility to these different tones can be characterized, usually expressed in the converter data sheet as the power supply rejection ratio (PSRR). However, the PSRR cannot represent the broadband effect on the converter's noise floor. The large currents generated in the switching power supply usually generate strong magnetic fields, which can couple with other magnetic components on the circuit board, including inductors in the matching network and transformers used to couple analog and clock signals. Care must be taken when routing the circuit board to prevent these magnetic fields from coupling with critical signals.

Power Savings (Efficiency Advantage)
While semiconductor companies continue to introduce more efficient ADCs, DACs, and amplifiers, these improvements pale in comparison to the total system power efficiency gained by replacing LDOs with DC/DC regulators. For example, a linear circuit that delivers 100mA, or 330mW, from a 3.3V supply would consume 500mW of power with a typical LDO that steps down 5V to 3.3V, with only 330mW providing useful power. The original power supply would have to be 51% larger than what was actually needed, wasting energy and increasing cost. By comparison, consider a DC/DC regulator that is 90% efficient. The total current requirement for the 5V supply would be 74mA (a much lower requirement), reducing both power and cost.

In systems such as wireless base stations, power is often provided by a single high current supply. This supply is often stepped down through a number of different buck stages before reaching the linear and mixed signal components. Although each buck stage is highly efficient, they also waste considerable power. Figure 2 shows a typical system where power is stepped down from a 12V rail using three or more buck stages to power ADCs and other analog devices. The last stage is typically an LDO, which is typically the least efficient of the buck stages. Even a high efficiency DC/DC regulator with 90% efficiency can only achieve 81% efficiency when cascaded twice as shown below, and even lower efficiency when the final regulator stage must be an LDO.

　　Figure 2 Typical system-level power supply

With the advancement of DC/DC power supply technology and the development of higher switching frequencies, DC/DC power supplies can directly power the ADC with greatly improved efficiency without performance loss . Figure 3 shows a typical buck circuit that eliminates the LDO.　

　Figure 3 Simplified system-level power supply

Additionally, many systems use a separate LDO for each ADC. Separate LDOs are used to provide noise isolation between different ADCs and reduce the power consumption of each LDO. This separate approach spreads the heat generated by the LDO and allows the use of LDOs in small packages. Because switching converters are more efficient, a single switch can power multiple ADCs and other linear components without generating excessive power and heat, which would occur with a single large LDO. Using filtering ferrite beads at the output of the switching power supply can provide isolation for components using the same power rail. Using a switching power supply reduces the need for a system regulator, which can significantly save power and reduce board cost by eliminating the redundant LDO and its associated circuitry.

Lab Circuits
16-bit, 125MS/s analog-to-digital converters such as the Analog Devices AD9268 are capable of achieving very low noise and 78dB signal-to-noise ratio (SNR) specifications. The very low C152dBm/Hz noise floor makes it ideal for evaluating switching power supplies. The additional noise or spurious amounts caused by the DC/DC converter can be easily seen in the converter's output spectrum. The converter is a companion product to the Analog Devices ADP2114 PWM buck regulator. This dual-output buck regulator has an efficiency of up to 95%, operates at a high switching frequency, and has low noise characteristics.

A laboratory study compared the performance of an ADC when using a linear regulator versus a switching regulator. These experiments were performed using a user evaluation board for the converter. The converter has two input supplies: AVDD to power the analog section and DRVDD to power the digital and output sections. For comparison, the converter was initially evaluated using two linear regulators (ADP1706 from Analog Devices) to provide the AVDD and DRVDD voltages, respectively. The setup for this test is shown in Figure 4. The converter was then powered using a switching regulator, as shown in Figure 5. The output of one switching regulator was provided to AVDD and the output of the other was provided to DRVDD.

Figure 4. Block diagram of linear power supply measurement using the ADP1708 LDO.

Figure 5. Block diagram of switching power supply measurement using the ADP2114 switching regulator.

In both setups, the analog input source is a Rohde & Schwarz (RS) SMA-100 signal generator and a KL bandpass filter. The analog input is provided through a double balun input network, which converts the single-ended output of the signal generator to the differential input of the ADC. The sampling clock source is a low-jitter Wenzel oscillator, also powered by the balun circuit for single-ended to differential conversion. The input power rail (before the regulator) is set to 3.6V for both measurements.　　

ADC Performance Measurement Results
The converter performance was measured for each power supply configuration to determine if the performance was degraded when using a switching power supply. The SNR and SFDR (spurious-free dynamic range) were measured over a range of input frequencies; the results are shown in Table 1, showing no significant changes in SNR or SFDR performance when using a linear regulator versus a switching power supply.

The switching regulator can operate asynchronously or it can be synchronized with the converter’s sampling clock without affecting the converter performance. Synchronization provides more flexibility in the application, which can be an advantage in the application.

FFT Spectra
Figures 6 and 7 show the FFT (Fast Fourier Transform) spectra of the AD9268 with an analog input frequency of 70MHz when using a linear power supply and a switching power supply , respectively.

　Figure 6  70MHz analog input using ADP1708 linear power supply

　Figure 7 70MHz analog input using ADP2114 switching power supply

Table 2 shows the measured efficiency of each power solution. With a 3.6V input voltage, the switching regulator improves efficiency by 35%, saving 640 mW. This power saving is for a single converter , and the savings will increase significantly in a system with multiple ADCs.

Figures
8 and 9 show the difference in heat dissipation in the power section of the board when using an LDO power supply versus the ADP2114. Both images use the same scale. The SP01, SP02, and SP03 measurement points in Figure 8 show the temperature of the linear regulator. SP06 in Figure 9 shows the temperature of the ADP2114, which is 10 to 15°C cooler than the linear regulator shown in Figure 9. SP04 shows the temperature of the AD9268, which is similar in both images. Also note that the total background temperature is higher in Figure 9, and a series blocking diode (not labeled) is handling the higher thermal load.　　

Figure 8. Heat dissipation image of the AD9268 evaluation board using a linear power supply.

Figure 9. Heat dissipation image of the AD9268 evaluation board using the ADP2114 power supply.

Figure
10 provides a detailed circuit diagram of the switching regulator configured to operate in forced PWM mode with the channels set to 2A individual outputs. The switching frequency of the regulator is set to 1.2MHz by placing a 27kΩ resistor between the FREQ pin and GND. In addition to the circuit shown, a ferrite bead is included between the switch and the ADC, and standard bypass capacitors are placed near the ADC power pins. This design achieves 220μV of switching ripple and less than 6μV of high-frequency noise at the output of the ADP2114. The ferrite bead and bypass capacitors placed near the AD9268 reduce the switching ripple to 300nV and reduce the noise at the ADC power pins to less than 3μV.

Figure 10 ADP2114 circuit configuration

The bill of materials and layout information are also provided. Note that in the layout, the switching inductors L101 and L102 are located on the back side of the board with the ADC and signal path components. This layout helps minimize voltage coupling between these inductors and components on the top side of the board, especially the baluns in the signal and clock paths. In the layout with switching converters , care should be taken to avoid magnetic or electric field coupling.　　

Figure 11 Relative positions of ADP2114 and AD9268

Conclusion
This article has demonstrated that, if design practices are carefully followed, an analog-to-digital converter can be powered directly from a switching power supply without loss of performance. The converter performance is not degraded when powered from the ADP2114 switching power supply compared to the ADP1708 linear power supply. Using a switching power supply can improve power supply efficiency by 30% to 40% and can significantly reduce overall power consumption (even more than simply choosing a lower power converter). In many systems, these devices need to operate continuously, so using a switching power supply can significantly reduce operating costs without sacrificing performance.