Bipolar Power Solutions for Precision Test and Measurement Systems

To ensure high accuracy, precision test and measurement systems require power supply solutions with low ripple and radiated noise that do not degrade the performance of the high-resolution converter signal chain. In these test and measurement applications, generating bipolar and/or isolated system power supplies presents system designers with challenges in board area, switching ripple, EMI, and efficiency.

Author: Alan Walsh

To ensure high accuracy, precision test and measurement systems require power supply solutions with low ripple and radiated noise that do not degrade the performance of the high-resolution converter signal chain. In these test and measurement applications, generating bipolar and/or isolated system power supplies presents system designers with challenges in board area, switching ripple, EMI, and efficiency. Data acquisition systems and digital multimeters require low-noise power supplies in order to provide the performance of a high-resolution ADC signal chain without being compromised by the ripple noise created by switching power supplies. Source meters (SMUs) and DC sources/supplies have similar requirements to minimize spurious output ripple on the high-resolution DAC signal chain. There is also a trend towards increasing the number of channels in precision test and measurement instruments to allow for increased parallel testing. These multi-channel instruments increasingly require channel-to-channel isolation in galvanically isolated applications, where power must be generated on each channel. This drive solution requires smaller and smaller PCB size while maintaining performance. Implementing a low noise power supply solution in these applications can result in a larger than desired PCB size and/or poor efficiency due to excessive use of LDO regulators or filter circuits.

For example, a switching power rail with 5 mV ripple at 1 MHz requires a combination of LDO regulator and ADC supply characteristics to achieve a power supply rejection ratio (PSRR) of 60 dB or more, reducing switching ripple at the ADC output to 5 μV or less. For an 18-bit high-resolution ADC, this is only a fraction of the LSB (and thus has no effect on the LSB).

Fortunately, the μModule can® d devices and related components to build a more integrated power solution to simplify this task. e.g. Silent Switcher® devices and LDO regulators with high power supply rejection ratio (PSRR), these solutions achieve higher efficiency while reducing radiated noise and switching ripple.

Many precision test and measurement instruments, such as source meters or power supplies, require multi-quadrant operation to acquire and measure positive and negative signals. This requires efficient generation of positive and negative supplies from a single, low-noise positive supply input. Let’s take the example of a system that needs to generate bipolar power from a single positive input power supply. Figure 1 shows a power supply solution that generates ±15 V and ±5 V and uses positive and negative LDO regulators to filter/reduce switching ripple, as well as other power rails such as 5 V, 3.3 V, or 1.8 V for signal conditioning circuits or ADC and DAC power supply.

Bipolar Power Solutions for Precision Test and Measurement Systems
Figure 1. Power solutions for non-isolated bipolar power systems (±15 V and ±5 V) with low power supply ripple.

The power rail solution shown here uses LTpowerCAD®System design tool design in . The LTpowerCAD® Design Tool is a complete power design tool program that can be used to significantly simplify power design tasks for many power products.

The LTM8049 and ADP5070/ADP5071 allow us to take a single positive input, boost it to the desired positive supply and invert to generate a negative supply. The LTM8049 is a μModule solution that significantly simplifies the component count required—just add input and output capacitors. In addition to simplifying the design challenges of selecting components and board layout for switching regulators, the LTM8049 minimizes the PCB size and bill of materials required to generate bipolar power supplies.To be at lighter loads (isolated bipolar power

When precision test and measurement instruments need to be isolated for safety reasons, it can be a challenge to efficiently supply sufficient power through isolation devices. In multi-channel isolated instruments, channel-to-channel isolation means a power solution for each channel. This requires a compact power solution that can provide efficient power supply. Figure 2 shows a solution for providing isolated power using bipolar supply rails.

Bipolar Power Solutions for Precision Test and Measurement Systems
Figure 2. Power solutions for isolated bipolar power systems with low power supply ripple.

The ADuM3470 and LTM8067 a allow us to efficiently deliver up to ~400 mA across the isolation at the 5 V isolated output. The LTM8067 is a µModule solution that integrates transformers and other components that simplify the design and layout of isolated power solutions while minimizing PCB size and bill of materials. The LTM8067 isolates up to 2 kV rms. For lower output ripple, the LTM8068 integrates an output LDO regulator, reducing output ripple from 30 mV rms to 20 μV rms at the expense of a lower output current of 300 mA.

The ADuM3470 family uses an external transformer to provide isolated power while integrating digital isolation channels for data transfer and control of the ADC and DAC. Depending on how the isolation solution is configured, the isolated power output can follow a power solution like Figure 1, which generates ±15 V rails on the isolated side from a single positive supply as shown in Figure 2. Alternatively, the ADuM3470 design can be configured to generate bipolar power directly without additional switch stages. This results in a smaller PCB area solution at the expense of efficiency. The ADuM3470 can isolate up to 2.5 kVrms, while the ADuM4470 family can be used for higher levels of voltage isolation up to 5 kVrms.

The CN-0385 is an example reference design implementing the ADuM3470 solution, as shown in Figure 2. The ADP5070 is used on the isolated side to generate bipolar ±16 V rails from the isolated 5.5 V. The digital isolation channels used by this reference design are also included in the ADuM3470. A similar design using the ADuM3470 is CN-0393. This is a multi-channel isolated data acquisition system based on the ADAQ7980/ADAQ7988 μModule ADC. In this design, the ADuM3470 is configured with an external transformer and Schottky diode full-wave rectifier to generate ±16.5 V directly without additional regulator stages. This allows for a smaller solution at the cost of reduced efficiency. Similar solutions are shown in CN-0292, a 4-channel data acquisition solution based on the AD7176 sigma-delta ADC, and CN-0233, which highlights the same 16-bit bipolar DAC Isolated power solutions.

These examples show how to provide isolated power to achieve the precision performance of isolated data acquisition or isolated power while maintaining a small PCB size or high power efficiency.

Silent Switcher Architecture for Efficient Buck and Low Noise

In the power scheme shown in Figure 1, an LDO regulator is used to step down from 15 V to 5 V/3.3 V. This is not a very efficient way to generate these low voltage rails. A high-efficiency solution for boosting down to lower voltages using the Silent Switcher, μModule regulator LTM8074 is shown in Figure 3.

Bipolar Power Solutions for Precision Test and Measurement Systems
Figure 3. Power solutions to step down to lower voltage rails with low EMI.

The LTM8074 is a Silent Switcher, µModule buck regulator in a small 4 mm × 4 mm BGA package capable of delivering up to 1.2 A with low radiated noise. Silent Switcher technology cancels out the stray fields generated by the switching currents, thereby reducing conducted and radiated noise. This µModule device is highly efficient and has very low radiated noise, making it an excellent choice for powering noise-sensitive precision signal chains. Depending on the PSRR connected to a mains powered component such as an amplifier, DAC, or ADC, it may be possible to power it directly from the Silent Switcher output, without the need for an LDO regulator to further filter the power supply ripple, which is required with conventional switches. The high output current of 1.2A also means it can be used to power digital hardware in systems such as FPGAs if needed. The LTM8074’s small size and high level of integration make it ideal for space-constrained applications while simplifying and accelerating switching regulator power supply design and layout.

If more customization is required at the expense of PCB area, discrete implementations of Silent Switcher devices can be achieved using products such as the LT8609S. These products include a spread spectrum mode, which spreads the ripple energy over a frequency band at the switching frequency. This reduces the magnitude of spurs present in precision system power supplies.

Combining Silent Switcher technology with the high level of integration in μModule solutions addresses the challenges of increasing density in precision applications such as multi-channel source meters without compromising the high-resolution performance system designers need to achieve level.

in conclusion

Isolated bipolar power systems that power precision Electronic test and measurement require a balance between system performance, maintaining small size, and power efficiency. Here we present some solutions and products that help address these challenges and allow system designers to make the right trade-offs.

Reference circuit

Balat, Fil Paulo, Jefferson Eco and James Macasaet. “Preventing Startup Problems Caused by Switching Converter Output Surge.” Analog Dialogue, January 2018.

Knoth, Steve. “Using Ultralow Noise LDO Regulators to Provide Clean Power.” Analog Devices, September 2018.

Limjoco, Aldrick. “Understanding Switching Regulator Output Artifacts to Speed ​​Power Supply Design.” Analog Dialogue, August 2014.

Luan, Austin. “Low EMI, Silent Switcher, 1.2 A µModule Regulator in 4 mm × 4 mm × 1.82 mm BGA Package.” Analog Devices, January 2019.

Morita, Glenn. “Understanding Low Dropout Regulators (LDOs) for Optimal System Design.” Analog Dialogue, December 2014.

Obaldia, Estibaliz Sanz and James Jasper Macasaet. AN-1359 Application Note, Low-Noise, Dual-Supply Solution Using the ADP5070 to Power a Precision AD5761R Bipolar DAC in a Single-Supply System. Analog Devices, March 2016.

Tompseet, Kevin. AN-1366 Application Note, Using the ADP5070/ADP5071 at VOUT Create positive and negative voltage supply rails in the case of IN. Analog Devices, July 2015.

Walsh, Alan. “Using High Efficiency, Ultra-Low-Power Switching Regulators to Power Precision SAR ADCs in Power-Sensitive Applications.” Analog Devices, Inc., March 2016.

author

Bipolar Power Solutions for Precision Test and Measurement Systems
Alan Walsh

Alan Walsh is an applications engineer at Analog Devices. He joined Analog Devices in 1999 and worked in the Precision Converter Applications Group in Wilmington, Massachusetts, USA. He holds a bachelor’s degree in electrical engineering from University College Dublin.

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