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 high-resolution ADC signal chains without being compromised by the ripple noise produced 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 footprints while maintaining performance. Implementing low noise power solutions in these applications can result in 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, this task can be simplified by building a more integrated power solution with μModule devices and related components. Solutions such as Silent Switcher devices and high power supply rejection ratio (PSRR) LDO regulators achieve higher efficiency while reducing radiated noise and switching ripple.

Figure 1. Power solution for non-isolated bipolar power systems (±15 V and ±5 V) with low power supply 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.

The power rail solution shown here was designed using the system design tool in LTpowerCAD. LTpowerCAD Design Tool is a complete power supply design tool program that can be used to significantly simplify the task of power supply design for many power supply 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 provide high efficiency at lighter loads (<~ 100 mA), the ADP5070/ADP5071 are a better choice. Although the ADP5070 solution requires more external components such as inductors and diodes, it allows for more customization of the power supply solution. Both the ADP5070 and LTM8049 have synchronization pins that can be used to synchronize the switching frequency and the ADC's clock to avoid switching the internal FETs during the ADC's sensitive period. The high efficiency of these regulators at load currents of hundreds of mA makes them ideal for precision instrument power supplies.
The LT3032 integrates a positive and negative voltage low noise, wide operating range LDO regulator in a single package. The LT3023 integrates two low noise, positive voltage LDO voltage regulators with a wide operating range. Both LDO regulators are configured to operate with minimal dropout (~0.5 V) for maximum efficiency while providing good switching power supply ripple rejection. Both LDO regulators are housed in a small LFCSP package, which reduces PCB size and simplifies bill of materials. If the LDO regulator requires higher PSRR to further reduce switching ripple in the MHz range, LDO regulators such as the LT3094/LT3045 should be considered. The choice of PSRR required in the LDO stage will depend on the PSRR of components such as ADCs, DACs, and amplifiers powered from the rails. In general, the higher the PSRR, the less efficient the LDO regulator will be due to the higher quiescent current.
CN-0345 and CN-0385 are two example reference designs that implement this solution using the ADP5070. These are designed for precision multi-channel data acquisition using precision ADCs such as the 18/20-bit AD4003 /AD4020. In the CN-0345, an LC tank circuit is used to filter switching ripple from the ADP5070, instead of using an LDO regulator, as shown in Figure 1. In reference design CN-0385, positive and negative voltage LDO regulators (ADP7118 and ADP7182) are used after the ADP5070 to filter switching ripple. An example of using the ADP5070 to power a bipolar 20-bit precision DAC such as the AD5791 can be found in the evaluation board user guide here.
These examples illustrate how to maintain high precision performance when using switching regulators such as the ADP5070 to generate bipolar power supplies in applications such as data acquisition and precision power/sourcing.
Isolated bipolar power supply

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 using bipolar supply rails to provide isolated power.
Figure 2. Power solutions for isolated bipolar power systems with low power supply ripple.
The ADuM3470 and LTM8067 allow us to efficiently deliver up to ~400 mA across the isolated 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 kV rms, while the ADuM4470 family can be used for higher levels of voltage isolation up to 5 kV rms.

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 the 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 the need for an additional regulator stage. 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 isolation of a 16-bit bipolar DAC 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.

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 stray fields generated by 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 traditional switches. The high output current of 1.2 A 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 a μModule solution 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.