By Alan Walsh, System Applications Engineer, Analog Devices
Developing bipolar and/or isolated power supplies is challenging for designers in terms of board area, switching ripple, EMI and efficiency. High-precision T&M instruments, and especially now with the growing trend of increasing the number of channels per instrument for parallel testing, need their high-resolution ADC signal chains to perform well, without being corrupted by spurious ripple tones from switching supplies.
Multichannel instruments have an increasing need for channel-to-channel isolation where power must be generated on per-channel basis. PCB footprints of each new generation of T&M instrument are expected to reduce, yet performance to increase. Implementing low-noise power solutions goes against that, with larger-than-desired PCB footprints and poor power efficiency from excessive use of LDO regulators and filter circuits. For example, a switching power supply rail with 5mV ripple at 1MHz would need a combined power supply rejection ratio (PSRR) of 60dB or greater from an LDO regulator, and an ADC to reduce the switching ripple seen at its output to 5μV or less – a fraction of a least significant bit (LSB) for an 18-bit ADC.
Luckily, there are solutions that can simplify this task through higher levels of integration, such as Analog Devices’s μModules and Silent Switcher devices.
Figure 1: Power solution for a non-isolated bipolar supply system (±15V and ±5V) with low supply ripple
Simplifying the design effort
Many precision T&M instruments require multi-quadrant operation to source and measure both positive and negative signals. This requires generation of both negative and positive supplies from a single positive supply input, with low noise and in an efficient manner.
Let’s consider a system that requires bipolar supply generation from a single positive input supply. Figure 1 shows an example that generates ±15V and ±5V and uses positive and negative LDO regulators to filter/reduce the switching ripple as well as generates additional rails of 5V, 3.3V or 1.8V for signal-conditioning circuits or ADCs/DACs. This solutions were designed using the system designer found in LTpowerCAD, a design tool with a complete power-supply-design program.
The LTM8049 and ADP5070/ADP5071 take a single positive input, boost it to the required positive supply and invert it to generate the negative supply rail. The LTM8049 is a μModule solution that in this case only needs input and output capacitors.
Where efficiency at lighter loads (< ~100 mA) is required, the ADP5070/ADP5071 is a better choice. Although it needs more external components (inductors and diodes), it can customise the power solution. Both the ADP5070 and LTM8049 have sync pins used for synchronising the switching frequency with the ADC’s clock to avoid switching the internal FETs in certain conditions.
The LT3032 incorporates both a positive and negative low-noise LDO regulator, and two low-noise positive LDO regulators, configured to operate with minimal headroom (~ 0.5V) to maximise efficiency whilst also delivering good ripple rejection from the switching regulator stage.
If much higher levels of PSRR are required from the LDO regulator to further reduce the switching ripple in the MHz range, then LDO regulators like the LT3094/LT3045 should be considered. The choice of how much PSRR is required in the LDO stage will depend on the PSRR of the components, like ADCs, DACs and amplifiers that are powered from the supply rails. Generally, higher PSRR LDO regulators are less efficient due to higher quiescent current.
Two reference designs examples, CN-0345 and CN-0385, implement this solution by using the ADP5070. The designs are for precision multichannel data acquisition using the 18-/20-bit AD4003/AD4020. In CN-0345, an LC tank circuit is used to filter the switching ripple from the ADP5070 instead of using an LDO regulator as shown in Figure 1. In the CN-0385 reference design, positive and negative LDO regulators (ADP7118 and ADP7182) are used after the ADP5070 to filter the switching ripple.
Isolated bipolar power supplies
When a precision T&M instrument needs to be isolated for safety reasons, this brings challenges in delivering sufficient power efficiently across the isolation barrier. In multichannel isolated instruments, channel-to-channel isolation means power per channel, which necessitates a compact solution; see Figure 2.
Figure 2: Power solution for isolated bipolar supply system with low supply ripple
The ADuM3470 and LTM8067 allow delivery of power over the isolation barrier up to ~ 400mA at 5V isolated output with high efficiency. The LTM8067 is a µModule solution integrating a transformer and other components that simplify the design and layout of the isolated power solution (to 2kVrms). For even lower output ripple, the LTM8068 incorporates an output LDO regulator that reduces it from 30mVrms to 20μVrms at the expense of the lower output current of 300mA.
Depending on how the isolation solution is configured, the isolated power output can be followed with a power solution similar to that in Figure 1, but shown in Figure 2 to generate ±15V rails on the isolated side from a single positive supply. Alternatively, the ADuM3470 design can be configured to generate bipolar supplies directly without an extra switcher stage, resulting in a smaller PCB area at the expense of efficiency. The ADuM3470 isolates up to 2.5kVrms, but the ADuM4470 family can be used for higher levels of voltage isolation up to 5kVrms.
CN-0385 is an example of a reference design that implements the ADuM3470 solution; see Figure 2. The ADP5070 is used on the isolated side to generate the bipolar ±16V rails from an isolated 5.5V. This reference design uses digital isolated channels, also included in the ADuM3470.
A similar design that uses the ADuM3470 is CN-0393. This is a bank isolated data acquisition system based on the ADAQ7980/ADAQ7988 μModule ADC. Here, the ADuM3470 is configured with an external transformer and Schottky diode full-wave rectifier to generate ±16.5V directly, without the need for an additional regulator stage. This reduces the solution’s footprint at the expense of lower efficiency.
A similar solution is shown in CN-0292, which is a 4-channel data acquisition setup based on the AD7176 ∑-Δ ADC, and CN-0233, which highlights the same isolated power solution of a 16-bit bipolar DAC.
The Silent Switcher architecture
In the power supply scheme of Figure 1, an LDO regulator is used to step down from 15V to 5 /3.3V, which is not an efficient way of generating low voltage rails. Figure 3 shows a solution to improve the efficiency of stepping down to lower voltages using the Silent Switcher, μModule regulator LTM8074.
Figure 3: Power solution for stepping down to lower voltage rails with low EMI
The Silent Switcher technology cancels stray fields generated by the switching currents, thereby reducing conducted and radiated noise. The high efficiency and low radiated noise of this module make it a great choice for powering noise-sensitive precision signal chains. Depending on the PSRR of the components connected to the output supply such as amplifiers, DACs or ADCs, it may be possible to power them directly from the Silent Switcher output without using an LDO regulator to filter the supply ripple, as with traditional switchers. Its high output current of 1.2A also means it could power a system such as an FPGA, if needed.
If greater customisation is required at the expense of the PCB area, then a discrete implementation of a Silent Switcher device can be created with LT8609S. These products include a spread-spectrum mode to spread the ripple energy at the switching frequency over a band, reducing the amplitude of spurious tones from the supplies.