Feature: T&M
Implementing low-noise power solutions in T&M applications can result in poor power efficiency from excessive use of LDO regulators or filter circuits
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 aſter 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 having power per channel, which necessitates a compact solution; see Figure 2. Te ADuM3470 and LTM8067 allow
delivery of power over the isolation barrier up to ~400mA at 5V isolated output with high efficiency. Te 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 ripple 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
Figure 2: Isolated bipolar supply system with low ripple
Figure 3: Stepping down to lower voltage rails with low EMI
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 at the expense of efficiency. Te 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. Te ADP5070 is used on the isolated side to generate the bipolar ±16V rails from isolated 5.5V. Tis reference design uses digital isolated channels, also included in the ADuM3470. A similar design that uses the
ADuM3470 is CN-0393. Tis 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. Tis 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. Te Silent Switcher technology cancels
stray fields generated by switching currents, thereby reducing conducted and radiated noise. Te 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 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 PCB area, then a discrete implementation of a Silent Switcher device can be created with LT8609S. Tese products include a spread-spectrum mode to spread the ripple energy at the switching frequency over a wide band, reducing the amplitude of spurious tones from the supplies.
www.electronicsworld.co.uk September/October 2020 53
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68
