Feature: T&M
Figure 1: Power solution for a non- isolated bipolar supply (±15V and ±5V) with low ripple
Bipolar power solutions for precision
test and measurement systems By Alan Walsh, System Applications Engineer, Analog Devices
D
eveloping 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 from switching supplies. Multichannel instruments have an
increasing need for channel-to-channel isolation where each channel requires its own power source. PCB footprints of each new generation of T&M instrument are expected to shrink, yet performance will 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 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 solutions.
Simplifying the design effort Many precision T&M instruments require multi-quadrant operation to source and measure both positive and negative signals. Tis requires generation of both negative and positive supplies from a single positive supply input, efficiently and with low noise. 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 generate additional rails of 5V, 3.3V or 1.8V for signal-conditioning circuits or ADCs/DACs. Te solutions were designed using Analog Devices’s LTpowerCAD, which includes 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 (<~100mA) is required, the ADP5070/ ADP5071 is a better choice. Although
52 September/October 2020
www.electronicsworld.co.uk
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 under certain conditions. Te 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 switching ripple in the MHz range, then LDO regulators like the LT3094/LT3045 should be considered. Te 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 their higher quiescent current. Two reference designs examples, CN-0345 and CN-0385, implement this solution by using the ADP5070. Te 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
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