Feature sponsored by Test & measurement I
n order to ensure high accuracy, precision test and measurement systems require power supply solutions with low levels of ripple and radiated noise so as not to degrade the performance of high resolution converter signal chains. In these test and measurement
applications, generating system supplies that are bipolar and/or isolated poses a challenge to system designers in terms of board area, switching ripple, EMI, and efficiency. Data acquisition systems and digital multimeters require low noise supplies in order to deliver the performance of high resolution ADC signal chains without being corrupted by spurious ripple tones from switching supplies. Source measure units and DC sources/power supplies have similar requirements for minimising spurious output ripple on high resolution DAC signal chains. There is also a trend toward a higher channel count in precision test and measurement instruments for increased parallel testing. In electrically isolated applications, these multichannel instruments have an increasing need for channel-to-channel isolation where power must be generated on a per channel basis. This is driving solutions that require a smaller and smaller PCB footprint while maintaining performance. Implementing low noise power solutions in these applications can result in larger than desired PCB footprints and/or poor power efficiency from excessive use of LDO regulators or filter circuits. For example, a switching power supply rail with 5mV of ripple at 1MHz would need a combined power supply rejection ratio (PSRR) of 60dB or greater from an LDO regulator and powered ADC to reduce the switching ripple
seen at the ADC output to 5µV or less. This would be a fraction of an LSB for a high resolution 18-bit ADC. Luckily there are solutions that are simplifying this task through higher levels of
Figure 1. Power solution for a nonisolated bipolar supply system (±15V and ±5V) with low supply ripple.
power solution integration with µModule devices and components that deliver greater
efficiencies while reducing radiated noise and switching ripple such as Silent Switcher devices and high power supply rejection ratio (PSRR) LDO regulators.
Many precision test and measurement instruments such as source measure units or power supplies require multiquadrant operation to source and measure both positive and negative signals. This requires the generation of both negative and positive supplies from a single positive supply input with low noise and in an efficient manner. Let us consider a system that requires bipolar supply generation from a single positive input supply. Figure 1 shows power solutions that generate ±15V and ±5V and use positive and negative LDO regulators to filter/reduce the switching ripple as well as generate additional rails like 5V, 3.3V, or 1.8V for powering signal conditioning circuitry or ADCs and DACs. The power rail solutions shown here were designed using the system designer found in LTpowerCAD. The LTpowerCAD design tool is a complete power supply design tool programme that can significantly ease the tasks of power
BIPOLAR POWER SOLUTIONS FOR PRECISION TEST AND MEASUREMENT SYSTEMS
By Alan Walsh, system applications engineer, Analog Devices 48 June 2023 Instrumentation Monthly
supply design with many power products. The LTM8049 and ADP5070/ADP5071 allow us to 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 greatly simplifies the number of components needed to do this - we just need to add the input and output capacitors. As well as simplifying the design challenge in terms of selecting components and board layout for switching regulators, the LTM8049 also minimises the PCB footprint and bill of materials needed for generating bipolar supplies. Where efficiency at lighter loads (<~100mA) is required, the ADP5070/ADP5071 is a better choice. Although the ADP5070 solution requires more external components, like inductors and diodes, it allows for more customisation of the power solution. Both the ADP5070 and LTM8049 have sync pins that can be used to synchronise the switching frequency with the clock of an ADC to avoid switching the internal FETs during sensitive time periods for an ADC. The high efficiency of these regulators at a load current of a few 100mA makes them ideal for precision instrument supplies. The LT3032 incorporates both a positive and negative low noise LDO regulator in a single package with wide operating range. The LT3023 incorporates two low noise, positive LDO regulators with a wide operating range. Both LDO regulators are configured to operate with minimal headroom (~0.5V) to maximise efficiency while also delivering good ripple rejection from the switching regulator stage. Both LDO regulators are available in small LFCSP packages that reduce the PCB footprint
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