8 tips for effective power-integrity probing
Imagine the following scenario; your latest hardware has been delivered ready for commissioning and you have successfully powered up your shiny new board, started functional tests, and are seeing rail droop, ground bounce, power-supply-induced clock and data jitter, as well as in-band spurs in the ADC/DAC output spectrum. Why is all of this happening? How can you identify the root causes and how bad are they? How sensitive are the loads to AC noise and ripple on their power rails and what is their PSRR? How quiet must the supply voltages have to be to deliver the required performance?
Measuring small levels of AC noise on DC power rails using an oscilloscope can be problematic: first, the instrument and probe add further noise leading to confusion between measurement or power-supply-induced noise. The amount of offset may be limited preventing you zooming-in to view and analyse AC interference riding on top of the DC supply. Furthermore, the input impedance of the oscilloscope may load the power rail and its bandwidth may be limited, masking high-frequency switching transients.
Given the increasing use of faster, higher-density FPGAs and broadband ADCs/DACs powered using lower supply voltages generated by switching regulators, together with bad design practices, engineers are requiring the ability to zoom-in on DC power rails to look for AC transients, noise, and ripple while they commission and debug their avionics hardware. An oscilloscope often does not have enough offset to be able to position a DC power rail in the centre of the screen for the required measurements. Placing a blocking capacitor in the signal path eliminates the offset problem but also masks important information, such as compression or low-frequency drift.
The latest, low-noise probes have good noise figure so they do not pollute the measurement of AC interference and ripple on a DC supply. An initial test is to short the inputs to verify that the probe and oscilloscope are suitable for the required measurement task. The plot below summarises the measurement of low-level AC noise on a DC rail and shows the reduction in baseline noise by using a power-integrity probe compared to a standard one.
Dynamic loading of a DC supply by an FPGA or an ADC/DAC occurs at the clock frequency and can generate high-speed transients and noise on the power rails. Designers need high-bandwidth tools to evaluate and understand broadband interference on supply voltages. Switching noise can generate transient frequencies that can easily exceed 1 GHz.
The length of the ground connection to the probe will impact the measurement of noise as shown below. The probe's internal capacitance and the ground lead form an LC circuit and a shorter return has less inductance. Ground can also act as an antenna for noise!
To successfully debug the latest avionics hardware, power-rail probes such as Keysight's N7020A offer a 1:1 attenuation ratio adding only 10% to the baseline oscilloscope noise, ±24 V of offset to allow you to centre the signal on the screen and zoom-in to analyse AC noise, 50 kΩ loading at DC, and 2 GHz of bandwidth to capture fast transients.
To summarise, the top tips for effective power-integrity oscilloscope measurements are:
- Start with a null measurement to understand the baseline floor of the oscilloscope.
- Choose the lowest noise 50 Ω path and reduce the attenuation ratio.
- Choose a probe with sufficient bandwidth; switching currents can cause transients with frequencies which exceed 1 GHz.
- Minimise the ground loop area.
- Use the probe offset to remove the DC offset to analyse AC noise and ripple.
- Minimise loading at DC.
- Use the frequency domain to identify possible noise sources.
- Use triggering and averaging to relate interference on supply rails to a noise source.
P.S. The first person to tell me how this last sentence fits with this post will get a free Courses for Rocket Scientists pen. Congratulations to Wojciech from Poland, a new reader of Out-of-this-World Design and the first to answer the riddle from my previous article on ARM-based processors.
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