Measurements verify power integrity

Engineers ought to measure and analyze energy integrity on the facility and floor planes of a board’s energy distribution community. Understanding energy integrity is essential when evaluating circuit energy high quality as a result of it has a direct affect on efficiency.

Energy integrity is a subset of sign integrity, whose evaluation ensures that indicators have appropriate amplitude, rise time, noise, transient, and different traits for correct circuit efficiency beneath all working situations. Energy integrity focuses on these similar traits for the facility distribution community (PDN) energy rails to make sure that units obtain provide voltages that fall throughout the system’s specified working ranges beneath all circuit working situations. Measurements play a big function in energy integrity.

Provide voltages within the PDN should be saved inside a specified tolerance of their nominal worth. A PDN’s noise, droop, and transients additionally want tight management. Cumulatively, this would possibly imply an allowable power-rail variation of lower than 1%, relying on the circuit. Subsequently, a 1 V energy rail would possibly have to have variations within the rail voltage restricted to ±10 mV (or much less). Furthermore, bandwidth of the noise and transient indicators on the facility rail can prolong past 1 GHz. Measuring the small (millivolt) variations of the facility rail at comparatively excessive bandwidths can show difficult.

Determine 1 exhibits a typical embedded system. Its PDN contains voltage regulator modules (VRMs), low dropout (LDO) regulators, board interconnects, PCB energy and floor planes, and bulk decoupling Capacitors. Packaged Semiconductor units have inner energy rails not proven in Determine 1 and are the main target of extra on-die energy integrity evaluation.

Noise and transients encountered in power-integrity measurements come up from many sources. Self-aggressive noise can come up from VRM to VRM, or the noise may originate at a processor core, I/O indicators, or the VRMs. Mutual crosstalk between any of the circuit parts can also contribute.

When making power-integrity measurements, your purpose is to guarantee the voltage ranges of the rails (regulation) and to characterize the noise on the rails (e.g., ripple, droop beneath transient load situations, air pollution from different sources, and many others.). Determine 2 exhibits an instance of typical power-integrity measurements.

This evaluation of a 1.5 V rail makes use of automated measurements to search out the imply rail voltage and the peak-to-peak noise amplitude. A Quick Fourier Rework (FFT) permits you to view the rail’s frequency spectrum. The frequency-domain view is a great tool for evaluation and troubleshooting. Most oscilloscope producers create a devoted spectrum analyzer person interface throughout the oscilloscope.

The important thing oscilloscope specs related to power-integrity measurements are amplitude decision, bandwidth, pattern fee, vertical offset vary, and reminiscence size.

Amplitude decision specifies the smallest amplitude distinction within the acquired waveform that the oscilloscope can resolve. It's normally specified by way of the variety of bits of the oscilloscope’s digitizer. Assume you’re measuring 5 mV of ripple or noise driving on a 1 V rail. The ripple voltage is 46 dB (200:1) decrease than the rail amplitude. A rule of thumb is that an oscilloscope’s theoretical amplitude dynamic vary is 6 dB per bit. An 8-bit oscilloscope could have a usable dynamic vary of 48 dB (at greatest), whereas a finer-resolution oscilloscope may have a wider dynamic vary. This distinction seems within the comparative sign readability of the 2 captures in Determine 3.

An oscilloscope’s bandwidth limits the vary of frequencies that the instrument can measure. Whereas the rail indicators are nominally at DC, rail noise and transients can have a large frequency vary. Usually, PDN power-integrity measurements require oscilloscope bandwidth from 500 MHz to 2 GHz to precisely seize noise and transient occasions on the board-level energy rail, with much more bandwidth required to measure nearer to or on the die rails.

An oscilloscope’s most pattern fee should be larger than twice its bandwidth. Most trendy devices have minimal oversampling of 4 or 5 instances the bandwidth, and this will increase the horizontal (timing) decision and the power to precisely seize high-speed transients. The pattern fee additionally units the upper-frequency restrict of the FFT at half the sampling fee.

The vertical offset vary units the utmost vertical sensitivity you need to use to measure an influence rail, relying on the probe you utilize. The oscilloscope’s offset shifts the DC rail voltage and facilities the facility rail sign vertically on the oscilloscope grid. Adjusting the oscilloscope’s acquire sensitivity to 10 mV/div or much less permits you to view small amplitude modifications on the facility rail. A typical oscilloscope could have 1 V of offset (or much less) on the highest sensitivities (e.g., beneath 100 mV/div or 50 mV/div) — not sufficient to heart the facility rail and consider transients on the highest sensitivities. Oscilloscopes optimized for power-integrity measurements present sufficient offset at increased sensitivities to not restrict rail measurements. Moreover, attenuating probes, equivalent to the ten:1 passive probe equipped together with your oscilloscope, prohibit the instrument’s offset vary as a result of the probe’s attenuation issue shifts the oscilloscope-plus-probe mixture offset vary by the attenuation issue.

An oscilloscope with an offset specification of 1 V at 5 mV/div will, for instance, be restricted to 1 V of offset at 50 mV/div (10 instances 5 mV/div) when utilizing a ten:1 passive probe. This restricts the best sensitivity acquire setting that you would use to view small voltage ranges. Utilizing a home-built coaxial rail probe or a specialised energy rail probe can reduce the impression of the offset vary both by lowering the attenuation or growing the offset vary.

The oscilloscope’s acquisition reminiscence size units the document’s most period, which is essential for operations equivalent to measuring energy sequencing at startup and shutdown. Giant quantities of acquisition reminiscence may help you perceive and debug complicated cause-effect eventualities in an operational system when taking a look at many energy rails directly. The utmost acquisition size additionally determines the FFT’s frequency decision.

Probing choices
Viewing energy rails requires a probe. There are a number of concerns within the alternative of an oscilloscope probe for power-integrity measurements. The primary is that the indicators of curiosity are sometimes small voltages atop bigger DC voltages, and the noise that the measurement system (oscilloscope plus probe) contributes should be smaller than the indicators of curiosity. The oscilloscope-probe mixture will need to have adequate bandwidth to not alter sign parts of curiosity. The PDN’s energy rail could carry solely a small quantity of present, and the probe shouldn't load the facility rail (draw an excessive amount of present compared to what it's designed to hold). Impedance variations between the probe and the PDN may also trigger reflections of high-frequency parts. Lastly, the instrument system ought to have adequate offset to match the rail voltage when the probe is related.

The primary probing possibility is the usage of the ten:1 passive probe normally equipped with the oscilloscope. This passive probe makes use of a 9 MΩ Resistor in collection with the oscilloscope’s 1 MΩ enter, forming a ten:1 attenuator. This attenuator is compensated by utilizing a small capacitor in parallel with the 9 MΩ resistor, forming a high-pass filter that compensates for the low-pass attribute of the oscilloscope’s enter. An adjustable capacitor within the probe compensates the attenuator by making the high-pass and low-pass filter cutoff frequencies equal. All the time compensate such a probe everytime you join it to an enter channel.

An oscilloscope’s 10:1 passive probe has some limitations. The ten:1 attenuator reduces the enter sign equipped to the oscilloscope enter, which the oscilloscope should then cut back up. Subsequently, the additive noise of the probe is ten instances, or 20 dB, larger than the precise noise.

The ten:1 passive probe typically comes with a wide range of tip equipment for various interconnection strategies, the selection of which may have an effect on its efficiency. Essentially the most typically used accent is the spring hook, which makes use of a protracted floor wire terminated with an alligator clip. This mixture works effectively for low-frequency indicators — increased frequency measurements will incur noise as a result of the probe’s floor wire varieties an antenna by means of the bottom loop. As a substitute, use a floor spring accent, which employs a spring wrapped across the probe’s floor ring with a really brief projection. This minimizes the high-frequency pickup. Determine 4 exhibits the variation in noise pickup for every technique.

A coaxial connector adaptor accent can also include the passive probe, or you should purchase it from third-party suppliers. It will possibly mount on the board, which reduces noise pickup.

You may as well use 10:1 transmission-line probes that terminate into the oscilloscope’s 50 Ω enter. These probes provide brief leads and broad bandwidth (larger than 1 GHz). Just like the 1 MΩ coupled 10:1 passive probe, the transmission line probe attenuates the enter and will increase inner noise scaled to the connection level. Exterior noise pickup is low, however such a probe has a decrease enter impedance of 500 Ω, which can trigger loading issues.

You may as well join a 50 Ω coaxial cable on to the facility rail to make use of as a probe. There isn't a attenuation with this connection, so there isn't any additive noise. If the coaxial cable is terminated into the 50 Ω enter of the oscilloscope, then the cable enter is a 50 Ω load on the connection level. This will trigger circuit loading issues, particularly if the facility rail provides very small currents. For instance, for a 5 V rail, the oscilloscope enter would draw 100 mA, and this might be a substantial load present for a battery-powered system. Most oscilloscopes additionally restrict the utmost voltage on the 50 Ω enter to five V or much less — be sure to don’t exceed this enter restrict or you would harm the oscilloscope’s enter amplifier.

Utilizing a coaxial cable terminated into the oscilloscope’s 1 MΩ enter may fit for low-bandwidth (beneath 500 MHz) measurements, however this association will lead to high-frequency reflections. When you can acknowledge the signature of those reflections (and ignore them in your circuit evaluation), this can be a very good, low-cost approach that minimizes loading and reduces additive broadband noise.

The ultimate probing technique is a power-rail probe. The facility rail probe, because the title implies, is designed particularly for measuring energy rails. It has a dual-path connection providing excessive enter impedance (50 kΩ) at DC to attenuate circuit loading and 50 Ω enter impedance above 1 MHz Bandwidth is often 2 GHz to eight GHz, relying on the provider. Its coaxial connections reduce pickup, with solely small (nominally 1x) attenuation in order that it doesn’t improve broadband noise by including acquire to an attenuated sign. The facility rail probe has a built-in inner offset within the vary of 24 V to 60 V. The draw back of this probe is that it does value extra, however it's general the best-performing resolution.

Desk 1 compares the efficiency of the 5 probing strategies described above when analyzing a 900 mV rail. The coaxial connection at 50 Ω and the facility rail probe present one of the best sign integrity. Each 10:1 probes present additive broadband noise, as anticipated. The coaxial connection terminated into 1 MΩ exhibits the reflections of the quick edges as a result of impedance mismatch.

Though the 50 Ω terminated coaxial connection produces glorious outcomes at 900 mV, it has a restricted offset vary (requiring adequate offset to be out there within the oscilloscope) and potential loading points attributable to its low impedance. If, nonetheless, the oscilloscope has adequate inner offset and circuit loading isn't a priority, it is a very low-cost and high-performance resolution to probing an influence rail.

A abstract of the probing choices for power-integrity measurements seems in Determine 5. With consideration to particular conditions, you need to use any of those probing strategies for power-integrity measurements. The facility-rail probe presents one of the best efficiency beneath virtually all situations. Sadly, it’s additionally the best value.

Energy-integrity software program
Many oscilloscope producers additionally provide specialised energy integrity software program to simplify the measurements, equivalent to that proven in Determine 6.

These software program packages seize energy rail indicators over 1000's of switching intervals and display statistical knowledge values of the required measurements. In addition they carry out a cycle-by-cycle evaluation of the rail voltages and plot the modifications in rail voltage over time as per-cycle waveforms. This vividly shows the conduct of the facility rails in a extremely intuitive and helpful method.

Suggestions for making power-integrity measurements are listed beneath:

• Do not forget that it’s straightforward to make a measurement, however it's laborious to make a measurement with out artifacts.
• Know the capabilities and limitations of the probe you select to make use of.
• When you selected to make use of a ten:1 probe, keep in mind to make use of the low-inductance suggestions and adaptors.
• For one of the best efficiency, use a power-rail probe if value isn't a problem.