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                                                   Evaluating Oscilloscopes for
                                                   Low-Power Measurements


                                                   Application Note




Increasing market demand for products that are portable, mobile, green,     gies for addressing the challenges of low-power measurements can be
and that can stay powered for long periods of time is driving a change      a huge boost in an engineer's ability to gain insight, understand, debug,
in new-product innovation. It's low power, not performance, that is king.   and characterize those designs.
The ever-increasing need for power reduction drives engineering teams
to devise innovative methods and architectures. The low-power mega          This application note articulates key low-power measurement attri-
trend has resulted in a changing landscape for devices, sub-systems,        butes for oscilloscopes including software features and probes. It will
and system-level products. A key requirement to fuel low-power innova-      use the Agilent Infiniium S-Series oscilloscopes and N2820A current
tion is the ability to measure and characterize device and sub-system       probe in examples. The principles in this application note can be
power consumption. Oscilloscopes designed with innovative technolo-         applied to all oscilloscopes used for low-power measurements.
Limitation on Getting Good Low Power Measurements

There are many factors that go into making a good low power mea-
surement. Some are inherent in the scope and some in the probe. The
basis for low-power consumption measurements are current probes.
Since power = V*I and voltage for many low-power applications is
steady, current measurements are a good proxy for power. Historically,
current probes have been designed to clamp around a power line,
and either measure the Hall Effect, use transformer technology, or a
hybrid between the two methods to continuously report an associated
current value for display on an oscilloscope's display.

Low-power measurements challenge testing in several ways. The
two biggest challenges are dynamic range (impacted by noise) and
sensitivity as shown in Figure 1.
                                                                                Figure 1. Vertical sensitivity is a key challenge for low-power measurements.
Traditional current probes have limited dynamic range, and the total            Scaling to see peak power and noise buries important signal details of low
dynamic range is a function of both the probe itself as well as the             power states.
scope to which it is connected. We are going to take a closer look at
each of these components and offer some best practices to help give
you the most accurate current measurement on a low power device
possible.




Noise

Let's look first at some low-power challenges related to the oscillo-
scope. Noise inherent to the oscilloscope diminishes the ability to see
small signal detail. At higher vertical scaling, oscilloscopes have more
absolute noise. At lower vertical full screen values, oscilloscopes have
lower absolute noise. Oscilloscope users will never be able to see
detail lower than the noise of the oscilloscope. Noise values are typi-
cally characterized and published by oscilloscope vendors. However,
plugging a current probe onto a oscilloscope will result in an increased
noise level. For oscilloscopes with high signal integrity, the overall
noise is typically more a function of the probe than the oscilloscope
itself. Users will be able to see signals, as long as these signal values
exceed the noise levels of the oscilloscope and connected probes. If
noise values exceed the smallest resolution, noise will be the limiting
factor in low-power measurements. Noise reduction techniques will
be covered later in this application note.



                                                                                Figure 2. If you've tried to zoom in on a power rail voltage to see additional
                                                                                detail, you've probably experienced noise issues. Noise buries signal detail.
                                                                                The key to success is reducing overall system noise to bring out signal
                                                                                characteristic from the noise.




                                                                            2
Resolution

Another limiting factor in seeing small current signals is the resolution
of your oscilloscope. Resolution is the smallest current value a scope
can measure for a specific full-scale vertical setting. Current measure-
ment resolution is calculated by dividing the full screen current value
by the number of quantization levels a scope offers. Oscilloscopes
with 8-bit ADCs offer 28, or 256 quantization levels. Oscilloscopes with
10-bit ADCs offer 210, or 1024 quantization levels.

The signal shown in Figure 3 shows a mobile device that moves from
a power conservation mode to a higher power state, then back to
sleep mode. To capture the signal, the oscilloscope must be scaled to
capture the highest power level. For this example, full scale is set
to a value of 200 mA/div, or full scale of 1.6 A. On an 8-bit scope,
resolution in this example equals 1.6A divided by 28 (256 quantization
levels), or 6.25 mA. The user will not be able to see detail smaller than
this value where power saving mode visibility is needed. On a 10-bit
scope like the Infiniium S-Series, in this example resolution will be
1.6 A divided by 210 (1024 quantization levels) or 1.56 mA. A user will              Figure 3. A mobile device requires vertical oscilloscope scaling of 200 mA/
get four times the resolution of an 8-bit scope, but still will not be able          div or 1.6 A full screen to capture the high power modes as well as the low-
to see signals smaller than 1.56 mA for this example.                                power modes. Oscilloscope resolution in this example equals 1.6 A divided by
                                                                                     the number of quantization levels the scope's ADC provides.


Vertical Scaling Impact on Resolution
In addition to the quantization levels and inherent resolution of your               moves into software magnification. Turning the scope's vertical scale to
oscilloscope, vertical scaling on the oscilloscope itself has an impact              a smaller number simply just magnifies the displayed signal and doesn't
on resolution. For example scaling the waveform to take the whole                    result in any additional resolution as the user would naturally expect.
display of the scope enables the scope's analog-to-digital (ADC)                     For a 1:1 current probe connected to the scope's 50  path, most tradi-
converter. If a signal is scaled to take up only 

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