On Apr 6, 2007, at 3:00 PM, Steve McKown wrote:
[OT] You mean like this one? ;^)
We coupled high-bandwidth precision power supply with a precision
resistor in
series with its output *before* the voltage feedback path. We
power a sensor
node with this guy, then use a high-speed DAQ to capture the
voltage drop
across the precision resistor and the PS output voltage at high speed
(>10MHz). We can then graph the voltage, current and power
consumption over
time. By having the sensor node twiddle a couple of digital
outputs sampled
concurrently, we also get a time reference for notable events.
Sure, like that. :)
We didn't build this to create power consumption models but to
measure power
state transistions. As an example, our board uses a lithium
battery, so the
design includes a voltage regulator. We initially selected a very
efficient,
low-Q part. But testing has shown the original VR was woefully
inadequate in
terms of bandwidth and couldn't maintain desired regulation during
many types
of power events. The moral for me has been: even though we're
talking about
small currents in sensor nodes, the relative magnitude of spikes
and slopes
in power draw can be significant.
That's definitely true; it also depends which side of the problem
you're attacking. If you're trying to design an ultra low-power
platform or develop an application on a specific platform, then the
exact slopes and magnitudes matter. If, in contrast, you're
developing a radio scheduling algorithm, then the exact values are
not as important, because you want to examine how it behaves across
many possible platforms. E.g., rather than say "for platform X with
values a, b, c, we find a q=0.213 is best," you want to say "the
optimal q values depends on a, b, and c, and can be derived as sqrt
(b^2 - 4ac)".
Phil
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