Hi, my name is Mike Hawes and I’m an application engineer with Agilent Technologies. Today
we’re going to demonstrate the seamless ranging capability of the N6705B DC Power
Analyzer on a medical device. We’re going to be using a pulse oximeter and stepping
through a simple five-step procedure to show you how to do this. If you have our evaluation
kit you already have this device with you. After we go through showing you how to do
this device, we’re going to give you some suggestions on how you might start with your
own device. So let’s get started. So Step One is configuring the hardware and connecting
our medical device. The hardware for our demo today is an N6705B DC Power Analyzer and it
has four separate modules. We’re going to only use Module Number One and that is an
N6781A source Measurement Unit or SMU. So we need to verify that we have the right SMU
in our DC Power Analyzer. I’m going to go to the “Menu” button and use the “Arrow”
keys to toggle down to the “About” selection and hit “Enter.” Here it describes the
different modules that are in our DC Power Analyzer. Again, I’m really only focused
on slot one. We do have a N6781A there and that’s what’s necessary for the demonstration.
So the hardware is set correctly, we’ll go hit “Back” and go to this Main Menu.
Secondly we want to connect our pulse oximeter and you can see that we have two twisted pairs
here. One is for the output and we want to make sure that the red wire is on top and
the other one is for the sense, because we do want to use remote sensing and sense down
where we’re connected to our pulse oximeter. We’re basically replacing the batteries
in this device. Now moving on to Step Two we need to configure the SMU. So we already
have “Number One” highlighted, we’re going to hit the “Settings” key and look
at some of the basic configuration. This is a two quadrant power supply, which allows
you to sync or source current with a positive voltage. We’re not worried about that or
most of the settings, but what we do want to do is simulate the three volt battery in
our pulse oximeter. So I’m going to use again this “Arrow” key to toggle down
to the “Voltage” section, select “Three Volts” and “Enter” that. If you wanted
too you could change the current limit, but I’m going to leave that as is. There is
one other place that I would like to make a selection and that is in the “Advance
Menu.” So I toggle over there and I go in and look at the advanced menu. I want to verify
that we are using the four-wire remote sense because we have it wired that way. I also
want to check and make sure that we have this output voltage bandwidth set to low. This
basically will assume a larger capacitive load on your pulse oximeter and give you a
slower response time for the output, which will minimize the opportunity for oscillations.
So those look like they’re set correctly. We’ll go down to “Close” this and “Close”
that and now we’ve got our SMU configured correctly. Moving on to Step Three we need
to set the data logger and configure it. So I’ll push the “Data Logger” button.
Then I come over to our “Properties” button and push that. By default the data logger
sets voltages to be displayed, but I want to look at current of the device. So I’m
going to toggle, “Remove Voltage” and go down and “Set Current.” I have two
other SMUs in there that are also set for voltage and I want to make sure that those
are turned off. So I’ll turn those off and we’ll go back to our data logger properties.
The other key thing that we need to set is the current log range. As I push this you
can see we have four current ranges, 10 microamp, 1 milliamp, 10 milliamps, and 3 amps. If you
can see on the slide seamless autoranging allows you to range between the 1 milliamp,
100 milliamps, and 3 amps ranges seamlessly and that means there’s no internal relay
switching, there will be no glitches. So this allows you to dynamically look at sleep currents,
which are typically in the microamp level and up to in the amp level or 100’s of milliamps
when you may have some kind of an excitation current needed. This is a patented technique
by Agilent and enables you to look at these dynamic currents in your medical devices.
So we’ll select “Auto” and then we need to go down and further decide how long do
we want to log data. I’m just going to choose “1 minute” and then come down to our sample
period and instead of doing it every 100 milliseconds, I’d like a faster sampling, so I’m going
to put in 0.2 milliseconds, okay. So it looks like our resulting file size that will be
stored inside our DC Power Analyzer will be about 1 megabyte. So we “Save” that. Now
we move on to Step Four. Step four is where we run our data logger. So I’ve got the
data logger set here. I’m going to make a few adjustments. You can see that on the
screen we’re at 50 milliamps per division, that is appropriate. But I do want to change
the horizontal to 10 seconds per divisions. So I’m going to go ahead and change that.
I also want the ground level down a bit so that we can see most of the wave form as we’re
going through the data log. So there we go. We’re set appropriate now to run. So what
I’m going to do is start the data logger with the “Run/Stop” key. And that is starting.
I’m going to let it go a second or two and then I’m going to turn on the three volts
for the SMU. Finally I’m going to go ahead and put on the pulse oximeter and turn it
on. This is going to take about a minute to complete, but you’re going to see the current
demand from this pulse oximeter as we go through our data log. I’m going to take the pulse
oximeter off at about 45 seconds and you’ll see that the current wave form is going to
become inconsistent as it’s lost the detection and eventually it will turn off. So we’re
done with our data log and we have a display here showing that we’ve saved the file.
We will close that. Now let’s move on to Step Five, where we analyze our data log record.
All I need to do is hit the “Data Logger” button again and you can see two markers appear.
These markers are used to help you analyze the wave form that you’re looking at. If
you look at the text at the bottom of the display you can see “M1” for Marker One
and “M2” for Marker Two; tell you what is the current at that place in the time scale;
the delta tells you the difference between marker one and marker two; and finally the
min, the average, and the max tells you in between those two markers what are the minimum,
average, and maximum values. So we’re going to use these to analyze the data. What I set
up here first is to look at that spike. That spike is when I turned on the pulse oximeter
and so it was the end-rush current that was needed to get it charged up. If we look at
the maximum or that peak of that spike in between the markers we can see that it’s
275 milliamps, so fairly high level. This would require us to be in the 3-amp range.
Again, this is the value of this SMU is these three dynamic ranges and being able to change.
So now let’s move these markers and see what was happening down at the sleep current.
So we had 3-volts power applied to the pulse oximeter, but it wasn’t turned on. So they’re
trying to minimize the amount of current here. If we look at the average of this current
it’s just 1.4 microamps, so very low drain on the battery. What I want to show is how
well and how accurately we can measure that level. So I’m going to change our vertical
resolution by 4-orders of magnitude and we’ll take a look at what kind of noise we have
down at that 1 microamp measurement that I just mentioned. I’m going to go back to
the previous display and make a few adjustments. I’m going to go up here and put our ground
in the middle of display and then we’ll start cranking the – down, okay, so it looks
like we’re down to about 500 microamps. Let me readjust the – this to the middle.
We’ll go down to 5 microamps. So now we’ve just moved 4-orders of magnitude and you can
see there is some noise here. But if I turn the markers back on, you can see that the
noise level is probably about not quite 10 microamps, so maybe 8 microamps of peak-to-peak
noise. So we’re able to resolve down to maybe 10’s of microamps at this low level,
as well as capture the peak. I’ve moved these markers over again and you can see we’re
still measuring this peak at 275 milliamps. So this is really the value of this SMU with
this seamless ranging and allows medical device designers to look at the dynamic characteristics
of their medical device and understand how this will deplete the battery. So now that
we’ve done that, what I want to do is go back through these five steps and give you
some suggestions of things that you might want to do as you look at your own medical
device. So first of all you want to make sure that you’ve got the right hardware. We talked
about that. You need an N6781 SMU. Then as you’re connecting your device I suggest
you use both source and sense; do the remote sensing out to the device so you get the most
accurate measurement. Second, Step Two, is configuring the SMU. The main thing to remember
when you’re configuring that is upping the voltage that you expect and making sure that
you have the output voltage bandwidth set to low. Step Three, configuring the Data Logger.
So when you’re configuring the data logger you’re going to need to set that autorange
for your current. You remember this is what sets the seamless rangings. So there’s setting
that autoranging is key to getting the measurement to work correctly. Then of course you want
to set the appropriate time frame and sample rate that you’re looking for. Step Four,
you want to go ahead and run the data logger. Again, you’re going to need to look to see
what is the appropriate vertical and horizontal ranging so that you get a nice display of
what you’re expecting to see. Then finally you turn on the markers in Step Five as you
analyze your wave form. And you can move those markers around to look at the peaks, the valleys,
and get a good idea of what’s going on as you’re using your medical device. So I hope
you found this demonstration valuable and thanks for watching.