Gas station without pumps

2016 December 11

FET Miller plateau with Analog Discovery 2

Filed under: Data acquisition — gasstationwithoutpumps @ 21:49
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I recently bought myself a birthday present: an Analog Discovery 2 USB oscilloscope. The device normally costs $279, but  I qualify for the academic discount, which brought the price down to $179—a very good deal.  This oscilloscope is better in every way than the Bitscope BS10 that I bought about 4 years ago: functions, resolution, bandwidth, software, … . The Analog Discovery 2 is also cheaper (at least with the academic discount).

I’ve been playing with it a little bit, and I decided to try to reproduce a few of the plots that I have done before.  This post is about creating the plot of the Miller plateau for an nFET (see, for example, More on nFET Miller plateau).  With the Bitscope, I had to filter the 5V power (which was just passed through the device, record many traces, process them with a program I wrote myself to remove the jitter in the triggering, average them, and plot with gnuplot).

With the Analog Discovery 2, I set the built-in power supply to 4V, set the function generator to a 1kHz square wave from 0V to 4V, and put the scope leads to measure Vgs and Vds for the following circuit:

The 100Ω gate resistor is to limit the current from the 1kHz square wave generator, so that the Miller plateau is stretched out in time.

This setup produced a very nice plot without any averaging—the Waveforms 2015 software that comes with the Analog Discovery 2 does the interpolation between samples to dejitter the waveform:

With a 100Ω gate resistance, the plateau is about 146ns long (easily measured with a pair of cursors not shown in this image).

With a 100Ω gate resistance, the plateau is about 146ns long (easily measured with a pair of cursors not shown in this image).

The average voltage is about 2.87V, so the current is (4V-2.87V)/100Ω=11.3mA, and the gate drain charge is 1.65nC, about half the 3.2nC on the data sheet (which was measured with different circuit parameters and is supposed to be a worst-case).  Note that this is a single trace, but multiple traces show almost no jitter, even though we are sampling at the full  100 Msample/second rate.  Averaging the traces would not make much difference in the signal.

If I replace the 100Ω gate resistor with a wire, I get a shorter Miller plateau:

With no gate resistor, the Miller plateau is only about 40.2ns long and averages 2.987V.

With no gate resistor, the Miller plateau is only about 40.2ns long and averages 2.987V.

If we assume 1.65nC in 40.2ns, we get a 41mA current, and (4V-2.987V)/41mA=24.7Ω for the output impedance of the function generator.  I may not have placed the cursors in exactly the same places on both curves, so this is a terrible way to estimate the output impedance of the function generator.

If I look at the square wave with nothing but the scope attached, then I see a voltage of about 4.005V.  With a 100Ω load, I see 3.44V, which gives an output impedance of 16.4Ω.

Of course, the square waves are not completely square, which may affect the Miller plateau measurement at high speed, but the edges are pretty sharp:

The falling edge takes about 50ns.

The falling edge takes about 50ns.

The rising edge takes about 50ns.

The rising edge takes about 50ns.

The edges are limited by the wiring, and I may be able to get better edges by using the BNC adapter board and 50Ω coax cable, rather than the wires provided, but I don’t happen to have any BNC coax cable handy.

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5 Comments »

  1. Are you telling me that this would be a viable replacement for the elderly Tektronix scopes we use in our first-year physics lab? What I saw when following that link was extremely interesting, particularly at that price.

    Comment by CCPhysicist — 2016 December 11 @ 23:10 | Reply

    • If you have laptops (Windows, Mac, or Linux) that are sufficiently recent to run the software, and you don’t need to go above 10MHz, then the Analog Discovery 2 should be a good replacement for an old oscilloscope. I don’t know how robust the devices are in school use, but probably not too bad.

      The reference manuals give the circuits and the API, but there doesn’t seem to be a User’s manual for Waveforms 2015, so there are a lot of features that I’ve not figured out how to use from the GUI.

      Comment by gasstationwithoutpumps — 2016 December 12 @ 07:38 | Reply

      • Yes, the only doubts I might have is whether it would stand up to school use AND whether they would be spoiled for life if this was the first oscilloscope they ever used. (And it might be hard for a total novice to see what it is doing.) But what is mind boggling to me is that it would replace almost everything we normally use in the lab (power supplies, DC and AC multimeter, and oscilloscope) in a single economical package. Only thing I can see is that we would have to change one lab because it can’t put out 1.00 A.

        After watching their demo video, I might have to get one just for lecture demonstrations. It can sweep frequency at constant voltage and display the output amplitude and phase? That looks magical! And I like that it would just plug into our bunker PC for use with our projection system. Thanks for writing about it.

        Comment by CCPhysicist — 2016 December 13 @ 21:07 | Reply

  2. […] in FET Miller plateau with Analog Discovery 2, I started posting about the Analog Discovery 2 USB oscilloscope, an oscilloscope with two […]

    Pingback by FET I-vs-V with Analog Discovery 2 | Gas station without pumps — 2016 December 12 @ 17:41 | Reply


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