Gas station without pumps

2016 December 26

Power waveform generator

Filed under: Data acquisition — gasstationwithoutpumps @ 21:53
Tags: ,

I was playing around a little more today with my Analog Discovery 2 USB oscilloscope, and found that one could set up the power supplies to be high-power waveform generators by setting configuration 6 in the device manager.  The power supplies are not great function generators, of course, as they are switching power supplies, but I can see some use cases for this functionality.

The power supplies are nominally limited to 700mA, so I wondered whether they had the same sort of sharp clipping that the function generator has (see FET I-vs-V with Analog Discovery 2).  I tested the power waveform generator with several different loads:

With no load, the power supply waveform generator has trouble with small voltages (no resistance to drain the capacitance) and has fairly high noise, but is nicely linear. At high load (1.8Ω) the voltage is substantially less than specified.

With no load, the power supply waveform generator has trouble with small voltages (no resistance to drain the capacitance) and has fairly high noise, but is nicely linear. At high load (1.8Ω) the voltage is substantially less than specified.

We can get a clearer idea of the behavior by looking at the difference between the power waveform generator and the normal waveform generator:

At low voltages, the regulator's output capacitor is not discharged fast enough without a load resistor. The regulator also does a better job of keeping the noise down with higher load.

At low voltages, the regulator’s output capacitor is not discharged fast enough without a load resistor. The regulator also does a better job of keeping the noise down with higher load.

With no load, the noise on the power waveforms is about ±4mV, but at high load, it drops to about ±1mV.

The noise is periodic with a frequency of about 1.024kHz, which is much too low a frequency for a switching regulator—it is actually from the sampling frequency for the waveform generator generating a 1Hz triangle wave (210 samples in wavetable). The usual waveform generator has four times as high a sampling frequency, so the error is mostly just quantization error from the power waveform generator, though a single step from the power waveform generator takes about 250µs to settle, so the 1Hz maximum frequency for the power waveform channels seems reasonable. With a 10Ω load, the settling time is reduced to about 50µs and the noise on each step is about 500µV RMS (not counting the quantization error).

With the 1.8Ω load, I let the current get as high as 927mA (well above the 700mA specification), and there is no sign of clipping. We can more reasonably model the power waveform generator as having an internal resistance. For the 1.8Ω and 10Ω loads, I plotted the equivalent internal resistance as a function of voltage (for the larger voltages):

The internal resistance is approximately 100mΩ—180mΩ, with the larger values at higher voltages. There is a clear anomaly at half the full-scale voltage (2.5V for 10Ω and 0.9V for 1.8Ω).

The internal resistance is approximately 100mΩ—180mΩ, with the larger values at higher voltages. There is a clear anomaly at half the full-scale voltage (2.5V for 10Ω and 0.9V for 1.8Ω).

I believe that the power waveforms will be useful for characterizing transistors, especially for sweeping a range of Vds voltages that require a substantial current.

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1 Comment »

  1. […] I played with sweeping the power supply (Power waveform generator).  In this post, I used that capability to plot Id vs Vds curves for different gate voltages (Vgs) […]

    Pingback by FET I-vs-V with Analog Discovery 2 again | Gas station without pumps — 2016 December 27 @ 12:59 | Reply


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