Yesterday, in FET Miller plateau with Analog Discovery 2, I started posting about the Analog Discovery 2 USB oscilloscope, an oscilloscope with two differential input channels, 2 arbitrary-waveform function generators, a dual regulated power supply, and a logic analyzer.
I want to modify something I said yesterday:
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.
I think that what I was seeing should not really be characterized as an output impedance, but as a current limitation. The AD8067 op-amp that is the output device for the waveform generator is specified to have a 30mA current limitation (for -60dB spurious-free dynamic range) and 105mA short-circuit current, and 3.44V/100Ω is 34.4mA. I can test this assumption by seeing what happens with a triangle-wave signal:
The triangle wave with a 100Ω load is clipped at approximately ±3.48V, corresponding to a current limitation of ±34.8mA.
With 100Ω, I get ±3.48V, for ±34.8mA. With 33Ω, I get ±1.475V, for ±44.7mA. With 18Ω, I get +1.014V, -0.8458V, for +56.3mA, -47mA. In each case, I am getting clear clipping, not scaling of the signal, so the best model is as a 0Ω output impedance, combined with current limitation, rather than as a non-zero output impedance. The current limitation is not quite constant—I can get more current at lower voltages.
Something else you can see in the image above is that the time axis is not limited to starting at 0—I can move the trigger point around either graphically or by typing into boxes that hold the trigger level and time position for the line in the middle of the screen.
What I really wanted to show today was not the waveform generator current limit, but Ids-vs-Vgs plots for an nFET (the same old AOI518 nFET that I was playing with yesterday). I can use the differential inputs to measure the gate-to-source voltage on one channel and voltage across a drain resistor on the second channel. It is easy to adjust the voltage range for a slow triangle wave driving the gate, and to look at an XY plot:
Voltage across 20Ω drain resistor to 5V for AOI518 nFET for a range of gate-to-source voltages. To get the large current, an external 5V wall-wart had to be connected.
It would be nice if there were a way to scale the voltages across the load resistor to plot currents on the XY plot, instead of just voltages. I can, of course, do this scaling with external programs, as I have with other measurement devices. I tried changing the resistors to get different current ranges, exporting the data in tab-delimited formats, and plotting superimposed I-vs-V plots. The results were not as good as I’ve gotten in the past using PteroDAQ:
The Ids-vs-Vgs curves do not superimpose as nicely as curves I’ve measured with PteroDAQ. I don’t yet understand why not.
I’m also not sure why there seems to be a 4µA leakage current. At the top end, I’m not hitting the current limit of the voltage regulator, which is 700mA when powered by an external power supply, as I did here.