# Gas station without pumps

## 2015 July 8

### Testing nFET body diode with function generator

Filed under: Circuits course — gasstationwithoutpumps @ 23:54
Tags: , , , ,

Continuing yesterday’s post on the i-vs-v characteristics of the PSMN022-30PL nFETs, I decided to look at the body diode characteristics. I changed the circuit only slightly from

Circuit for measuring I-vs-V characteristics of nFET.

to

The circuit is essentially the same, but some of the wires are swapped to make measurements all be in 0–3.3V range.

The plots I got were the exponential dependence of current on voltage that I expected:

To get the straight lines to fit so nicely, I had to do two things: throw away any points where E20  was less than 5mV or E21 was greater than 3.29V (the voltages may have been going outside the ADC range of 0V–3.3V, and the last few counts at each and of the range are always a bit suspect), and subtract a tiny offset from the E20 voltage (1.2mV).

I did run into one surprising phenomenon: the max current was not as high as I expected.  With a 47Ω output impedance from the FG085 and a 10V maximum voltage, I expected the 27Ω resistance to give me a current of about $(10V - 0.7V)/ (27\Omega + 47\Omega) = 126mA$, but I only got 77mA.  I investigated further and looked at the voltage across a 27Ω resistor with no FET:

Both currents are show as positive, for convenience in plotting (as the KL25Z only records positive voltages—I swapped leads on the function generator to examine negative swing.

The triangle waves, which look ok with no load or with large impedance loads, get very distorted at high currents.  For triangle waves going positive, the current starts getting limited at about 72mA, and strong clipping kicks in at about 80mA.  For triangle waves going negative, the current starts getting limited at 50mA, and strong clipping kicks in around 65mA.  The clipping actually gets stronger as the voltage of the source attempts to rise further, so the waveform isn’t flat on top but dips.

Unfortunately, the current limits do not seem to be a simple thing.  Using a 3Ω load instead of a 27Ω load gets different values for the limits:

With a 3Ω load, the ultimate current limits are substantially higher.

For positive wave forms, the limitation starts at around 74mA with strong clipping at 93mA, and for negative transitions limitations start around 52mA with strong clipping at 80mA. The values where the current limiting starts are about the same as before, but the strong clipping limits have gone way up.   It is probably reasonable to treat the FG085 as really having  either ±50mA current limits or –50mA,+72mA limits.  The specs for the LM6172 op-amp that drives the output is typical ±60mA, min ±50mA, so treating the limits as ±50mA seems reasonable.

Note that the bad glitches from the cheap fake DAC used in the FG085 are very evident in these plots—the glitches are at each of the high-order bit transitions, with the really big glitch in the middle of the output range (switching from 0x7F to 0x80).  The glitch there is about 3.2LSB (that is, the voltage for 0x80 is slightly less than for 0x7D), which is really terrible for an 8-bit DAC. The 0x40 transition has about a 1.5LSB glitch, and the 0x20 transition about a 0.7LSB glitch. Using a cheap DAC0800 multiplying DAC would have added about 50¢ to their parts and assembly cost (a big deal for a \$50 retail product), but allowed them to increase the sampling rate and avoided the non-linearities.  Switching to a better microprocessor (like the KL25 from Freescale could have gotten them more bits, better linearity, and a fast sampling rate at no higher parts cost than the ancient ATMega processor they chose).

I noticed one other problem with the FG085 function generator when doing these measurements—the buttons have very weak springs and often don’t return after being pressed, because the caps rub against the holes in the panel (they made the holes a bit too small or used buttons with crummy springs).  If the button stays down, subsequent button presses look to the firmware like the earlier button pressed again.  I often had to flick the button back up by pressing it up from below the panel—not an ideal user interface!

## 1 Comment »

1. […] its output impedance. I noted that the output impedance was only 46.7Ω, instead of 50Ω, and in Testing nFET body diode with function generator that the output current was limited to about ±5omA, which is ok for 50Ω at 5V peak-to-peak […]

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