[sdiy] MOSFET leakage.

[Ian Fritz]

Hi Rene --

>>Following up on the VCO using a MOSFET switch (BS170) that I recently 
>>described, I decided to have a try at directly measuring the MOSFET 
>>leakage current.  Wasn't there some discussion here recently on how to go 
>>about this?  Here's one way:  :-)
>
>Weren't we discussing cap leakage?

Ah, yes.  Same technique should work, though, unless the leakage is so 
small that the instrumentation current is much larger that what you are 
measuring.


>>To make the measurement I used a low leakage opamp (OPA602, I_bias < 
>>4pA), with a 10nF cap (feedback configuration) followed by a gain-of-five 
>>amplifier.
>
>Did you follow any of the advice that was given in that earlier thread?
>(Dead-bug, Air and Teflon insulation or the like...)

No!  Just plugged it all into my whiteboard. :-)

Plastics are actually very good insulators.  I remember a friend showing me 
that if you pot the contacts of a coax connector with 5-min epoxy then 
measure the resistance with an electrometer that the resistance is over 
10^14 Ohm.  That's roughly the point where you have to go to teflon, etc.

The main worry would be conduction along the surface, but it doesn't seem 
to be an actual problem.


>>The leakage current is then the capacitance times the ramp rate, which 
>>for the BS170 turned out to be 1.8 pA.  (The opamp bias current came out 
>>at 0.4 pA.)  The corresponding channel resistance is 10^13 Ohms.  Yes, 
>>you *can* measure these levels at home!
>
>Thats really not bad! Very good! Even more so if you consider that a 
>MOSFET can switch more current than a JFET at the high end. So you can 
>just raise the cap proportionally and you would still get a very large 
>sweep range. Also this shows that one has to interpret the values in 
>the  datasheets really carefully. Here the current is 500 times lower than 
>what you would expect from the datasheet.

What I've been looking at  is the speced "drain cutoff current" which is .5 
uA.  So 2 pA is a factor or 250,000 smaller.


>I have experimented with BS108s as discharge switches in my VCO3, but 
>didn't like the effects of the channel capacitance.
>I had to put a small resistor in series with it, since the discharge was 
>faster than the integrators OPs slew rate. That tamed the overshoot a bit 
>but it was still somewhat dirty.

Yeah, that's what I'm working on.  Instead of your series R trick, I'm 
using  pulse shaping to try to tame the overshoot.  Keeping the comparitor 
from going into deep saturation seems to help a lot.  Also I want to use a 
device with less capacitance.  The BS170 and 2N7000 units have 60 pF input C.


>>I spent some time yesterday looking for better devices to use for the 
>>switch.  Speed isn't much of an issue, but lower capacitance would be 
>>nice.  I found interesting thru hole devices from Supertex (VN2106 and 
>>VN0104) and many interesting smd parts from Fairchild and others (BSS138 
>>and FDV301N).  Anyone have experience with these or any other 
>>suggestions?  Thanks.
>
>With a larger cap the gate to channel capacitance would have less impact.

Sure.  I'm using a 10 nF cap and a 1 V ramp.  I don't like to go over 0.5 
mA from the expo converter, so that determines my cap size.


>Also don't forget that one could neutralize the capacitance, by putting a 
>second one in parallel, which gets a reset signal of opposite direction.

Interesting idea.  I'll need to think about that some more.  You mean 
generate counter-glitches using a second cap?

   Ian