SV: Re: [sdiy] MOSFET leakage.

[karl dalen]

I fill in with the 2N7002.Available in sot and TO92

KD
--- Grant Richter <grichter at asapnet.net> skrev:

> I have used a 2N7000 with OK results.
> 
> The VCO for the Terrain module replaces the analog comparator with a  
> 20 MSPS video A/D.
> 
> The sawtooth peak is detected as the "all ones" condition with an  
> eight input AND gate.
> This sets a 4013 which drives the 2N7000 across the timing cap.
> The "all zeros" reset signal is generated by an eight input NOR gate,  
> which resets the 4013 and starts the next cycle.
> 
> The rest of the circuit is Terry's sawtooth core.
> 
> The advantage is that you have simultaneous analog and digital  
> sawtooth outputs.
> 
> On Dec 17, 2005, at 2:34 PM, Ian Fritz wrote:
> 
> > Hello all --
> >
> > 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:  :-)
> >
> > 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.
> >
> > Here's an old lab-rat trick for measuring a signal with a  
> > background present: First connect the MOSFET from the opamp to the  
> > (+) power supply rail and measure the ramp rate of the voltage  
> > output.  Then connect the MOSFET from the opamp to the (-) rail and  
> > do the same.  The MOSFET leakage current changes sign with the  
> > reversed connection, but leakage from other sources such as the  
> > opamp bias current do not.  So the device leakage is obtained by  
> > subtracting the two results and dividing by two.  The other leakage  
> > contributions are obtained by averaging the two results.
> >
> > 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!
> >
> > The measured leakage level would correspond to a cycle time of  
> > nearly two hours for my VCO, assuming that the integrator's offsets  
> > were trimmed out.  I don't think I'll be bothering with that.  :-)
> >
> > 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.
> >
> >
> >   Ian
> 
>