[sdiy] MOSFET leakage.

[Grant Richter]

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