[sdiy] integrator / capacitor leakage

[Ian Fritz]

At 08:29 AM 12/3/05, René Schmitz wrote:
>Hi Jürgen,
>
>do an experiment!

A brave suggestion!  But if you are careful, why not?


>Use the intended OP as an electrometer amp (voltage follower) and measure 
>the time constant of the discharge of a cap for various voltages. Charge 
>to 2V and take the time for the drop to 1V. Then try 10V and measure the 
>time for 5V... You'll have to do that several times when timing with a 
>stopwatch, and average the results.

Yes, with care you can measure leakage that way.  But input bias current of 
the amp can be tricky to sort out, especially if it is highly temperature 
sensitive. Rather than use the intended opamp, I would suggest using the 
*best* opamp you can get your hands on.  Also the test setup must be 
carefully insulated from leakage to the outside world.  I recommend 
building the circuit in air using teflon standoffs.


>I remember an experiment I once did using a piece of RG58 as cap, and a 
>similar arrangement, but I was interested in the input resistance of the 
>amp. Could be the larger contributing factor here. If you try several caps 
>you can divide the contributions from cap and amp.

Why not use that electrometer opamp that was mentioned here yesterday?


Still, after all this, you have only characterized leakage, not all the 
time-dependent problems caps have. :-)

   Ian