[sdiy] integrator / capacitor leakage

[René Schmitz]

Hi Jürgen,

do an experiment!

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.

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.

Cheers,
  René

JH. wrote:
> Polystyrene is no problem.
> So, same configuration, same type of capacitor, what would be better?
> C/10, or level/10 + amplification?
> 
> JH.
> 
> 
> 
>>imho the weak link is the capacitor...
>>
>>if the 'smaller' cap allows you to get into a high-quality type
>>like a polystyrene, I'd go that direction.  Its hard to get large
>>value, low leakage caps.
>>
>>Good amplifiers are easier to find.
>>
>>H^) harry
>>
>>
>>
>>jhaible at debitel.net wrote:
>>
>>
>>>When I'm building an integrator for a triangle or sine wave oscillator
> 
> with an
> 
>>>amplitude of 20Vpp, I have two options:
>>>
>>>a) run the integrator at 20Vpp, or
>>>b) run the integrator at a lower voltage, and amplify the signal
>>>   with an extra amp.
>>>
>>>Now, for a certain current into the integrator, the integration
> 
> capacitor
> 
>>>in case (a) will be much smaller. (Larger voltage to pass in the same
> 
> time
> 
>>>at th esame current.)
>>>
>>>What is better, in terms of precision / leakage?
>>>
>>>At first glance, I'd say leakage is mostly leakage _currents_, so
>>>it will be the same in both cases.
>>>At second glance, I'd say if the leakage currents are not entirely
>>>independent with voltage, case (a) will be worse.
>>>
>>>Is this right? Are there other things to consider?
>>>
>>>Background: VCO which not only will run from 0.03 Hz to 20kHz,
>>>but which will also produce very precise waveforms over that
>>>whole range.
>>>
>>>Any ideas / hints welcome.
>>>
>>>JH.
>>>
>>>-------------------------------------------------
>>>debitel.net Webmail
>>
>>
> 
> 

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159