[sdiy] integrator / capacitor leakage

[Magnus Danielson]

From: jhaible at debitel.net
Subject: [sdiy] integrator / capacitor leakage
Date: 2 Dec 2005 13:56:22 +0100,Fri,  2 Dec 2005 13:56:22 +0100
Message-ID: <1133528182.439044762ead5 at www.debitel.net>

Hi Jürgen,

There are many sides to this...

> 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?

Let me point out that these may not be the same. You are trying to reach
precision, and leakage is one of several things to battle.

> 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?

If we talk leakage, there are two sources, internal and external.

Internal is naturally the leakage trough the dielectrum. Assuming a linear
low-epsilon material (i.e. good plastic), this leakage is fairly linear with
the charge and also recall that we also tend to use these caps in a fraction of
their supported voltage range, makes the linear assumption fairly safe.
Hint: Stay of (most) ceramics.

External sources of leakage includes creep-voltages from power-supplies, signal
etc. These are due to the leaking in the PCB and eventually also on the dirty
surface. Some of this can be solved by providing a guard-ring on both sides of
the mounting-hole, keeping the same voltage as the pin to be guarded. For an
integrator this would typically be the ground, which is what the positive input
of the op-amp is hooked to. It also helps to make the surfaces of the PCB and
cap clean, please apply isopropanol here, and mounting it inside a tight box
which keeps dirt from comming (you don't smoke near your system, do you?) in
makes it safer yeat.

If you think about it, a higher voltage of the output makes the drive-current
for the same frequency lager than for a lower voltage, and then will the same
external creep-current from other sources less influential. However, you will
suffer if that means that you run closer to saturation of the expo-transistors,
and then that will be your main problem, but in a different range of the
frequency range, since you are already running a higher current, so leakage is
more of a low-freq-problem due to current-ratio is smaller.

You also have the leakage from the op-amp.

If you really think you are hurting from leakage, you can compensate for it by
bringing a small contra-current to the summing point which compensate for both
the voltage (use the output of the integrator) and for the DC current.
It is fairly simple to do this, but it should be done only after you have
chosen capacitor, done the guard-ring and concluded that this does not suffice.

I also assume that you have taken all forms of care to handle things like
reset-time, non-linearities in expo-transistors (bulk resistance compensation,
near saturation compensation, etc. etc.). If you have a "perfect" temperature
compensation expo you still need to account the temperature dependence of other
parts, especially when they do not drift relative each other so that the
relation between values change.

I think one does not have to look at the more esoteric non-linearities of
capacitors if you just avoid the worst dielectrics in this sense, which you can
do fairly easilly and at no major cost. Actually, active compensation for
leakage is probably not needed, but it depends on just how much precission you
need.

> 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.

That is 19,23 octaves... which is possible to acheive, but care needs to be
taken. I should clean up my ASM-1 board and re-measure it again. Should try to
figure out a good way to measure temperature so I get a good temperature
dependence measure. I've got some LM-135s lying around somewhere.

> Any ideas / hints welcome.

I hope I have contributed some with my initial comments. I have only briefly
seen the rest of the thread, so I wanted to go back to the top and take it from
there.

Cheers,
Magnus