Mail Archives: geda-user/2023/05/11/06:34:51
Nice suggestion, and yes, I have spare opamps, but a capacitance
multiplier needs a proportionally higher voltage at the internal side.
In other words: for a tenfold capacitance and a charge level up to 15
volts, this would require 150 volts on the other side of the cap.
I think I'll suggest using a couple of 10uF/50V X5R MLCCs with a 1M
timing resistor, but only 1V maximum charging voltage. This should still
give acceptable linear capacitance behavior, and the voltage can then be
amplified to the desired maximum level.
Regards,
Richard
Op 11-05-2023 om 01:11 schreef Stephen C. Menasian (menasian AT ptd DOT net)
[via geda-user AT delorie DOT com]:
> I don't know exactly what your circuit looks like; but you might consider a
> capacitance multiplier circuit. A factor of 10, or even 100 might be
> easy. If you have a spare FET input op amp, it could be, essentially,
> free.
>
> On Wed, 10 May 2023 23:07:44 +0200
> "Richard Rasker (rasker AT linetec DOT nl) [via geda-user AT delorie DOT com]"
> <geda-user AT delorie DOT com> wrote:
>
>> Hi Bob, and Frank and Gabriel,
>>
>> Thank you for your insights.
>>
>> This part of the application is very simple: it's a linear audio
>> compression filter with a long release time constant (some 45 seconds),
>> with space constraints making smallish timing capacitors desirable.
>> Foil capacitors are used for reasons of low leakage current and good
>> linearity, but these quickly become quite bulky with capacitance
>> exceeding 1 uF. Most MLCCs with X-type dielectric have serious
>> capacitance derating with increased voltage (15 volts maximum), so
>> these were avoided.
>>
>> However, from your answers I gather that the chosen solution with 22 M
>> resistors is also not optimal, and that potting or coating does not
>> guarantee trouble-free long-term operation.
>>
>> Maybe the best solution here is to use high-capacitance MLCCs after all
>> with a maximum 1 M timing resistor, but only use a tiny charging
>> voltage (e.g. 0.1 volts), and amplify this voltage with an extra opamp.
>>
>> Best regards,
>>
>> Richard
>>
>>
>> Op 10-05-2023 om 21:17 schreef Bob Paddock (graceindustries AT gmail DOT com)
>> [via geda-user AT delorie DOT com]:
>>> On Wed, May 10, 2023 at 7:45 AM Richard Rasker (rasker AT linetec DOT nl)
>>> [via geda-user AT delorie DOT com] <geda-user AT delorie DOT com> wrote:
>>>
>>>> ... 22 Meg ...
>>>> Would it be better to use a larger resistor (e.g. 1206), or forego
>>>> SMD altogether? I can of course also specify a moisture-resistant PCB
>>>> coating, but that is a rather expensive extra production step.
>>> Coatings are not hermetic seals.
>>>
>>> They are good at keeping out particle contamination.
>>> However moisture will still seep through over time.
>>>
>>> As it is now devoid of contamination it will never be noticeable in a
>>> digital circuit, due to its high impedance;
>>> perfectly pure water is an isolator.
>>>
>>> However in high impedances, or RF tuning circuits, the dielectric
>>> changes. Perfectly tuned transmitters are returned in six months for
>>> being off frequency. Been there, done that. :-( Water ran off the
>>> boards in removing the coating.
>>>
>>> This is why some high impedance related parts. such as Electrostatic
>>> Grade Op-Amps, say "expose to air" in their data sheets.
>>>
>>> As Frank pointed out, flux is an issue.
>>>
>>> We really try hard to avoid QFNs and anything that is hard to clean
>>> under in our low current (uAs) standby products,
>>> Any residual flux will build up contamination over time, which could
>>> be measured in months or years.
>>> A 10 uA poorly cleaned board built and measured today, might measure
>>> 100 uA in six months.
>>> Been there, done that too.
>>>
>>> If there are no other through-hole parts consider putting two 10 Megs
>>> in series. To avoid the labor costs of the stuffing charges.
>>>
>>> Knowing more about the overall circuit may lead to better/different
>>> answers.
>> Met vriendelijke groeten,
>>
>> Richard Rasker
>> --
>> Linetec
>> Akkerstafhof 15
>> 7544SP Enschede
>> 053-4350834
>> http://www.linetec.nl/
>>
>>
Met vriendelijke groeten,
Richard Rasker
--
Linetec
Akkerstafhof 15
7544SP Enschede
053-4350834
http://www.linetec.nl/
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