The key properties relevant to long-TC integrator capacitors are
insulation resistance (leakage) and dielectric absorption (DA). (One
response on this thread opined that DA is not important in this circuit,
but that is not correct. DA is most definitely a factor in long-TC
integrators, including this one.) If maintaining an absolute time
constant is important, then tempco enters the picture (but the absolute
time constant of the A9 integrator is not a critical parameter).
Consider polypropylene (PP) capacitors as the baseline for long-TC
integrator applications.
Polycarbonate (PC) capacitors are very scarce these days, because the
sole source of PC capacitor film discontinued it over 15 years ago.
Even if they were available, the advantages of PC over PP in this
application are small.
PPS has a lower tempco than PP, and may have lower leakage (depending on
construction and processing). They are generally not so easy to find,
particularly in larger values, but Robert already directed you to the
Kemet SMR series, which are available in suitable values and are
conveniently packaged.
Polystyrene (PS) is the best dielectric for long-TC integrators except
for Teflon (PTFE), but PS caps are extremely hard to find in large
values, are generally huge compared to PP and PPS, are easily damaged
during soldering, and are not conveniently packaged.
PTFE is the best dielectric for long-TC integrators, but PTFE caps are
expensive and huge, and they are unnecessary for this application.
Your best choices are PP and PPS. The DA of PP and PPS are similar. The
leakage depends as much on the construction and processing of particular
caps as on the bulk resistivities of PP and PPS, so look at the
datasheets carefully to see what is guaranteed by the manufacturers.
Best regards,
Charles
On 2/24/2018 8:40 PM, cdel...@juno.com wrote:
Well I hunted up the change pages for the A9.
The cap is polycarbonate, the original A9 was polyester.
If I can't find the polycarbonate I'll look into the PPS.
Cheers,
Corby
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