N
[email protected]
- Jan 1, 1970
- 0
Does anyone make SMD polypropylene capacitors? I need a type with low
dielectric absorption for use in an integrating photodiode amplifer,
value 100pF - 10nF. 5% tolerance is OK as the product will be
calibrated before shipping; tempco also isn't much of an issue as we
can measure the ambient temp and compensate.
I've built a prototype with a through-hole polypropylene cap which
works well; now I need to build the production version with all SMD
parts, as there isn't much space to work with.
Cost isn't much of an issue; it's a low-volume, high-value product.
I'm basing my choice of dielectric on this app note:
http://www.national.com/rap/Application/0,1570,28,00.html
There is some long-time-constant effect visible with the circuit I have
built but it's negligible compared with the signal. Just for an
experiment I tried an X7R SMD ceramic, the dielectric absorption made
it totally unusable.
The nearest SMD type I can find is polypropylene sulphide from several
different manufacturers; they all *claim* low dielectric absorption but
don't provide any figures. I can't find any comparisons between PPS
and polypropylene along the lines of the graph at the end of the
National app note.
Teflon would obviously be better still but I can't find *any* of these,
leaded or SMD - have they been banned under RoHS or something?
TIA
dielectric absorption for use in an integrating photodiode amplifer,
value 100pF - 10nF. 5% tolerance is OK as the product will be
calibrated before shipping; tempco also isn't much of an issue as we
can measure the ambient temp and compensate.
I've built a prototype with a through-hole polypropylene cap which
works well; now I need to build the production version with all SMD
parts, as there isn't much space to work with.
Cost isn't much of an issue; it's a low-volume, high-value product.
I'm basing my choice of dielectric on this app note:
http://www.national.com/rap/Application/0,1570,28,00.html
There is some long-time-constant effect visible with the circuit I have
built but it's negligible compared with the signal. Just for an
experiment I tried an X7R SMD ceramic, the dielectric absorption made
it totally unusable.
The nearest SMD type I can find is polypropylene sulphide from several
different manufacturers; they all *claim* low dielectric absorption but
don't provide any figures. I can't find any comparisons between PPS
and polypropylene along the lines of the graph at the end of the
National app note.
Teflon would obviously be better still but I can't find *any* of these,
leaded or SMD - have they been banned under RoHS or something?
TIA