Re: Multilayer ceramic chip capacitor for op amp active fiter
- From: "Tom Bruhns" <k7itm@xxxxxxx>
- Date: 17 Dec 2006 20:52:18 -0800
On Dec 13, 10:39 am, Steve <s...@xxxxxxxxxx> wrote:
On 12 Dec 2006 12:08:35 -0800, "Tom Bruhns" <k...@xxxxxxx> wrote:
Steve wrote:
I am designing low and high pass op amp (+/- 15V) based active filters
near 1 Hz that require capacitor values between 1 and 10 uF. Are
there any reasons to not use surface mount multilayer ceramic chip
capacitors in such a design ? Is capacitance change with voltage an
issue ? The filters will be used at room temperature with little
temperature variation.
Multilayer ceramic chip capacitors are available in values to 10 uF or
higher with +/- 10% tolerances and lead spacings of a few millimeters
for $1. Is it better to use metalized polypropylene or metalized
polyester even though they are more expensive and have much larger
lead spacing (up to 1 inch) ? Examples inlcude Panasonic ECQ-E(B) or
ECQ-E(F) series and Epcos MKT series, available at 5% or even better
tolerance.
Thanks,
Steve
Can you use higher resistor values, so you can use lower capacitor
values? If so, C0G ceramics may become possible. You can get them in
excess of 0.1uF these days. They tend to be pricey.
Beware that capacitors with dielectric absorption will add to the
non-ideal behaviour at low frequencies. AFAIK, C0G are low dielectric
absorption, as are polypropylene. I came across another dielectric
that I'm not remembering right now that's also good--not one of the
"usual suspects." Mylar/polyester are not particularly good. The
non-ideal behaviour in this case is that the capacitors will look
smaller in value to high frequencies than to low; it's still generally
a linear effect, at least, unlike the voltage-dependence of the high-K
ceramics.Tom,
I am using Texas Instrument's Filter Pro software and designing a 4
pole high pass MFB filter. With R1 seed at 100 k, it generates
capacitance values of 15 uF. and feedback R1 values of 174 k and 412
k. WIth R1 seed as 1 M, the generated capacitance values drop to 1.5
uF and the R1 increases to 1.74 M and 4.12 M. I am using a quad OPA
4277 op amp. Any sugestions on op am parameters to watch out for when
dealing with such high feedback resistor values ?
How high a frequency is needed before capacitors with dielectric
absorption begin to show lower capacitance ? i.e. If I am working
with 1 Hz filter frequencies, are the offending frequencies always so
high that they will always be beyond the filter's cutoff frequency
even if the capacitance value changes and effects the actual cutoff
frequency ?
(Sorry for the delayed reply...I missed seeing your followup before.)
When you go to higher resistances, voltage noise gets worse. If that's
an issue, you'll need to put some amplification ahead of the filter
stage, to get the signal far enough above the noise. Op amps with
extremely low current noise help: those will generally be JFET-input
op amps. There's also the advantage that input bias current is low, so
that higher resistance values won't cause too much additional DC offset
voltage. Look out, too, for 1/f noise corner frequencies that are too
high. If offsets and very low frequency noise are issues for you, you
could consider chopper-stabilized CMOS amplifiers, but they get
expensive I suppose. In laying out the circuit, work to keep circuit
areas small so that noise has less chance to be picked up. Some sort
of electrostatic shielding may be in order. You mentioned it's a HPF,
so of course, you need to maintain bandwidth out to whatever your
application dictates.
Dielectric absorption can be modelled as an ideal capacitor in parallel
with several copies of a series R-C, where each C is small compared
with the ideal cap, and the R-C time constants of each vary over a
range generally from milliseconds to many seconds. So the effect is
seen as a gradually changing capacitance versus frequency, if you look
at it in the frequency domain.
I'm not sure what topology your filter software is using, but you may
find some advantage to using a state variable topology. The
integrators can be high impedance, and their low-pass characteristic
will keep noise out of the feedback path. The feedback path itself can
be kept low impedance, to keep wideband noise down and to make the
circuit less sensitive to electrostatic coupling from outside. The
higher parts count may be worth it! It will take four integrators to
get your four poles: four capacitors, four op amps. Add to that the
amps to sum the feedback with the stage's input signal, for two stages.
Cheers,
Tom
.
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