[sdiy] Those AS33xx chips again...

Wed Nov 29 19:43:00 CET 2017

Amplitude and phase responses change very quickly for high-Q resonant circuits (small fractional bandwidth), so the actual percentage drift in the capacitor (or inductor value for that matter!) due to thermal effects could still have been quite small.

It's even worse for power applications where many kilowatts of power flowing through inductors and capacitors can cause very significant self heating!

-Richie,

Sent from my Xperia SP on O2

---- Nils Pipenbrinck wrote ----

> The discussion of temperature coefficient of capacitors just reminded me
> of something.
> 
> Last year I was doing some RF development and in a high resonance LC
> resonator even the temperature coefficient of C0G caps showed up as
> detuning of the resonance frequency. I'm only speaking of a drift of 10°
> to 15° degrees here. That was quite a surprise for everyone because the
> textbook sais C0G don't do this. Well - they did nonetheless.
> 
> 
> To solve the problem I did some research on temperature compensating
> capacitors. There are a lot of choice with different coefficients from
> single digit picofarads up to half a microfarad.
> 
> If your design has a systematic but small temperature drift you can
> often just put one of them in parallel to your integration cap.
> 
> (we ended up with a solution of just directing the heat away from the
> cap by doing a ground-plane split in the PCB. That was good enough for us).
> 
> /Nils
> 
> 
> 
> On 11/29/2017 02:49 PM, Roman Sowa wrote:
> > thank you for your comments, here are mine:
> > 
> > 1. I know it's not the only source, that's why I put a little note at
> > the bottom I plan to build another board witch few changes, in specific:
> > get rid of trimmers, and use lower voltage reference for current
> > reference in expo pair as there's not great variety of 25ppm resistors
> > in megaohm range. Also will try different capacitors. Currently there is
> > Panasonic ECH-U which is PPS film.
> > According to their poorly scaled graph in datasheet I can assume tempco
> > is around 250 ppm. For some reason I thought they were far better. Maybe
> > I'll just replace with COG, or build another one with 20 times more
> > expensive mica.
> > Funny thing, I was already holding the COG 1n cap in order to solder it,
> > then I thought "naaah, let's make it better and use super fancy ECHU cap!".
> > 
> > 2. I don't think I'll ever test more than a handful of chips, so we'll
> > leave that uncertainity open for future generations if the ringbump is
> > trademark of AS33 or simply manufacturing variation. Anyway, the bump is
> > small, merely 120mV for half a microsecond. Easily flatten by following
> > opamp. What is more disturbing is the step at lower frequencies. Here
> > triangle corner is missed by 40mV, while CEM - 20mV. That also could be
> > chip-to-chip variation and may turnout the other way.
> > 
> > 3. couldn't agree with you more
> > 
> > Roman
> > 
> > W dniu 2017-11-29 o 10:45, Steve Lenham pisze:
> >> Very interesting and useful - thanks Roman! My immediate thoughts:
> >>
> >> 1. Re the tempco, the 3340 is not the only potential source of
> >> variation. What dielectric are you using for the timing cap? C0G/NPO
> >> is fairly blameless, but others have significant tempco (I remember
> >> the tempco of a polystyrene timing cap being used to cancel an
> >> opposite tempco in a classic National Semiconductor appnote). Even
> >> trimpots have thermal drift.
> >>
> >> 2. Re the different voltages at the bottom of the triangle, this will
> >> be set by the offset voltage of the internal comparator. One would
> >> need to test multiple examples of each type to determine whether there
> >> really is a difference between the CEM and AS parts or whether it is
> >> simply chip-to-chip variation. The original CEM3340 datasheet
> >> specifies that the triangle bottom point can vary between +15mV and
> >> -15mV.
> >>
> >> 3. I don't know how much they matter in real applications, but the
> >> larger glitches on the triangle midpoint and bottom look like the
> >> biggest differences between the two devices. They look like the sort
> >> of thing you get when the opamp/comparator has slightly too much
> >> bandwdth - a case of modern IC technology being a bit too good? For
> >> example, there are certain vintage sawtooth VCO designs where you have
> >> to use a 741 for the main opamp, otherwise you get a colossal spike at
> >> the sawtooth reset; the sluggish 741 simply isn't capable of doing it.
> >>
> >> Cheers,
> >>
> >> Steve L.
> >> Benden Sound Technology
> >>
> >>
> >> On 29/11/2017 02:04, David G Dixon wrote:
> >>> Interesting!  I would have expected better performance.  The
> >>> waveforms look a little dicey and the tempco is definitely not too
> >>> great.
> >>>
> >>>    
> >>> ------------------------------------------------------------------------
> >>>     *From:* Synth-diy [mailto:synth-diy-bounces at synth-diy.org] *On
> >>>     Behalf Of *Roman
> >>>     *Sent:* Tuesday, November 28, 2017 2:34 PM
> >>>     *To:* synth-diy at synth-diy.org
> >>>     *Subject:* [sdiy] Those AS33xx chips again...
> >>>
> >>>     During few last days I've spent wonderful time with AS3340 VCO,
> >>>     giving it a closer look, and also a few measurements. Nothing
> >>>     spectacular, just something that any geeky nerd would find
> >>> interesting.
> >>>
> >>>     It's about vintage chips, so it seems appropriate to have it written
> >>>     in ancient vintage static html, basicaly the 90's style.
> >>>     http://www.sowa.synth.net/synthchip
> >>>
> >>>     Roman
> >>>
> >>>
> >>>
> >>> _______________________________________________
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> >>> Synth-diy at synth-diy.org
> >>> http://synth-diy.org/mailman/listinfo/synth-diy
> >>>
> >>
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