[sdiy] Those AS33xx chips again...

[Roman Sowa]

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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> 
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