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Test Equipment Anonymous (TEA) group therapy thread

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ch_scr:

--- Quote from: Vince on March 22, 2023, 08:00:20 pm ---
--- Quote from: ch_scr on March 22, 2023, 05:45:32 pm ---Hi Vince, I've looked and IMHO you "might as well buy a few of these" . Yeah, thick film, 2% but at the price they are well worth it to "have a play", at least!
Don't forget what comes "before and after" - the logic buffer to drive the bits (it's output transistors in particular) influence dac performance - make sure to go CMOS, TTL won't cut it. Also, you probably want to buffer the DAC output - take care that you would need either an opamp with "rail-to-rail in and output" or a negative supply to make it "go to zero". Good luck!

--- End quote ---

Yes I will probably do that.
I checked all the ladders on mouser.... they don't have that many all in all, and you quickly realize that they really only have one brand and one series... Bourns series 4000, same datasheet for all, all 8 bits or nothing. The choice is more about value and package type.

So if even Mouser has only 8 bits ones, no need to waste my time trying to find 9 bit jobbies, they either don't exist or can't be bought by a hobbyist.  So I will have to make one myself using an 8 bit one and adding a couple 0.1% resistors to extend it and make up the ninth bit. I checked the price of such resistors on Mouser (so I can order everything from the same place...), and they are freaking expensive in leaded / through-hole type, but way, way cheaper in SMD packages. In 0805 (easy to solder) 1/8W (same power rating as the ladders), they are 35 cents a piece from Bourns, and only 17 cents, less than half price, in 10 quantity.

https://www.mouser.fr/ProductDetail/Bourns/CRT0805-BY-1002ELF?qs=h2IHEVivlqCjNQtH%252BVLkLw%3D%3D

As for buffers, hopefully I don't need any. Won't be using TTL for the reason you said.

Basically, what I want to do is have binary counter chip, 74HCxxx, drive the DAC.
So I thought, if I use a 10K or 100K ladder, it should not load the 74HC outputs enough to cause trouble.
At any rate, even if it did, every output would behave the same, so it should not affect the linearity of the DAC should it. Might affect the output voltage, I would get a bit less than 5V full scale, but I could not cat less in my application.
All I want is a DAC, any DAC... just convert the counter output to analog and that's it, no fuss. I am not designing a metrology grade instrument to say the least.... I guess anything would do.

Buffering the output of the DAC, might no even be necessary, it will drive 1Mohm loads.
And again even if the voltage dropped, I don't care, I can adjust full scale deviation afterward, no big deal.

I will post somewhere on the forum to gather suggestions about DACs that can be used without a CPU, maybe there are some out there that exist, or maybe not. The experts will tell me...

--- End quote ---
Don't want to pee in your coffee, but where will the 5V for the buffers come from? If they drift, your "full scale" (or any divided part of it) will drift accordingly! All I'm saying is, maybe get a few TL431 as well, and maybe use one just to power the buffers - If you do, get the specific Onsemi one's linked (TL431BC) as they're pretty good (lowest noise).

Vince:
No worries, you can't pee in my coffee because I don't drink the stuff, I hate it  8)

You are complicating things... it's not looking simpler to use a "proper" DAC.

As I said I only need a crude DAC, don't care about full scale value or stability, could be +5V or -17V or +27.34V, doesn't matter.

Now I think of it, I remember some gizmo available for sale, that does something similar to what I want to do, and it uses just a SIL ladder connected to the I/O pins of an MCU, nothing else, and it works perfectly...
So I guess I am again guilty of overthinking things...

Hell maybe I will reconsider my position and use an MCU instead of discrete logic chips. Would make the design more compact and easier to modify/fine tune.
I will check the timing requirements of my project and see if an AVR MCU could cope with it or not.
Only reason for discrete logic initially, is to try to fo more with less... it's more satisfying and elegant. But at the end of the day I want this project to come to life, I kinda need it as stepping stone for a larger project, so if an MCU makes more practical sense then I can't rule it out...

I don't want to disclose my project, so I will PM you so you can understand the situation better.... but you will have to sign an NDA first !  :-DD



m k:

--- Quote from: Vince on March 22, 2023, 06:23:23 pm ---
--- Quote from: m k on March 22, 2023, 10:57:11 am ---
--- Quote from: Vince on March 20, 2023, 10:36:35 pm ---This a Tek 7504 scope.... from my big TE haul from a few months ago.
It's a parts unit in extremely poor shape...

--- End quote ---

So you lost a perfectly good sink wash demo possibility?

--- End quote ---

I don't know what that is  :-//

--- End quote ---

Yes, probably means nothing in selected language.

I remember you were not very comfortable with Tek's washing method.
“We no longer consider it necessary to remove the CRT, shields, vacuum tubes, etc. to do a thorough cleaning job. Experience has shown that warm water and detergent under pressure penetrates these areas adequately without completely exposing them.”

If you don't have a tub just flip your old refrigerator on it back.
Assuming here that for now you're at least on a brink of replacing it.

Vince:
Oh, OK !  :-DD

Washing that 7504 did not occur to me.... it's not hollow-state technology, it was not dirty to begin with either, and it was so far gone that restoring it never ever even remotely crossed my mind....

No tub here, and my fridge is doing just fine thank you !  :horse:

OK I need to replace the thermostat (still runnong on my home made electronic timer for now... works like a charm), and the door lock of the freezer compartment at the top, does not close anymore... so I can't buy any frozen food.
But, other than than it works like a champ ! It's only 30/35 years old, and my dad replaced the compressor 25 years ago with a top quality one, a Swedish brand. He said back in the day that brand made top notch compressors...   
So I will probably keep it until the compressor dies, because I know that if I buy a brand new fridge it will come with a crappy compressor that will give up the ghost in just a few years.

m k:

--- Quote from: Vince on March 22, 2023, 09:01:19 pm ---Washing that 7504 did not occur to me.... it's not hollow-state technology, it was not dirty to begin with either, and it was so far gone that restoring it never ever even remotely crossed my mind....

--- End quote ---

My mental image also included a belly up machine handled from the frame with long yellow gloves, a brush resting aside and Bobby Darin rolling on the turntable.

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