electro-music.com   Dedicated to experimental electro-acoustic
and electronic music
 
    Front Page  |  Articles  |  Radio
 |  Media  |  Forum  |  Wiki  |  Links  |  Store
Forum with support of Syndicator RSS
 FAQFAQ   CalendarCalendar   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   LinksLinks
 RegisterRegister   ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in  Chat RoomChat Room 
 Forum index » DIY Hardware and Software
Help needed. Buffered output on CMOS / PIC?
Post new topic   Reply to topic Moderators: jksuperstar, Scott Stites, Uncle Krunkus
Page 1 of 1 [19 Posts]
View unread posts
View new posts in the last week
Mark the topic unread :: View previous topic :: View next topic
Author Message
rjh



Joined: Jan 20, 2013
Posts: 12
Location: Sweden

PostPosted: Sun Jan 20, 2013 5:04 am    Post subject: Help needed. Buffered output on CMOS / PIC? Reply with quote  Mark this post and the followings unread

Hi! Most of my Google quires lead to this forum so I figured I join! Seams like a good place for this kind of stuff.

Recently got in to DIY:ing some modules for my modular synth (mostly Modcan A). Most of the projects are based around logic chips (4000 series) and PIC:s. What I'm wondering is, before I let these modules play with my other modules, what precautions should I take for the input / output. Most circuits I come across has either or both a buffer before the output (usually an op-amp) and a protective resistor (usually 10-100k). Is this considered common practice?

When reading up on the CMOS logic chips, I came across this document discussing buffered (suffix B) and unbuffered (suffix UB) logic ICs. I'm curious to know if this would eliminate the need for a buffer afterwards.

Bonus question: If the last stage of a circuit is an op-amp (a mixer for example), it doesn't make any sense buffering that output right?

Sorry for the total newbie questions, trying to learn. Smile

Would be happy to post some circuit diagrams if that could help.

Any help is greatly appreciated!

edit: grammar / stupidity
Back to top
View user's profile Send private message
elmegil



Joined: Mar 20, 2012
Posts: 1516
Location: Chicago
Audio files: 14

PostPosted: Sun Jan 20, 2013 10:39 am    Post subject: Reply with quote  Mark this post and the followings unread

I usually use 1k on outputs, following the example of a lot of the stuff I've built by Thomas Henry. Can't say so much about inputs, Most of the stuff I've thrown together myself has been CV generators only (from arduino and such).

I can't say I know much about Buffered/Unbuffered CMOS, so I'll defer that to someone else.

I agree, buffering a mixer output doesn't seem like it should be necessary.
Back to top
View user's profile Send private message
corex



Joined: Mar 02, 2010
Posts: 114
Location: Las Vegas

PostPosted: Sun Jan 20, 2013 12:36 pm    Post subject: Reply with quote  Mark this post and the followings unread

It's bad practice to rely on the internal buffers because they can only source limited current. This is fine for CMOS-to-CMOS connections, but in a modular synth context you can't be sure where the output will go. So generally we re-buffer with an opamp, and then after that a current-limiting resistor (1K generally) to protect against shorts.

At the inputs, there is a different problem: logic levels. At 12 volts, CMOS will switch to logic high at around 8.4 volts (70%), so this won't switch at all from a 5V gate. Commonly, transistors are used here to boost the input signal. Ideally you want it to switch at a pretty low level for easy compatibility with a variety of signal sources... perhaps 1.2 to 2.5 volts.

Another problem at the inputs is that CMOS cannot take any voltage outside of its supply rails, and its supply is single-sided -- on a bipolar +/-12V supply you normally give the CMOS subcircuit 12V and GND. Therefore, any negative voltage reaching the CMOS input pin will be routed through the chip's power protection network, and those tiny diodes can't handle much. You will burn up the chip. Therefore, it must also be protected against negative voltages. Typically this is solved with an inline diode.
Back to top
View user's profile Send private message
rjh



Joined: Jan 20, 2013
Posts: 12
Location: Sweden

PostPosted: Sun Jan 20, 2013 3:59 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks a lot for the info! Very informative. You just saved a few small diodes from burning. They will be grateful.

Just out of curiosity.. I came across this CGS diagram while searching for info. In the upper right corner, there's something that looks like a transistor buffer(?). Most transistor buffers I've seen takes a Vref which is added to the base. Would something like that be enough for the outs of a CD4xxxxB? I have a lot of transistors laying around and since I'm only dealing with CVs right now (0-5v) I can live without bipolarity.

Again, thanks a lot for the help!
Back to top
View user's profile Send private message
corex



Joined: Mar 02, 2010
Posts: 114
Location: Las Vegas

PostPosted: Mon Jan 21, 2013 10:26 am    Post subject: Reply with quote  Mark this post and the followings unread

Yeah that's a buffer. I'm not sure what the trade-offs are between a transistor and an opamp in this application. No DC offset should be added, though (no VRef).

Edit, from an EE friend: "Either work fine. For logic transistors are way easier to use. For analog scaling op amps are easier. Also keep in mind that that circuit [CGS] can drive a crapload of current just by changing the transistor and adding cooling."
Back to top
View user's profile Send private message
rjh



Joined: Jan 20, 2013
Posts: 12
Location: Sweden

PostPosted: Mon Jan 21, 2013 1:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

Cool, thanks! Seems like every time I think I got the hang of something, a ton of other questions pop-up. Still a long way to go Smile

Anyway, this is what I've come up with so far. The first stage amplifies the signal and then passes in into a Schmitt trigger. Most input will be CV but I do want the option to pass in AC. Any remarks / suggestions are of course appreciated.


outputbuffer.png
 Description:
output stage
 Filesize:  10.11 KB
 Viewed:  3012 Time(s)

outputbuffer.png



inputstage.png
 Description:
input stage
 Filesize:  16.45 KB
 Viewed:  58 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

inputstage.png


Back to top
View user's profile Send private message
elmegil



Joined: Mar 20, 2012
Posts: 1516
Location: Chicago
Audio files: 14

PostPosted: Mon Jan 21, 2013 1:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

Some circuits (I'm thinking of a few different Thomas Henry circuits I've seen) put a switchable cap across the input to let you limit to AC only. Open switch, the signal goes through the cap and is AC coupled, closed switch, the cap is shorted and the signal is DC coupled.
Back to top
View user's profile Send private message
rjh



Joined: Jan 20, 2013
Posts: 12
Location: Sweden

PostPosted: Mon Jan 21, 2013 2:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

hmm not sure I get the point of AC coupling in this circuit. Should be able to handle both AC and DC.. I guess the only point would be to have the option to eliminate some DC offset?


edit: spelling

Last edited by rjh on Mon Jan 21, 2013 2:46 pm; edited 1 time in total
Back to top
View user's profile Send private message
elmegil



Joined: Mar 20, 2012
Posts: 1516
Location: Chicago
Audio files: 14

PostPosted: Mon Jan 21, 2013 2:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

Right, that was what I thought you meant when mentioning AC. Overall, yes, you don't have to filter it, assuming you're not going to be sending in an AC signal of say, 10vpp at a DC offset of 5+, it ought to be fine.

Edit: not even 10vpp at 5V is really a problem, but I get the point across too.
Back to top
View user's profile Send private message
corex



Joined: Mar 02, 2010
Posts: 114
Location: Las Vegas

PostPosted: Mon Jan 21, 2013 7:53 pm    Post subject: Reply with quote  Mark this post and the followings unread

You may not need a Schmidtt trigger on the inputs, in which case you maybe could reduce parts count by using transistor buffers for the input network.

For example, http://yusynth.net/Modular/EN/DIVIDER/index.html

Oh wait, that example is a Schmidtt trigger... still looks like less parts.

Also, Yves uses a diode from ground to prevent negative voltages here (instead of inline). That could be a good idea. It will still allow a small negative voltage though.
Back to top
View user's profile Send private message
rjh



Joined: Jan 20, 2013
Posts: 12
Location: Sweden

PostPosted: Tue Jan 22, 2013 2:05 am    Post subject: Reply with quote  Mark this post and the followings unread

ah, surprised I didn't find that circuit before. Looks pretty much like what I want to do (though the purpose of this build is more to learn than anything).

Speaking of part count. I guess one alternative would be to just use a regulator and operate the whole circuit in the 0-5v range. Then use 5v zener diode at the input to protect against anything above 5v or negative (using the configuration in the diagram you posted but with a zener). This would eliminate the need for boosting the signal, leaving just the schmitt trigger at the input stage.. if the schmitt trigger has a high enough input impedance that is. More interestingly, the output buffers could then just be kept to a op-amp buffer without the need for any scaling.
Back to top
View user's profile Send private message
Boogdish



Joined: Sep 21, 2009
Posts: 122
Location: Bloomington, IN

PostPosted: Tue Jan 22, 2013 7:55 am    Post subject: Reply with quote  Mark this post and the followings unread

For PIC inputs, I usually put a 1k resistor between in the pin and the input, and have schottky diodes going to +5V and ground in parallel with the pin. In general, schottky's act faster than zeners.
Back to top
View user's profile Send private message Visit poster's website
wmonk



Joined: Sep 15, 2008
Posts: 489
Location: Enschede, the Netherlands
Audio files: 13

PostPosted: Tue Jan 22, 2013 11:52 am    Post subject: Reply with quote  Mark this post and the followings unread

I breadboarded the discrete schmitt trigger, where I put 5V on the collector of the second transistor, and it works. Smile I think this is a great approach for clock signals.

I wonder how much power different approaches use.

_________________
Weblog!
Back to top
View user's profile Send private message
corex



Joined: Mar 02, 2010
Posts: 114
Location: Las Vegas

PostPosted: Tue Jan 22, 2013 1:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

Boogdish wrote:
For PIC inputs, I usually put a 1k resistor between in the pin and the input, and have schottky diodes going to +5V and ground in parallel with the pin. In general, schottky's act faster than zeners.

Yeah, I think the 1N4148 in the YuSynth design should probably be replaced with a Schottky for that reason -- faster.
Back to top
View user's profile Send private message
rjh



Joined: Jan 20, 2013
Posts: 12
Location: Sweden

PostPosted: Wed Jan 23, 2013 4:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

wmonk wrote:
I breadboarded the discrete schmitt trigger, where I put 5V on the collector of the second transistor, and it works. Smile I think this is a great approach for clock signals.


Sounds like an interesting approach, but not sure I follow you completely. 15v on the first transistor and 5 on the other?
Back to top
View user's profile Send private message
rjh



Joined: Jan 20, 2013
Posts: 12
Location: Sweden

PostPosted: Wed Jan 23, 2013 4:44 pm    Post subject: Reply with quote  Mark this post and the followings unread

Boogdish wrote:
For PIC inputs, I usually put a 1k resistor between in the pin and the input, and have schottky diodes going to +5V and ground in parallel with the pin. In general, schottky's act faster than zeners.


thanks, will give that a try next time. What does the schottsky's diode to +5 do?
Back to top
View user's profile Send private message
wmonk



Joined: Sep 15, 2008
Posts: 489
Location: Enschede, the Netherlands
Audio files: 13

PostPosted: Wed Jan 23, 2013 5:08 pm    Post subject: Reply with quote  Mark this post and the followings unread

rjh wrote:
Boogdish wrote:
For PIC inputs, I usually put a 1k resistor between in the pin and the input, and have schottky diodes going to +5V and ground in parallel with the pin. In general, schottky's act faster than zeners.


thanks, will give that a try next time. What does the schottsky's diode to +5 do?

It protects the output (of that circuit segment) from going higher then 5V. If it gets higher, the schottky will conduct, in effect clamping the output to 5V.

_________________
Weblog!
Back to top
View user's profile Send private message
rjh



Joined: Jan 20, 2013
Posts: 12
Location: Sweden

PostPosted: Thu Jan 24, 2013 4:01 am    Post subject: Reply with quote  Mark this post and the followings unread

I'm probably missing something, but wouldn't the schottky to ground take care of that? I.e anything above the breakdown voltage will go to ground. Does schottky diodes work differently from zener in other ways than the switching time?
Back to top
View user's profile Send private message
wmonk



Joined: Sep 15, 2008
Posts: 489
Location: Enschede, the Netherlands
Audio files: 13

PostPosted: Thu Jan 24, 2013 5:25 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes, a zener has a so called 'zener voltage', a voltage applied in reverse when the zener begins to conduct. Where a normal diode won't conduct in reverse, a zener will above that voltage.
A schottky doesn't have that property, but has a lower forward voltage drop. Where a normal silicon diode has a forward voltage drop of around 0.7V, for a schottky that's about 0.2V, allowing it to switch faster.

_________________
Weblog!
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic Moderators: jksuperstar, Scott Stites, Uncle Krunkus
Page 1 of 1 [19 Posts]
View unread posts
View new posts in the last week
Mark the topic unread :: View previous topic :: View next topic
 Forum index » DIY Hardware and Software
Jump to:  

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum
e-m mkii

Please support our site. If you click through and buy from
our affiliate partners, we earn a small commission.


Forum with support of Syndicator RSS
Powered by phpBB © 2001, 2005 phpBB Group
Copyright © 2003 through 2009 by electro-music.com - Conditions Of Use