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 » Developers' Corner
Offset biasing in OTA's (LM13700) in DC-coupled applications
Post new topic   Reply to topic Moderators: DrJustice
Page 1 of 1 [10 Posts]
View unread posts
View new posts in the last week
Mark the topic unread :: View previous topic :: View next topic
Author Message
codemode



Joined: Oct 14, 2013
Posts: 5
Location: Boston

PostPosted: Mon Oct 14, 2013 1:52 am    Post subject: Offset biasing in OTA's (LM13700) in DC-coupled applications
Subject description: designing VCA's for synth control
Reply with quote  Mark this post and the followings unread

I have spent a couple days trying to design a VCA to have CV control over the modulation amounts of envelopes and LFO's. Basically, the control voltage to the VCA will control the depth of the modulation source.

One of the main destinations for modulation will be a VCO. The problem I have is with offset biasing in the OTA. As the control input is varied, the DC offset in the output varies. I can try to minimize this by trimming the input of the OTA but it doesn't get rid of it by any means. There can be 100mV or more of offset depending on the control voltage, and it doesn't seem to vary linearly (there's a big bump in offset right in the middle of CV range). Since the VCO is 1V/Oct (83mV per semitone) I don't want more then 5mV offset or the fundamental pitch of the VCO will change several cents or even semitones depending on the modulation amount!

I have tested some circuits based on the LM13700 both in LTSPice and in the lab. The results for both sim and proto are exactly the same - too much offset. I want to use the LM13700 because they are cheap and still available, and they have a linear control input. The basic circuit is attached with description below.

Here's what I have done to this point.
    *scaled resistor values so that the 5Vpp envelope is attenuated to 25mV at the OTA input.
    *scaled the CV input so that the 0-3.3V control voltage gives 0 to 0.5mA to the OTA control input.
    *using an external op amp instead of LM13700 buffers. Feedback resistance is scaled for 1:1 output at full scale (5V VCA output for 3.3V CV input)
Certainly hundreds or thousands of you have done this before me! Is there any way to improve this? I've seen many commercial preset analog synths that use LM13700's for control VCA's, presumably with modulation destinations going to VCO's, or even controlling glide amounts. There must be some way to get the offset low enough?


vca_circuit.jpg
 Description:
 Filesize:  53.32 KB
 Viewed:  150 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

vca_circuit.jpg


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



Joined: Feb 27, 2013
Posts: 161
Location: San Diego

PostPosted: Mon Oct 14, 2013 2:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

Oh boy, I'm looking to do almost exactly the same thing, but I'm not quite as far along. Now you've got me worried.

Have you tried a 500ohm trimmer across the + and - inputs with wiper to ground (instead of your 220's and offset trimmer) as shown in the datasheet?

_________________
My synth build blog: http://gndsynth.blogspot.com/
Back to top
View user's profile Send private message
gdavis



Joined: Feb 27, 2013
Posts: 161
Location: San Diego

PostPosted: Mon Oct 14, 2013 7:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hmm, I'm playing with your circuit in LTSpice and not really seeing any offset issue that can't be fixed by the trimmer.

Couple things I did notice:
The 220k makes the trimmer a bit touchy, 1M seems to work fine.

60k seems to be to large for your 500uA Iabc to get Vout/Vin = 1. According to the math I think it should be 34k but 37k seems to work in simulation.

dunno

_________________
My synth build blog: http://gndsynth.blogspot.com/
Back to top
View user's profile Send private message
codemode



Joined: Oct 14, 2013
Posts: 5
Location: Boston

PostPosted: Mon Oct 14, 2013 7:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Have you tried a 500ohm trimmer across the + and - inputs
yes I have, as you would expect it achieves the same end.
Today I simulated and tested a circuit based on the Mike Sims improved VCA that I've seen a lot on these pages. The idea is to improve THD and CV feedthrough. In its basic form it works ok. I was able to improve performance and get 5mV max offset by doing the following.
    * based on Mike Sims matched-OTA circuit
    * remember my design is scaled for 0 to 3.3 CV so components scaled appropriately.
    * keep Ibias below 0.3mA max. If I don't the offset starts going non-linear. Basically I find the offset jumps from 0V at a 0V CV input to 120mV at 3V CV input and down to -50mV or something like with a 5V CV input (equivalent to Ibias of 0.5mA). So if I keep Ibias between 0 and 0.3mA the offset is kind of linear.
    * Next, I feed a little bit if the CV to the inverting input of the OTA to scale the offset for increasing CV input. Doing this I can get an offset of 5mV max.
I have no idea if this is an appropriate solution or if there's an easier way or whatever but please let me know if you have any input.


vca_circuit2.jpg
 Description:
 Filesize:  98.33 KB
 Viewed:  146 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

vca_circuit2.jpg


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



Joined: Oct 14, 2013
Posts: 5
Location: Boston

PostPosted: Mon Oct 14, 2013 8:04 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
60k seems to be to large for your 500uA Iabc to get Vout/Vin = 1. According to the math I think it should be 34k but 37k seems to work in simulation.

hmmm, I bet you're right.
As I mentioned in both posts, I kept getting non-linearity in the CV bleedthrough when the Ibias goes above 0.3mA or so. Assuming this error maybe the offset would be somewhat linear when operating Ibias in the appropriate range. I'll test it as well.
Back to top
View user's profile Send private message
codemode



Joined: Oct 14, 2013
Posts: 5
Location: Boston

PostPosted: Tue Oct 15, 2013 12:57 am    Post subject: Reply with quote  Mark this post and the followings unread

OK so I backed off on the control current and it works very well. You can see Figure 4 in the datasheet showing Vos versus Iabc and it does start to take off above 0.2mA so I think it makes sense. I tested the circuit as shown below and was able to calibrate the offset until it was inaudible with a raw square wave. For testing I grounded the VCA input and varied the CV from 0V to 3.3V. I actually had the CV sent to the pitch CV of a SEM module I had on the bench so I could also hear the results. Here's the actual offset I measured at various CV's...

0V = 100uV
1V = 360uV
2V = 200uV
3V = 110uV
3.3V = 140uV

so less than one cent in musical terms - which is great.


vca_circuit4.jpg
 Description:
 Filesize:  70.89 KB
 Viewed:  70 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

vca_circuit4.jpg


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



Joined: Feb 27, 2013
Posts: 161
Location: San Diego

PostPosted: Tue Oct 15, 2013 1:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

Glad to hear it. I remember some time back deciding on a max Iabc of 200uA but couldn't remember why Embarassed and the audio VCA circuits that I found on the net where hitting 500uA but I guess it's not really an issue for AC.
_________________
My synth build blog: http://gndsynth.blogspot.com/
Back to top
View user's profile Send private message
codemode



Joined: Oct 14, 2013
Posts: 5
Location: Boston

PostPosted: Tue Oct 15, 2013 1:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yeah exactly, the datasheet and all applications I've seen take advantage of Ibias up to 0.8mA to maximize performance in regards to SNR and THD. And for audio (AC) it's probably best case. For control VCA's I guess I was getting too cute.
Back to top
View user's profile Send private message
gdavis



Joined: Feb 27, 2013
Posts: 161
Location: San Diego

PostPosted: Tue Jan 21, 2014 10:53 pm    Post subject: Reply with quote  Mark this post and the followings unread

Well there it is. I've finally seen this offset in my CV VCA. With trimming the best I can do is a hump in the middle of the CV range just like described above. And apparently at some point I decided Iabc of 500uA was a good idea, so I seem to be getting somewhere around 16 cents off at the VCO. Based on this discussion I'll try adjusting my circuit for 200uA Iabc, hopefully I'll get the same improvement.
_________________
My synth build blog: http://gndsynth.blogspot.com/
Back to top
View user's profile Send private message
gdavis



Joined: Feb 27, 2013
Posts: 161
Location: San Diego

PostPosted: Wed Apr 16, 2014 8:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

gdavis wrote:
Well there it is. I've finally seen this offset in my CV VCA. With trimming the best I can do is a hump in the middle of the CV range just like described above. And apparently at some point I decided Iabc of 500uA was a good idea, so I seem to be getting somewhere around 16 cents off at the VCO. Based on this discussion I'll try adjusting my circuit for 200uA Iabc, hopefully I'll get the same improvement.


Well that didn't work at all. I reduced Iabc and increased the output resistor value to maintain the same output voltage, and the offset was MUCH worse. Apparently any offset improvement is more than counteracted by the increased gain of the current to voltage converter. Maybe I should compensate for less Iabc by increasing the differential input instead of increasing gain on the output... Or is the input distortion going to be a problem then?

_________________
My synth build blog: http://gndsynth.blogspot.com/
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic Moderators: DrJustice
Page 1 of 1 [10 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 » Developers' Corner
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