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codemode
Joined: Oct 14, 2013 Posts: 5 Location: Boston
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gdavis
Joined: Feb 27, 2013 Posts: 359 Location: San Diego
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Posted: Mon Oct 14, 2013 2:26 pm Post subject:
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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/ |
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gdavis
Joined: Feb 27, 2013 Posts: 359 Location: San Diego
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Posted: Mon Oct 14, 2013 7:16 pm Post subject:
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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.
_________________ My synth build blog: http://gndsynth.blogspot.com/ |
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codemode
Joined: Oct 14, 2013 Posts: 5 Location: Boston
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codemode
Joined: Oct 14, 2013 Posts: 5 Location: Boston
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Posted: Mon Oct 14, 2013 8:04 pm Post subject:
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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. |
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codemode
Joined: Oct 14, 2013 Posts: 5 Location: Boston
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gdavis
Joined: Feb 27, 2013 Posts: 359 Location: San Diego
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Posted: Tue Oct 15, 2013 1:10 pm Post subject:
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Glad to hear it. I remember some time back deciding on a max Iabc of 200uA but couldn't remember why 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/ |
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codemode
Joined: Oct 14, 2013 Posts: 5 Location: Boston
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Posted: Tue Oct 15, 2013 1:35 pm Post subject:
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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. |
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gdavis
Joined: Feb 27, 2013 Posts: 359 Location: San Diego
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Posted: Tue Jan 21, 2014 10:53 pm Post subject:
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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/ |
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gdavis
Joined: Feb 27, 2013 Posts: 359 Location: San Diego
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Posted: Wed Apr 16, 2014 8:16 pm Post subject:
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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/ |
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dougcl
Joined: Feb 08, 2008 Posts: 33 Location: Portland OR
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Posted: Tue Dec 15, 2015 11:25 pm Post subject:
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Hi folks, I had pretty good luck following the advice in this thread and scaling back Iabc to 0.5mA max.
The problem I have now though is with the input jack floating (no cable connected) I want the input to default to 0 volts. Unfortunately the LM13700 input floats quite a bit. This shows up pretty accurately in the simulator. I tried a 1M pull down resistor on the input, but it can't drain enough current. In fact, it looks more like a 1k is needed to get an acceptable level, but that would give me a crazy low input impedance at the input jack. Seems like an op amp buffer on the input will solve the problem, but is it necessary? That's a lot of op amps. |
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brock
Joined: May 26, 2011 Posts: 112 Location: Canada
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Posted: Thu Dec 17, 2015 7:11 pm Post subject:
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I think an op amp on the input is the only way around the low impedance problem. I don't think I've seen an LM13700 VCA that doesn't have all inputs buffered is some way. |
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dougcl
Joined: Feb 08, 2008 Posts: 33 Location: Portland OR
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Posted: Fri Dec 18, 2015 11:38 pm Post subject:
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brock wrote: | I think an op amp on the input is the only way around the low impedance problem. I don't think I've seen an LM13700 VCA that doesn't have all inputs buffered is some way. |
Thanks yeah I am looking at this now and it seems to make the whole circuit healthier. |
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dougcl
Joined: Feb 08, 2008 Posts: 33 Location: Portland OR
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Posted: Mon Apr 04, 2016 5:04 pm Post subject:
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dougcl wrote: | Hi folks, I had pretty good luck following the advice in this thread and scaling back Iabc to 0.5mA max. |
On second thought, I am not so certain I solved anything. I think I merely retuned the input network to a much larger input voltage. This reduced the offset because I used less makeup gain, but led to unacceptable distortion.
I am unable to get both the distortion and offset down to acceptable levels simultaneously. Meanwhile the TI SPICE model is giving me wonderful predictions on both, but somehow off by a factor of 15x from the actual circuit. Worse however, the Iabc of one OTA is affecting the offset trim of the other (both OTAs in the same package). This happens with Iabc max of 500uA or 1mA. I haven't seen any mention of this online or in the datasheet. It is a deal breaker for me and the LM13700. |
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