[sdiy] CV input op-amp circuit

[Oren Leavitt]

Yep. I like to put the current limit R inside the feedback loop to avoid 
loading errors. A couple examples from a MIDI to CV I'm working on:

On 12/4/20 7:11 PM, David G Dixon wrote:
> 
> If I'm generating CVs that are meant to be precise (as, for example, 
> VCO pitch-controlling voltages from a keyboard or sequencer), then 
> I take the opamp output from the other side of the current-limiting 
> resistor.  I call this resistor an "innie" and it doesn't affect the 
> output voltage of the circuit (the opamp compensates for it).  I don't 
> want that damned 1k resistor dividing my carefully calibrated voltage.
>
> ------------------------------------------------------------------------
> *From:* Ian Fritz [mailto:ijfritz at comcast.net]
> *Sent:* Friday, December 04, 2020 3:47 PM
> *To:* David G Dixon
> *Cc:* synth-diy at synth-diy.org
> *Subject:* Re: [sdiy] CV input op-amp circuit
>
> [*CAUTION:* Non-UBC Email]
> Most of mine come from 1k output resistors.
>
>> On Dec 4, 2020, at 4:29 PM, David G Dixon <dixon at mail.ubc.ca> wrote:
>> 
>> Also, most CV sources are coming directly from opamps,
>> ------------------------------------------------------------------------
>> *From:* Synth-diy [mailto:synth-diy-bounces at synth-diy.org] *On Behalf 
>> Of *David G Dixon
>> *Sent:* Friday, December 04, 2020 3:24 PM
>> *To:* 'Ian Fritz'
>> *Cc:* synth-diy at synth-diy.org
>> *Subject:* Re: [sdiy] CV input op-amp circuit
>>
>> [*CAUTION:* Non-UBC Email]
>> Just to clarify, my design did not require an inverting opamp.  The 
>> operation was all done in a single opamp.  I just showed two opamps 
>> in the picture because one was processing -5V and other +7V, to show 
>> that the proper voltages were obtained.
>>
>> ------------------------------------------------------------------------
>> *From:* Ian Fritz [mailto:ijfritz at comcast.net]
>> *Sent:* Friday, December 04, 2020 1:26 PM
>> *To:* David G Dixon
>> *Cc:* synth-diy at synth-diy.org
>> *Subject:* Re: [sdiy] CV input op-amp circuit
>>
>> [*CAUTION:* Non-UBC Email]
>> IMO, everyone should at some point go through the derivation of the 
>> equations for the generalized opamp summer. This often makes it easy 
>> to avoid using unneeded inverting stages. One thing to watch, though, 
>> is that the resulting equations assume zero impedance voltage sources 
>> for inputs.  Usually you have to take source impedances into account.
>>
>> Ian
>>
>>> On Dec 4, 2020, at 12:03 PM, David G Dixon <dixon at mail.ubc.ca> wrote:
>>>
>>> 
>>> Hello Christian,
>>> It seems to me that your circuit will invert the CV, which is not 
>>> what you want.
>>> Here's how I would do it:  First, I calculated that the range of -5V 
>>> to +7V is 12V, and the range of 0 to 3V is 3V, so you need a gain of 
>>> 25%.  This alone would change the range to -1.25V to +1.75V.  Hence, 
>>> this needs to be shifted by +1.25V.  So, you need a circuit that 
>>> will apply a gain of 25% and a shift of +1.25V.  I am going to 
>>> assume that you have a -5V reference source available (or an 
>>> inverted +5V reference). So, the -5V reference requires a gain of 
>>> -25%.  So, what circuit will apply a (non-inverting) gain of 25% to 
>>> one input, and an (inverting) gain of -25% to another input?  This 
>>> one, with 5% resistors:
>>> <CVShifter.png>
>>>
>>> Or, a slightly more accurate version with 1% resistors:
>>> <poop.png>
>>> The CV comes into the + input through a 4:1 voltage divider which 
>>> applies a gain of 20%.  However, the 1:4 ratio of feedback to 
>>> inverting input resistors applies a gain of 125% to the 
>>> non-inverting input, and (125%)(20%) = 25%.
>>> The -5V reference comes into the - input through feedback/input 
>>> resistor ratio of 1:4, which applies an inverting gain of -25% to 
>>> that voltage, creating a level shift of +1.25V.
>>> The convenient aspect of this is that both pairs of resistors have a 
>>> 4:1 ratio.  The closest 5% standard values are 33k and 8.2k.  The 
>>> closest 1% values are 102k and 25.5k.
>>> Cheers,
>>> Doc Sketchy
>>>
>>> ------------------------------------------------------------------------
>>> *From:* Synth-diy [mailto:synth-diy-bounces at synth-diy.org] *On 
>>> Behalf Of *Christian Maniewski via Synth-diy
>>> *Sent:* Friday, December 04, 2020 5:31 AM
>>> *To:* synth-diy at synth-diy.org
>>> *Subject:* [sdiy] CV input op-amp circuit
>>>
>>> Hi all!
>>>
>>> I’m trying to come up with an op-amp design for a CV input. I want 
>>> to transform a signal ranging from -5V to +7V to a more MCU 
>>> digestable 0-3.3V. I came up with the circuit you’ll find attached.
>>>
>>> I have seen other approaches, where an offset reference is injected 
>>> in the feedback loop, while the positive op-amp input is grounded. 
>>> Are there any disadvantages to my approach or is it also valid?
>>>
>>> Thank you so much!
>>>
>>> I’ve been following this email list for some time now. This is my 
>>> first question and first email entirely. Please bear with me.
>>>
>>>
>>> Chris
>>>
>>>
>>>
>>> _______________________________________________
>>> Synth-diy mailing list
>>> Synth-diy at synth-diy.org
>>> http://synth-diy.org/mailman/listinfo/synth-diy
>>> Selling or trading? Use marketplace at synth-diy.org
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy
> Selling or trading? Use marketplace at synth-diy.org
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20201204/53c8d4ed/attachment.htm>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: unity-non-inv-CV-buffer.png
Type: image/png
Size: 13949 bytes
Desc: not available
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20201204/53c8d4ed/attachment.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: inv-cv-out.png
Type: image/png
Size: 14619 bytes
Desc: not available
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20201204/53c8d4ed/attachment-0001.png>