[sdiy] CV input op-amp circuit

[Brian Willoughby]

I was going to suggest that "take the opamp output from ..." would be clearer if phrased "take the opamp feedback from ..." but Oren's  mention of the feedback loop is where it's at.

Brian

On Dec 4, 2020, at 18:27, Oren Leavitt <obl64 at ix.netcom.com> wrote:
> 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
>> Sent: Friday, December 04, 2020 3:47 PM
>> To: David G Dixon
>> 
>> 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: David G Dixon
>>> Sent: Friday, December 04, 2020 3:24 PM
>>> To: 'Ian Fritz'
>>> 
>>> 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
>>> Sent: Friday, December 04, 2020 1:26 PM
>>> To: David G Dixon
>>> 
>>> 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: Christian Maniewski via Synth-diy
>>>> Sent: Friday, December 04, 2020 5:31 AM
>>>> 
>>>> 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