[sdiy] CV input op-amp circuit

Fri Dec 11 15:52:20 CET 2020

 Hi Chris,
I was just referring to a circuit that I think others have posted, I think I just described it differently.  Below is a screenshot of such a circuit:

Incidentally, I've used something similar to the above circuit many times (the MCP664 is fancier than I'd normally use).  It has shipped inside of instruments with no customer complaints.  Usually I'd use an MCP6004 running on a 3.3V rail.  I've also used a TL074 here running on +/-10 or 12 with diode overvoltage protection on the output.  After trying several ways of doing this, I've settled on this one due to low cost/parts and usually the rails I need are in the system.  As others have said, you invert the signal once it's digital.  No diode based OVP is needed since you're driving an inverting node of a RRIO low voltage part.  If AC coupling is needed, I usually include it on the output to have more control over the corner frequency and avoid huge value.
Let me know if you come up with something clever for the jack switch, it's trickier than one would think.  I have a solution that works but it feels like there may be something more elegant out there.  Make sure you include OVP for the switch input.  Upon plug-ing in, many jacks will allow conduction between the plug tip and switched contact BEFORE the switch is actually physically displaced (I suppose mechanically it would hard NOT to do this).  Anyways, I've blown MCU inputs this way weeks before a production run (fun times), so let my folly be a lesson.  You'd want separate TVS, series R + BAT54S or something similar.
Ben    On Friday, December 11, 2020, 04:23:43 AM EST, Christian Maniewski <chris at manie.ws> wrote:  

   Hey Ben,
thanks a lot for your helpful input here! I’m a bit confused about the technique to sink current from the inverting node. Can you point me to some references on this? Or just something I can google, to find reading material or example circuits?
Regarding the jack switch: I’m plannign to do some sort of plug detection there, so this will most likely go to an MCU output.
Thanks again,
Chris 

  On Friday, Dec 04, 2020 at 5:16 PM, ben gebhardt via Synth-diy <synth-diy at synth-diy.org> wrote:

Hi Christian,
I have used this opamp for the exact circuit you're using it for.  Here are my recommendations:

   - As Tom stated, if this is to be a eurorack input you'd want to double the impedances (47k and 13k)
   - The issue with your arrangement is that you will have DC voltage (about 1.5V) at the opamp inputs.  This will cause some DC current to flow back into whatever you plug into it.  Not the end of the world if your source is well designed, but not ideal. 
   - If you had some type of negative rail, what you could do is sink current from the inverting node.  This would bias the signal up to 1/2 your converter supply.  This is typically what I do.  (you'd save a resistor!)

   - A liability of such an arrangement:  You'd need a relatively quiet and accurate negative rail or reference otherwise you'd inject noise/offset into the output signal.

   - I would also ground that jack switch.  No reason to inject noise into the ADC when nothing is plugged in if you don't have to right?
   - If your converter isn't 5V tolerant, I would agree with Tom that this should be powered from 3.3V--it'd be safer.  This opamp can swing w/i 25mV of the rail which is usually good enough.  

Hope that helps.
Ben    On Friday, December 4, 2020, 09:39:37 AM EST, Tom Wiltshire <tom at electricdruid.net> wrote:  

 Hi Chris,
I’ve seen various approaches to this problem including:
1) Half-wave precision rectifiers to remove inputs below zero volts, with zeners or similar to limit the highest voltage
2) Op-amp clipping with op-amps run from +/-12V that then have their maximum output scaled down and offset to the correct range.
3) Op-amp clipping on single-ended op-amps run at the processor voltage (like yours)
I’m sure there are others. Honestly, I think (3) is pretty simple and straightforward, although I like the precision rectifier circuit too (just because it’s cool!) but the limitation there is that it doesn’t control the higher voltage as neatly.
I notice you have the MCP6002 on 5V supply, but you said you wanted 0-3.3V output. Is that right?100K input impedance is typical for CV inputs on modular gear, so you might want to double your resistor values round the op-amp.

HTH,Tom
 ==================
       Electric Druid
Synth & Stompbox DIY
==================

On 4 Dec 2020, at 13:31, Christian Maniewski via Synth-diy <synth-diy at synth-diy.org> wrote:

   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 

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