[sdiy] CV input op-amp circuit

[Gerry Murray]

Hi Christian

I'm working on an analog input for the synthimuse Version 2.

It's untested but here's my first draft of the input that ranges from 
-5.6 V to +5.6V and scales it to just inside the 3.3V Range.
Changing the scaling resistors slightly and adjusting the offset should 
hopefully get you the range you're looking for.
Schematic image below, LTSpice file attached.

I'm happy if anyone wants to rip it to pieces as I'd rather find out my 
folly now, instead of when my prototype stuffed boards come back. ;-)

Best regards
Gerry





On 04/12/2020 13:31, Christian Maniewski via Synth-diy 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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Version 4
SHEET 1 880 680
WIRE -592 -112 -704 -112
WIRE -320 -80 -448 -80
WIRE -32 -64 -192 -64
WIRE -704 -48 -704 -112
WIRE -320 -48 -320 -80
WIRE -80 -16 -112 -16
WIRE -112 16 -112 -16
WIRE -192 32 -192 -64
WIRE -144 32 -192 32
WIRE -32 48 -32 -64
WIRE -32 48 -80 48
WIRE 48 48 -32 48
WIRE 128 48 48 48
WIRE -704 64 -704 32
WIRE -576 64 -704 64
WIRE -320 64 -320 32
WIRE -144 64 -320 64
WIRE -448 128 -448 -80
WIRE -576 144 -576 64
WIRE -112 144 -112 80
WIRE -320 208 -320 144
WIRE -448 320 -448 208
WIRE -320 336 -320 288
FLAG -576 144 0
FLAG -448 320 0
FLAG -320 336 0
FLAG -592 -112 +3V3
FLAG -80 -16 +3V3
FLAG -112 144 0
FLAG 48 48 Vout
SYMBOL voltage -704 -64 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value 3.3
SYMBOL voltage -320 192 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V3
SYMATTR Value 2.25
SYMBOL voltage -448 112 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V4
SYMATTR Value 1.65
SYMBOL Opamps\\UniversalOpamp2 -112 48 R0
SYMATTR InstName U1
SYMBOL res -336 -64 R0
SYMATTR InstName R6
SYMATTR Value 91k
SYMBOL res -336 48 R0
SYMATTR InstName R7
SYMATTR Value 33k
TEXT -736 440 Left 2 !.dc v4 -5.6 5.6 .1