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<DIV dir=ltr align=left><SPAN class=945072518-04122020><FONT size=2>Hello
Christian,</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=945072518-04122020><FONT
size=2></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=945072518-04122020><FONT size=2>It seems to
me that your circuit will invert the CV, which is not what you
want.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=945072518-04122020><FONT
size=2></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=945072518-04122020><FONT size=2>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:</FONT></SPAN></DIV>
<DIV><SPAN class=945072518-04122020><FONT size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=945072518-04122020><IMG border=0 hspace=0 alt="" align=baseline
src="cid:945072518@04122020-0AA9"><FONT size=4><BR></FONT></SPAN></DIV>
<DIV><SPAN class=945072518-04122020><FONT size=4><FONT size=2>Or, a slightly
more accurate version with 1% resistors:</FONT></FONT></SPAN></DIV>
<DIV><SPAN class=945072518-04122020><FONT size=4></FONT></SPAN> </DIV>
<DIV><SPAN class=945072518-04122020><FONT size=4><IMG border=0 hspace=0 alt=""
align=baseline src="cid:945072518@04122020-0AB0"></DIV></FONT></SPAN>
<DIV><SPAN class=945072518-04122020><FONT size=4></FONT></SPAN> </DIV>
<DIV><SPAN class=945072518-04122020><FONT size=2>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%.</FONT></SPAN></DIV>
<DIV><SPAN class=945072518-04122020><FONT size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=945072518-04122020><FONT size=2>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.</FONT></SPAN></DIV>
<DIV><SPAN class=945072518-04122020><FONT size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=945072518-04122020><FONT size=2>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.</FONT></SPAN></DIV>
<DIV><SPAN class=945072518-04122020><FONT size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=945072518-04122020><FONT size=2>Cheers,</FONT></SPAN></DIV>
<DIV><SPAN class=945072518-04122020><FONT size=2>Doc Sketchy</FONT></SPAN></DIV>
<DIV dir=ltr align=left><FONT size=4></FONT><BR></DIV>
<DIV dir=ltr lang=en-us class=OutlookMessageHeader align=left>
<HR tabIndex=-1>
<FONT size=2 face=Tahoma><B>From:</B> Synth-diy
[mailto:synth-diy-bounces@synth-diy.org] <B>On Behalf Of </B>Christian Maniewski
via Synth-diy<BR><B>Sent:</B> Friday, December 04, 2020 5:31 AM<BR><B>To:</B>
synth-diy@synth-diy.org<BR><B>Subject:</B> [sdiy] CV input op-amp
circuit<BR></FONT><BR></DIV>
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src="https://read-receipts.canarymail.io:8100/track/C49266956199D83F011285EF840B71C7_75E3E7C7EDA0AF42FB4C4E434016155C.png"
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<DIV id=CanaryBody>
<DIV>Hi all!</DIV>
<DIV><BR></DIV>
<DIV>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.</DIV>
<DIV><BR></DIV>
<DIV>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?</DIV>
<DIV><BR></DIV>
<DIV>Thank you so much!</DIV>
<DIV><BR></DIV>
<DIV>I’ve been following this email list for some time now. This is my first
question and first email entirely. Please bear with me.</DIV>
<DIV><BR></DIV>
<DIV><BR></DIV>
<DIV>Chris</DIV>
<DIV><BR></DIV></DIV>
<DIV id=CanarySig>
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