Hi everyone! I bought some 3340 VCO chips and I wanted to add some LFO to the CV input. But knowing that LFO oscillate between positive and and negative voltage I said to myself “maybe inserting it into the CV input might kill the chip?”.
Well if you understand me I’d like to know what can happen when you apply negative voltage to its CV input 
You’re good. Well, I don’t know what your build looks like but negative supposed to work exactly like a positive voltage but it will pull the frequency down
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Okay thank you! Maybe I can put a “flyback” diode between the CV input of the chip and the ground so all neg voltage are filtered
You can put a negative voltage up to -6 V on the CV pin. But of course in normal use you’ll sum several control voltages at that pin, so the LFO might contribute to the total’s varying between +1 and +3 V for instance. Or, on the other hand, -1 to -3 V. It’s all good.
It’s all in the datasheet http://www.alfarzpp.lv/eng/sc/AS3340.pdf.
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Okay thank you. I should have read the datasheet! Well okay, and anyway the 100k resistors are lowing down the voltage I guess
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Oh, of course you’re right. That’s a virtual ground summing node, the voltage there will be 0 V. I always forget that point!
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Wait is it 0V ? Then how can it get the CV voltage?
Current.
The summing node consists of an opamp that’s wired to keep its inverting input at zero volts. To do that, it needs to generate a non-zero output, corresponding to the sum of all input currents. See:
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Interesting I will read this once I’m done with chores
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I wrote a post about summing OpAmp circuits here (showing how the simple mixer worked):
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Thank you !!! Sharing knowledge is one of the best gift 
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