<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">+1 agree. I’d do it Olivier’s way too, for the reasons given. One op-amp mixer per CV input isn’t a big deal and a reference voltage is not hard to rustle up either.<div class=""><br class=""><div class="">
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<div><br class=""><blockquote type="cite" class=""><div class="">On 14 May 2018, at 17:57, O Gillet <<a href="mailto:ol.gillet@gmail.com" class="">ol.gillet@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class=""><span style="color:rgb(34,34,34);font-family:sans-serif;font-size:13px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline" class=""><span class="">> </span>guess I'm having a bit of difficulty figuring out exactly what the voltage will be at the MORPHADC and SPREADADC pins.<span class=""> </span></span><br class=""><div class=""><span style="color:rgb(34,34,34);font-family:sans-serif;font-size:13px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline" class=""><span class=""><br class=""></span></span></div><div class="">For this you have to know the input impedance of the MORPHADC pin.</div><div class=""><br class=""></div><div class="">I'm not a big fan of this kind of circuit. I prefer the way I do it (with a rail to rail op-amp in inverting configuration): the op-amp has a low output impedance, which is good for driving ADC inputs; the input impedance can be set to 100k precisely, which works nicely with some passive modules; you can do some low-pass filtering too. Costs an op-amp per input, and a voltage reference for your whole board.</div></div><br class=""><div class="gmail_quote"><div dir="ltr" class="">On Mon, May 14, 2018 at 6:25 PM Corey K <<a href="mailto:coreyker@gmail.com" class="">coreyker@gmail.com</a>> wrote:<br class=""></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr" class="">I've been studying a few open source circuit designs for digital eurorack modules, and I've come across one that I'm having a little difficulty analyzing. It is for the 4ms spectral multiband resonator (SMR). In particular, I'm looking at circuits for the MORPHCV and SPREADCV inputs, which are on sheet 4 of the schematic (left hand side): <div class=""><br class=""></div><div class=""><a href="https://github.com/4ms/SMR/blob/master/hardware/smr-1.0.1-schematic.pdf" target="_blank" class="">https://github.com/4ms/SMR/blob/master/hardware/smr-1.0.1-schematic.pdf</a><div class=""><br class=""></div><div class="">I understand the basic parts of the circuit: the input passes through and op-amp buffer (gain=1), and the output is connected to a 5v source via a trim pot. The input to a microcontroller is clipped to an acceptable range [0, 3.3V] with two diodes. </div><div class=""><br class=""></div><div class="">I guess I'm having a bit of difficulty figuring out exactly what the voltage will be at the MORPHADC and SPREADADC pins. I tried to do some nodal analysis, but things weren't making sense to me.</div><div class=""><br class=""></div><div class="">My intuition is that this is some type of "summing circuit" (the current from the 5v source gets added to the current from the op-amp). If so, why wouldn't the designer have used a traditional op-amp summing circuit? What might be preferable about this configuration?</div><div class=""><br class=""></div><div class="">Thanks,<br class="">Corey</div><div class=""><br class=""></div><div class=""><br class=""></div></div></div>
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