This is used to take in pitch control voltage and feed it to an ADC. Pitch CV is usually in the range of 0 to 8v, but, in Eurorack voltages can be anywhere from -12 to +12 volts. The ADC needs to see 0 to 3.3v. So this is where this circuit comes in play.
-
-
Prikaži ovu nit
-
You might think that you can use a simple voltage divider here, and you could, but, that has three important disadvantages: 1. You have to scale the full 0 to +12v range to 0 to +3.3v for your ADC, even though pitch cv is only 0 to 8v. You lose resolution.
Prikaži ovu nit -
2. You still have to deal with negative voltage, so you'll have to add diodes or something. 3. The input impedance needs to be 100k for best compatibility. So enter this two-stage active attenuator. It's brilliant. Let's look at how it works.
Prikaži ovu nit -
The first stage, IC9B, is a TL074 in an inverting configuration. It's power supply is -12 and +12. The input resistor (R54) sets the impedance at 100k (solving issue #3). The feedback resistor (R53) sets the gain to -0.25.
Prikaži ovu nit -
So after this stage the signal is now somewhere between -3v and +3v. +12v on the input gives -3v on the output, and -12v gives +3v. So we've scaled it down, but we still need to deal with the bipolar voltage.
Prikaži ovu nit -
The second stage is where this magic happens. IC7A is an MCP6004. It's setup as a inverting amp with a gain of 1 so you'd think you'd still have the bipolar voltage problem- but nope, we take advantage of some of the characteristics of this amp to fix that.
Prikaži ovu nit -
It's powered by a different supply than the first stage. It's power is 0v and 3.3v. It's a rail to rail op amp so that means it can *only* output 0v to 3.3v. So, it takes that -3v to +3v input, flips it upside down, and drops anything outside of its output range.
Prikaži ovu nit -
So this fixes issue #2. To recap: Stage 1 attenuates -12v to +12v to +3v to -3v (note the inversions) Stage 2 inverts stage 1 and then clips it to 0v to 3.3v. And just like that you have a very nice active attenuator and clipper.
Prikaži ovu nit -
Here's a simulation you can play with: http://tinyurl.com/v2sl9yz
Prikaži ovu nit
Kraj razgovora
Novi razgovor -
-
-
This is really interesting and I won't even pretend I understand all of it. I would like to get into building guitar pedals one day, but my lack of EE knowledge is a bit of a hindrance.
-
You can learn! I'm totally self taught!
Kraj razgovora
Novi razgovor -
Čini se da učitavanje traje već neko vrijeme.
Twitter je možda preopterećen ili ima kratkotrajnih poteškoća u radu. Pokušajte ponovno ili potražite dodatne informacije u odjeljku Status Twittera.
CircuitPythonista