CV Voltage Limiting on Mutable Instruments Modules

Looking at the schematic for Marbles (but I think this applies to most of the more recent MI modules) on the Inputs page, I can see how the MCP6004 op-amp is used to scale and possibly clip incoming CV signals, so that the module can accept signals in the -5v to 5v range, and the microcontroller only received 0v - 3.3v.

It seems to me that these inputs work as follows: Each op-amp is set up as a mixer which mixes an offest into the incoming CV signal, which is then scaled by setting the gain of the op-amp using the feedback resistors. The op amp is a rail to rail op amp, with the supply voltage set to be the same as the microcontroller, so the op amp can’t produce an output outside the range that the microcontroller can accept.

Looking at the MCP6004 datasheet under absolute maximum values it lists the following:

Analog Inputs (VIN+, VIN-)††… VSS – 1.0V to VDD + 1.0V

Which means that the chip can only accept input voltages between -1V and 4.3V.

So my question is: what happens when someone plugs something into the module with a high or low voltage, i.e. -10v, +12v etc.?

I have learnt an absolutely huge amount from looking at the Mutable Instruments schematics but I’ve not been able to figure this one thing out!

You are mixing up two things:

  • The input voltage of the module.
  • The input voltage of the op-amp.

Check the schematics and you’ll see that they are not the same. The input voltage of the op-amp is 0V for both VIN+ and VIN-.

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Is this because of the feedback loop? I.e. whatever voltage gets fed in, the op amp will force it very close to ground?

Does this mean that the maximum voltages for the inputs on the datasheet only apply if you’re using the op amp as a comparator (or similar)?

This is because the op-amp is used in an inverting configuration, in which the V+ input is grounded, and the V- input behaves as a virtual ground.

It depends on the circuit in which the op-amp is used. Some circuits directly exposes the op-amp pin to an external voltage (eg: buffer/voltage-follower, summing amplifier, non-inverting amplifier, comparator); while some other circuits are designed to keep both op-amp pins at ground level.

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Thank you so much - this is very clear.

Honestly your schematics are one of the best (if the THE best) resources for learning electronics I’ve ever seen! I’ve learnt so much from them. (in fact I spent so long staring at the Marbles schematic I actually got the urge and bought it yesterday!)

I also have no idea how you manage to reply to my posts so quick and still get so much done. You’re a machine!

Thanks again for your help - it’s very much appreciated.

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