I'm designing an audio signal frontend to take an electric guitar analog output and deliver it to an ADC channel on a microcontroller. The µC/ADC are
3.3V devices (although there is a
5V regulated voltage source available on its carrier board). The purpose of this frontend is to take the AC waveform coming out of the guitar, which has a P-P voltage dynamic range between a few dozen
mV and up to
1V, offset the signal by
1.65V (half the ADC range), amplify it up to the ADC's rails, and apply an antialiasing lowpass filter beyond
I would prefer to use a low voltage, single supply, rail-to-rail amplifier that can directly run off the MCU's
3.3V supply without me having to mess around with additional voltage clamp circuitry or even a dual supply which would require a charge pump. I want to use only components that I have on hand and not buy anything extra. The only single-supply
3.3V amp that I have is the
LT6200-10 (product page; datasheet). I have 2x units of this IC, enough to create one preamp and one active filter.
Before printing and assembling the PCB by hand, which always runs the risk of damaging components, I wanted to simulate the preamp portion to make reasonably sure that it will do what I want. Since the heart is an LT part, I begrudgingly used the awful LT-SPICE simulator to capture the schematic and run a transient simulation. Here are the results.
- [OK] The input waveform
V(in)is a steady
- [OK] The bias voltage divider
V(divider)emits a clean
- ---> The noninverting input
V(ninv)to the amp appears to be centered around
~2.3Vrather than at the expected bias voltage of
- ---> The output waveform
V(out)is clipped at exactly
3Vand bottoms out at around
~1.1Vrather than near
At first I suspected that the
-10 variant of the
LT6200, which, according to the datasheet, appears to be intended for gains of >10, may not be compensated for the low gain of this circuit. So, as a test, I replaced the
LT6200-10 with the regular
LT6200 and re-ran the simulation. However, I observed an identical graph.
I think the fact that
V(ninv) is offset
650mV higher than expected is a huge giveaway. As another test, I disconnected the noninverting input and re-ran my simulation. I was struck by the fact that
V(ninv) now is perfectly centered around the bias voltage:
So it seems that the input to the amp – which is, presumably, at a very high impedance – nevertheless dramatically alters the incoming signal. My analog knowledge is pretty weak so I can't understand why this is happening. I know that an ideal amp will always attempt to keep the inverting and noninverting inputs at the same voltage, so this is a hint to me on what's going on, but I'm stuck in trying to do further circuit analysis. I've fiddled a bunch with various orders of magnitude for all of the surrounding passives but nothing seems to cause a trend towards my intended behavior.
What am I missing? Is my topology wrong, or maybe one of the component values? I'm equally interested in filling the gaps in my theory knowledge as I am in a practical solution to this problem so that I can actually build the circuit.
Thanks for any help!