# Op Amp adds spectral components to amplified output signal

I'm using this simple circuit to amplify a small signal:

simulate this circuit – Schematic created using CircuitLab

Input voltage is defined by: Vin = 1.5V + 5mV*sin(wt) with w = 5 kHz / (2*Pi).

The op-amp I use is the MCP6142 (application requires very low current consumption). I use the second op-amp only to probe the output signal from first op-amp (as scope impedance - 1 MOhm - is much higher than the resistances I use).

The problem I have is that some extra spectral components are added to the output signal:

Here is a capture of the input and output signals over time:

We can see that the input signal - blue - is "pure" as it is generated by the waveform generator of the scope. However the output signal - yellow -, though it is correctly amplified, has some extra spectral components, whose frequencies are multiples of the input frequency. Do you have any idea why this happens?

• Notice the gain is also much less than you would predict. Look at the output waveform, I think you will see it's clipping, i.e. the output is hitting a supply rail (or both). How close to the supply rails is this opamp (no link to datasheet) spec'ed to go? Oct 11, 2016 at 21:16
• You may also be close to the slew rate limit of the op-amp. I calculate if the op-amp operates ideally, you'd have 19 V/ms slew rate at the output, while the datasheet spec is 24 V/ms, so you might be getting close enough to that to cause some distortion. Oct 11, 2016 at 21:21
• @BrianDrummond Output is not clipped, as the GBW of the op-amp is rather small (see datasheet fig 2.14 page 9). At this frequency, gain is only 30dB. Oct 11, 2016 at 21:21
• It's picking up hum, and that's causing some of your problems. The vin- terminal is a "virtual earth" sourced from very high impedance, but drawn on the schematic as a huge area. Make it as small as possible in physical area and, if necessary, put a copper foil screen over it. (If that's with the new, lower, impedances, then the hum's coming from somewhere else) Oct 11, 2016 at 21:55
• Did you put a guard ring around the input terminals (see page 18 of the datasheet)? You might be picking up noise from elsewhere in the circuit. Oct 11, 2016 at 22:00