I would like to use an Si514 as a tuneable oscillator driving later stages with 50 ohm impedance. So I put an opamp on it and attempted to configure it as a saleen-key low pass filter (to round the squares). The problem I have is, although it works, the output is 5mVpp if my cheap oscilloscope is to be believed. My fluke multi-meter on DC says 1mV at J2 with no coax hooked up.

The first thing I know I did wrong is I accidentally swapped the + and - of my opamp when transcribing the circuit. An online circuit simulator doesn't seem to care about this fact. Nevertheless, is this the issue or is something else wrong?


Channel 1 is a coax cable attached at J2. Channel 2 is a probe touching the oscillator side of R1.

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    \$\begingroup\$ The opamp has full POSITIVE feedback - swap the input nodes. \$\endgroup\$ – LvW Feb 6 at 9:48
  • \$\begingroup\$ 1. As @LvW says, your inputs should be swapped. 2. Can you plot the input signal on Ch 2 so we can correlate it with the output? 3. Can you share a well-lit, detailed photo of your board + probing setup? \$\endgroup\$ – pion Feb 6 at 9:58
  • \$\begingroup\$ 1. Falstad Circuit Sim didn't show this as an issue but NI multisim seems to. Is multisim right? 2. Added 3. I forgot nice test points so holding a probe on an 0805 part takes all of my attention much less taking a photo. \$\endgroup\$ – foreverska Feb 6 at 17:29

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