Hey guys,
I was having problems with a circuit oscillating today. I've posted the relevant part of the schematic.
First of all, forgive me if this thing is a mess. This is a circuit board that has gone through a few changes by a few different people. Basically, there is a sensor (P12) at the top that outputs 2.5 > 1V DC and it gets scaled and offset (with pot RV5) so the output at P21 is approx -.2V - 10V.
P66 is a point that was added to include an external DC signal. This was added to the non-inverting side of the op-amp because of the nature of the signal and how it needs to control the output voltage is a non-inverting way.
Wires to P66 and P21 are run approx 12" bundled next to each other. This is what I noticed was causing the circuit to oscillate today. When the wires were bundled tight, oscillation would happen (about 60khz) at the output of the op-amp U5A. When I loosened the wire bundle, the oscillation would stop.
The confusing part about this was that this has been built before, and no oscillation has ever occurred. One thing that I changed though was the OP-AMP manufacturer being used, from an STM TL074CN to a TI TL074CN. But, I figured they'd be exactly the same. The op-amp is bypassed with .1uF caps right next to the chip.
After poking around and trying a few different things, the oscillation stopped when I put a .1uF cap across R55 (or from P66 to ground).
Sure that's great that adding a capacitor seems to have stopped the oscillation, but I don't know why it does. So that's why I'm posting
I was having problems with a circuit oscillating today. I've posted the relevant part of the schematic.
First of all, forgive me if this thing is a mess. This is a circuit board that has gone through a few changes by a few different people. Basically, there is a sensor (P12) at the top that outputs 2.5 > 1V DC and it gets scaled and offset (with pot RV5) so the output at P21 is approx -.2V - 10V.
P66 is a point that was added to include an external DC signal. This was added to the non-inverting side of the op-amp because of the nature of the signal and how it needs to control the output voltage is a non-inverting way.
Wires to P66 and P21 are run approx 12" bundled next to each other. This is what I noticed was causing the circuit to oscillate today. When the wires were bundled tight, oscillation would happen (about 60khz) at the output of the op-amp U5A. When I loosened the wire bundle, the oscillation would stop.
The confusing part about this was that this has been built before, and no oscillation has ever occurred. One thing that I changed though was the OP-AMP manufacturer being used, from an STM TL074CN to a TI TL074CN. But, I figured they'd be exactly the same. The op-amp is bypassed with .1uF caps right next to the chip.
After poking around and trying a few different things, the oscillation stopped when I put a .1uF cap across R55 (or from P66 to ground).
Sure that's great that adding a capacitor seems to have stopped the oscillation, but I don't know why it does. So that's why I'm posting
