Hello,

I'm building a "precision rectifier" using an op amp and some diodes. The schematic is attached. On a breadboard, this circuit behaves as expected, producing a clean, rectified sine wave. When I transfer the circuit to a protoboard, I get the signal shown in the attached scope capture. One side of the sine looks clean and the other is noisy. I've done this twice now, with the same results! Any ideas?

One possible culprit... I'm driving the input with an iPhone signal generator app, because I don't have a proper function generator. Maybe it's not up to the task? It seemed to work fine when the circuit was on the breadboard. I've rung the circuit out multiple times with a multimeter and I'm still not sure what's going on here.

 

The circuit appears to be oscillating. Are your input and output signals well separated on the protoboard?

I don't see the schematic, only the 'scope waveform.

 

Thanks for the reply, Zapper. I've edited the post to include the schematic.

The components were much more spread out on the breadboard than the protoboard, so you may be on to something.

 

Here are some photos of the protoboard, if it helps at all.

 

I don't see any power bypass caps. Add 0.01 to 0.1uF or so at the power pins: + to - is usually all you'll need (but since you didn't draw the power connections....).

 

Your construction technique is an open invitation for all kinds of noise, oscillations and instability.

 

Your construction technique is an open invitation for all kinds of noise, oscillations and instability.

Haha. Fair enough. I'm a software guy. I'll try to shorten up the wires.

 

I don't see any power bypass caps. Add 0.01 to 0.1uF or so at the power pins: + to - is usually all you'll need (but since you didn't draw the power connections....).

Thanks for the tip. I had bypass caps on my first board but they didn't seem to help. I'll add the caps to my second board and see if things improve.

 

One 0.1μF cap directly from the + pin to ground and one cap from - pin to ground is best.

 

Haha. Fair enough. I'm a software guy. I'll try to shorten up the wires.

That and avoid output signal wires being close to any input signal wires.