| Electronics > Beginners |
| Op amp not working out as planned |
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| netdudeuk:
--- Quote from: Hero999 on June 12, 2018, 01:06:24 pm --- --- Quote from: netdudeuk on June 11, 2018, 12:06:52 pm ---I added a series input capacitor and amended RF to 2K. So, the gain should be 2. However, the scope traces still don't look right (especially as it isn't inverting). Either there is still something wrong with the circuit or I'm measuring it wrong |O --- End quote --- That doesn't look right. Which trace is which? Where is the 'scope's 0V connection going to? Please post a complete schematic and a photograph, showing how it's physically connected up. --- End quote --- The yellow trace is connected to the output of the DDS before the series input capacitor. The blue trace is the output of the op amp. So, yes, it doesn't look right. The scope grounds, the ground for the 5 volt supply (Rigol DP832) and the ground for the 10 volt supply (the same DP832) are all connected together. I'll need to get a photo and a schematic later. Thanks |
| netdudeuk:
Schematic and setup photos. Thanks |
| Zero999:
--- Quote from: netdudeuk on June 12, 2018, 04:18:17 pm ---Schematic and setup photos. Thanks --- End quote --- That circuit looks fine. Such a high speed op-amp shouldn't be used on breadboard, but it doesn't appear to be oscillating. If all the connections are good, then I'd suspect it's the op-amp. Can you change the op-amp for the TL071 or TL081? If not, you could try the old '741 but the frequency would need to be lowered to 10kHz and a larger DC blocking capacitor used: try 1μF. |
| StillTrying:
The op amp might have more gain if you connected pin 4 to 0V. :) |
| netdudeuk:
--- Quote from: StillTrying on June 13, 2018, 11:10:09 pm ---The op amp might have more gain if you connected pin 4 to 0V. :) --- End quote --- It is working somewhat better now thanks :) I wanted to create an adjustable offset on the output so I connected the non inverting input to another rail on my PSU instead of the 5V. The ground for this was connected to all the others. However, if I dialled in anything more than a few hundred mV, I had what I'm guessing we'd describe as unwanted oscillation. The basic sine wave was still there but rather than a line that traced the rise and fall of the signal, think of it as a many columned bar graph where the numbers followed the sine wave. I don't have access to the equipment at the moment but hopefully it is clear what I mean. Why would this be ? Thanks again. |
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