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Old 09-16-2014, 02:43 PM   #1109 (permalink)
e*clipse
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Woo Woo! I'm making some progress.

I got the sine wave I was looking for a couple of nights ago - 9V p-p @ 50kHz. The only problem was the power op-amps I am using (those ST L-165 obsolete ones) were getting awfully hot.

So, I decided, what the heck, try it anyway, just monitor the temperature. My lab was 33.1C (yes, really it's pretty hot here) and the temperature of the power amps just tied to the 'scope was running 55C. A discussion about this heat problem led me to check the load impedance. Myprobes resistance is about 245Ohms; I've heard the capacitance is significant, but I have no way to check it. So, knowing that the impedance of the resolver is 1152 Ohms at 50kHz ( I calculated that previously ) I felt it was safe to plug the circuit into the resolver.

Well, the good news is that the signal is VERY stable, and did not attenuate at all due to being plugged into the resolver. The power op-amps heated up to about 60C while driving the resolver. The output is exactly what is predicted by the transformer ratio, or 2.2V p-p.

Done with that, right? Well, not sooo fast....

1) I'm very unhappy with the hot power amp. It should not overheat (yes, I think it's overheating) with no load. Perhaps I should not focus on a solution for this amp, as I'm not going to use it in the real circuit anyway.
2) There is a LOT of attenuation in the circuit before the power amp. The input is 5V p-p, and the signal the power amp finally sees is about 0.35V p-p. Is that a big deal? Considering op-amps can support gains over 100, perhaps the gain of 13 isn't. Also, highly amplified low amplitude signals are big noise problems. The low output amplitude seems weird, so I looked into that a bit.

I guess I could add a point to the "why bother when you can buy one" list for Astro. Sometimes the best way to force yourself to learn something is to do a project.

Here's what I learned - there are hundreds of op-amps out there, all with different specs that vary subtly from on another. In this case, I was using two different CMOS op-amps from Microchip, the MCP602 and the MCP6002. Both are low power, high speed precision op-amps intended for instrumentation. The MCP6002 is a newer version, with lower power consumption and a slightly worse Gain-Bandwith product. The MCP6002 GBwP is 1MHz, while the MCP602's GBwP is 2.8MHz. I figured that for this circuit, I just wanted a gain of 1 from each stage, since I was primarily doing waveform shaping. A Bandwidth of 1Mhz should be fine for a circuit needing a 50kHz signal, right?

NO. Not even close. The MCP6002 (which I was using exclusively for this circuit) cannot do the simple task of buffering the 50kHz input signal. It puts out a sawtooth (triangle) waveform with a peak of about 75% of the input. It also won't follow a sine wave above about 30kHz; there's lots of signal lag, leading to significant phase lag between the input and output. WTF?? It turns out that you have to look at a number of other specs as well. In this case the MCP6002 has a terrible slew rate - 0.6V/us. This means that it can barely track the square wave at pedestrian speeds below 10kHz. This also means that I basically built a circuit to make a sine wave out of a triangle wave, and that the waveform distortion DUE TO THE OP-AMP was significant enough to help create a sine wave. Meanwhile, the MCP602 has no problem accurately tracking the 50kHz square wave. Its slew rate? 2.3V/us. Meanwhile the power amp's slew rate is 8V/us.

Back to the drawing board. I really don't want the success of something to be due to a major screw-up. Well, it was a good lesson...
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MPaulHolmes (09-16-2014)