Turning the Prop into a simple oscilloscope?
MarkS
Posts: 342
I'd like to do this. With a Prop running at 80MHz, I could create a scope, in theory, with a few dollars in parts that would rival a several hundred dollar scope.
The only issue I am having with the design, which currently only resides in my head, is the voltage divider circuit. I want to use a rotory switch connected to a voltage divider to allow testing of signals above the 3.3v limit of the Prop. A voltage divider circuit is simple, but what if·I wanted to test a signal less than 3.3v? How do you multiply a voltage? I don't think I've ever seen a circuit that does that. All I can come up with is a multi-wind transformer. That doesn't seem like a very ellegant solution though.
The only issue I am having with the design, which currently only resides in my head, is the voltage divider circuit. I want to use a rotory switch connected to a voltage divider to allow testing of signals above the 3.3v limit of the Prop. A voltage divider circuit is simple, but what if·I wanted to test a signal less than 3.3v? How do you multiply a voltage? I don't think I've ever seen a circuit that does that. All I can come up with is a multi-wind transformer. That doesn't seem like a very ellegant solution though.
Comments
It's already been done. You've got an existing design that you can build on: http://forums.parallax.com/showthread.php?p=692191.
This sort of range thing is normally done with a voltage divider as you expect, but with an op amp acting as a buffer. You can use the same op amp as an amplifier for signals less than 3.3V.
Look at the attached picture, and you have seen one.
Andy
Otherwise it can be considered as a piece of software, busy regulating its output so that the the VOLTAGE at the + and the - input become exactly equal.
So there MUST be a feedback.
All kinds of opamp circuits can be understood and parametrized following that simple rule!
you can set the AGC·signal so that it·would always be attenuated to either boost or compress gain to 3.3v full scale
use some signal diodes to clamp your signal voltages between the rails also
this way you can keep the threshold right in the middle of full scale signals
this circuit , I believe, includes an op amp IC within the feedback loop of another op amp IC, so its more of a specialty circuit than a single·op amp buffer or such
youre going to wind up with a compressed signal , so you might need amplitude modulation on the output to readjust the gain compensated signal back into its appropriate voltage levels.·
OR you could rise to the level of a god and use a 6922 Twin Triode vaccuum tube amplifier setup.· Putting a lightbulb resistor in these circuits attenuates the gain naturally, due to the ratio of temperature to heat loss (q=eoT^4), as the signal increases in strength, the bulb dissipates more and more power, exponentially, effectively compressing the circuit.· This is old school, but NO semiconductors! no switching. Ancient compression.
oh yeah, you dont necessairly need feedback. Most op amps require feedback loop closure because the open loop gain is in the order of 10's of thousands.·Like 741's. ·HOWEVER,· Theres alot of op amps out there that provide open loop gains of lower values , like 4 or 10 , so that you can use the amp as a comparator· ( not having to virtually ground the output by feedback to the input )·.· It keeps the system working more like a buffer, where each input terminal of the amp has just one trace , keeping the input impedance closer to infinite, which is a good thing.· It will, however , suffer from excessive output offset error without feedback. You also wouldnt have to add external resistors to set the gain in one of these open loopers.·
·
Something with 4 or 10 is
- either an internally compensated package
or
- no opamp at all.
That's why they are marketed as "comparator".
You could make a salami sandwich and under the right conditions you could still call it an op amp.· Its just got really bad
·output offset and really bad common mode rejection and really bad thermal linearity......
The salami sandwich, you see, ideally has infinite open-bun gain , but today the gain is .001 for anything spectrally outside of 420hz +/- .00001.·
keep in mind the salami is doped