Re: Frustrated Modbus
Look at the attached schematic. Whenever you design a circuit around an opamp you should use a compensation cap in the feedback loop as well as good power supply decoupling. In most documents this information is "left out" because it is a "given" (the schematic gets very crowded showing every decoupling cap).
The formula for calculating the "Ccomp" cap is shown in the schematic with a very simple explanation (it really is beyond this forum to explain -3db break points etc.). The point is, you can design yourself a very nice oscillator if you ignore Ccomp and bypassing (another term for decoupling), especially when using breadboards and flying wire construction.
In the example, figure out what the highest frequency is that you will be passing though the circuit and calculate the value needed and then use the closest common value available. If this is varying DC I wouldn't get too worried about it...
There are often overlooked parameters using opamps (and other devices, even by "pro's") such as CMRR and PSRR (a little homework for you) that do not work properly without a bypass cap. A 0.01-0.1uF cap is mandatatory on every IC in a design. The additonal cap such as the 1uF Tantalum (notice one cap is ceramic, the other is tantalum, or could be electrolytic - another homework assignment) shown is optional but you as the designer of the circuit are responsible for deciding that (ever looked at a circuit board and seen caps soldered across pins on parts; they "decided" incorrectly).
Consider these things as "law" and you will find that your circuits behave correctly not only on the bench but in actual use, over time and temperature changes. Ignore them and one day your circuit may "flake" on you; by the time you get it back to the bench it'll work "correctly" again... until the next time...
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