OK, gang, I got my breast in the blender for not reading the data sheet closely enough. And the board is already done for twenty of my students and reordering boards is not an option this late in the semester.
Here is the problem: I have a raw +17 volt power supply (transformer, bridge rectifier, large filter capacitor) feeding an LM317 regulator to provide a regulated 1.5-15 volt output.
In between the raw +17 supply and the regulator is a 0.25 ohm current sense resistor (2 0.5 ohm 1/4 watt carbon film in parallel) so that the sense differential voltage goes from 0 to 0.25 volts (absolute 17 to 16.75) as the current drawn from the supply goes from 0 to 1 amp.
The problem, as some of you will be quick to note, is that I used an LM358 opamp as a diff amp to make this 0 - 0.25 volts go to 0-10 volts (g = 40) to drive the front panel voltmeter so that the student could monitor either voltage or current with the same analog pointer meter via toggle switch select. Regular old diff amp configuration ... series 2.7K to the (+) noninverting input with a 100K from input to ground and another 2.7K to the (-) inverting input with a 100K feedback resistor from output to (-) input
LM358 does NOT like the inputs to be equal to the supply volts. It (after a thorough study of the data sheet) likes the inputs to be at least 2 volts below supply. Damn. Blew that one out my ass, I did, I did.
So the boards are done with the LM358 footprint. I need an opamp that can stand the inputs to be at or within a few hundred millivolts of supply. I'd prefer a popcorn part since the meter only has to indicate very roughly what the current is. Using 5% resistors doesn't guarantee precision.
THere you have it. Any suggestions for a cheap and easily obtainable opamp replacement for the 358 (yes, it is a SMD in the SOIC configuration so the kids get a taste of microsoldering)?
Jim