I was looking at the schematic for Dave Jones' uCurrent, and I can't seem to make sense of why there are 270 ohm resistors on the inputs and outputs of the op amps in his circuit. The resistance is low enough that it wouldn't affect the filter in the feedback loop, and is negligible compared to the input resistance of an op amp.
Any ideas why these resistors are put in series with the input/output of these op amps?
Here's a snippet of the main section I was looking at, with resistors R12 and R8. The full schematic can be found here: Full uCurrent Schematic. Here's a link to full description of the project: Full uCurrent Design Article. The schematic is on page 16, and a short sentence on the resistor in question can be found on page 8 (search for "R10").
Dave mentions that he uses the resistors to provide "output stability". How exactly does this small resistor provide stability, and why?
Best Answer
Regarding R10, the virtual ground is driving stray capacitance. If you loaded the output of U2 directly with that capacitance it would probably oscillate.
R8 actually hurts stability of U4. It would improve it only if C4 was connected to pin 1, however in most cases users will not load the output capacitively (eg. by attaching a long cable) so it's probably meant to limit the short-circuit current from the amplifier, and keep it from driving the virtual ground too far.
R12 limits current in case of a fault such as a large voltage applied to the input- he uses 270R to reduce the number of different part values- but a larger value might be appropriate too. Also perhaps it acts as a jumper for his single-sided layout. ;-)