some questions about operate amplifier!!!

[ChillTerry]

first, thank you very much for the answer about last question, i am really a freashman about this field but eager to revive your project, and the procceeding is going to 50% and now i have some theoretic question about the amplifier, why do you choose such value of risitors and capacitors to amplifing the thermometer value, (if it is possible, may you give a mathematical deduction?) and why does the diode directly connect to the GND? what do you think of the digtal GND and analog GND, why do not you use them to make a segragation so that to supress noise?

thanks again!

[wagiminator]

Looks cooool! D1 (on your schematic) limits the voltage on the positive input of the opamp to around 0.6V (forward voltage of the diode). If the soldering tip is powered, the voltage would be otherwise 24V, which would destroy the opamp. 0.6V is more than enough, the voltages created by the thermocouple are far below. R8 limits the current passing the diode. (I <= (24V - 0.6V) / R8 = 2.34mA). C11 and C12 reduce the noise. C12 must be quite low, because it can only discharge via R8 (RC discharge time ~ 5 * R8 * C12 = 110µs). C11 was determined empirically. R6+R7 and R9 form the negative feedback of the opamp, setting its gain to (R6+R7+R9) / R9 = 201. The board is quite small in order to be able to sufficiently separate the analog from the digital area; at least I wasn't able to do this at the time - at that point I had only been dealing with electronics for about half a year. Before that I didn't even know what an opamp was ;-)