Re:Ask HN: What's the purpose of the resistor in this circuit?
by mindslight on Friday March 20, 2009 @07:39AM
(#524773)
I don't know the Arduino specifics, but if it's just an analog input on the Atmel chip, the datasheet says that's at least 100M ohm. (A pic18 actually has a very low recommended source impedance - 2.5k ohm, which can bite you if you're not careful, but that doesn't seem to be a concern here.)
If you're going to use an op-amp, you'd be better off using a different configuration, say with the thermistor as the feedback resistor, to get a linear response.
Taking a look at the circuit on your page, it seems like you could drop R9 and R3 (while changing the value of R8), as the function of those 3 resistors is only to feed a fixed current into the node of pin 2 (which is always held at 0V by the op amp). In fact, if you play around a bit, you should be able to get the proper range using only one op amp, at which point you can use a 5V rail-to-rail amplifier and get rid of that dual supply.
If you're going to use an op-amp, you'd be better off using a different configuration, say with the thermistor as the feedback resistor, to get a linear response.
Taking a look at the circuit on your page, it seems like you could drop R9 and R3 (while changing the value of R8), as the function of those 3 resistors is only to feed a fixed current into the node of pin 2 (which is always held at 0V by the op amp). In fact, if you play around a bit, you should be able to get the proper range using only one op amp, at which point you can use a 5V rail-to-rail amplifier and get rid of that dual supply.