Oznog,
Your post is very close to the info I need for my project, but I'm still confused about what exactly source impedance refers to, and how to measure it.
With your example of 2 x 5kohm resistors to divide the 8v voltage, i don't understand why the impedance at the pin is only 2.5kohm. I would have thought it would be 5kohm, because there is only one 5kohm resistor between positive rail and the pin. So, to provide current required for AD, this is only ever drawn from the positive rail, through a 5kohm resistor, to the pin. So, why is this not considered 5kohm input impedance?
Also, would you be able to expand on ways to circumvent this limiation: I am trying to measure a signal from a force load cell (varies impedence with applied force, where increasing force leads to decreasing resistance) and its minimum impedance is still a lot above 10kohm. So, should I be op-amping my signal? Do different op-amps have different output impedances? Is there a standard op-amp design for interfacing with PICs?
Thanks,
Brian
Don't recall the name of the electrical law, but the impedance for a node is all the impedances leading to DC voltages, in parallel. So if you want to measure a 12v battery voltage and divide it with a 10k resistor and a 4.7k resistor, the impedance seen
at the ADC pin is 10k in parallel with 4.7k ~3.2k. Whereas the battery sees a 14.7k load impedance for the divider.
Honestly? I doubt the PIC's ADC will give you acceptance load cell resolution. How much % does the load cell resistance change from your max and min load conditions, and how many codes do you need to realize in that range?
A load cell is just a resistance bridge, and works fine at voltages below its max rating. So DON'T give it 8v. Just give it 5v!
An op amp will not only remove the impedance issue as seen by the PIC, but can also greatly amplify the range. So what was +- 20mV may become +/- 2.5v. However, lemme warn you. It is difficult to get a differential amp balanced. If it's not balanced, the PIC won't get 2.5v=no load. If you try to software-adjust the center for say 4v=no load, then the usable dynamic range is screwed. Normal resistor tolerances can easily throw off this balance, and all amps of this type using external resistors must manually be adjusted with a trim pot. Trim pot adjustment is a minor pain and temperature drift in the resistors and pot itself can affect the center voltage.
Generally you want the ADC, op-amp, and PIC running on the same supply voltage. If you don't do that, the op-amp or ADC pin could get >Vdd on the input pin which is a "bad thing".
I prefer an actual differential load cell ADC, if at all possible, like the LTC2410. That'll give incredible accuracy and drift is essentially nonexistent. It's fairly cheap, and the more accurate multi-turn pots are actually not cheap. You'll need some sort of breakout board though.