Wow some great insights. Clearly the capacitor was huge and I still can't find any of the examples with the mux that even mention this. I'm wondering if I also need one for other ICs like the ADS1115? The information I've been reading also talk about additional capacitors of other sizes too?
Grumpy_Mike:
Well look at the data sheet for the one you are using ( the actual manufacturers data sheet ) but typically there is a 150R or so series resistance.
The board is from Sparkfun, and is just a breakout of the chip from TI. The only datasheet provided is from TI and says "Low on resistance" 60-70 ohm but I don't doubt that there is more including the breakout board. ...guess I'm fortunate I didn't get this one from ebay.
I'm not sure how to calculate how much the additional resistance would affect the voltage reading going thru the mux vs going directly since it's not a textbook voltage divider circuit. Give me a little break, I'm just a software guy.
Grumpy_Mike:
That would imply that either the resistor values on the potential divider is wrong ( not the ratio of the resistors but the absolute size ) or something else is not right.
I did double check the resistor values on the voltage divider. They are 47k and 22k (sparkfun) for a factor of about .319. I doubt they are exact and my multimeter is not precise enough to confirm. However they do correspond to the actual (multimeter) voltage measurements. I don't understand how the values of the voltage divider resistors ... which I thought only important for the ratio that caused the output leg voltage to be lower, would make any difference when combined with the addition of the mux resistance...not that I disagree at all I just don't understand that science. Can you give me a concept to google or research because it's way past my education on this. ...and then there is the likelihood that it's ALSO something else that I've likely mucked up. I guess I was under the misunderstanding that since this was only a voltage measurement no current was actually flowing ...so therefore resistance would have very little affect on the voltage.
Grumpy_Mike:
I think it is time to post a schematic. Just a point, I hope you are not trying to do this on solderless bread board are you? They are notorious for bad joints.
Re the schematic - I haven't done this yet because they are just basic duplication of the example circuits ...so I'll have to install software to do this I guess. And even for a software guy this circuit was pretty straight forward. It will probably take a little time to do this. I'm certainly not disagreeing and greatly appreciate your willingness to have spent the time you have already spent in assisting.
Guilty as charged. Yes doing it on bread board...which is how I could use same sketch and move from mux to direct and back easily. ...but I'm not planning on relying on the data while on the breadboard just prototyping the circuit. ...and yes I agree my last major hair pulling session was caused by a bad connection so I probably just need to re-connect everything a few more times.
Grumpy_Mike:
The circuit is not actually how you describe it.
Ouch! OK it is certainly possible that it was not executed as designed and deserves to be reinspected by quality control.
Many thanks for your help.