How many pots can I wire to an LM4040, again...

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alexdry
 
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How many pots can I wire to an LM4040, again...

Post by alexdry »

I had started this thread last summer viewtopic.php?f=8&t=181113 but now it's locked and I can't go on making posts there (and I was too late to thank Mike for all the info), so I'm starting a new one. I'm still trying to get my head around Mike's lesson on electronics. I want to have a variable setup with a bunch of ADS1115 breakouts reading potentiometers, and an LM4040 voltage reference. Since the LM4040 breakout has fixed resistors that don't work in my case, I'm trying to not use this breakout. According to Mike, if I have understood his post, when powering up my project with 5V, and connect the 4.096 reference of LM4040 to one pin of the potentiometers, if I want to have a variable number of pots, between 1 and 30, the resistor I should choose is 68 Ohms. I have concluded to this value based on the following calculations:
5V - 4.096V = 0.904V
0.904V / 14mA (maximum current to flow through the LM4040) = 65 Ohm (choosing the next value from the E12 series, which is 68)
0.904V / 68 Ohm = 13.3mA
13.3mA - 1mA (so that the LM4040 always has at least 1mA flowing through it) = 12.3mA
12.3mA / 409.6uA (if this is the correct value of current across a 10K pot with a 4.096V ref) = 30

If all the above is correct, I can get a variable number between 1 and 30 pots with one LM4040 with a 68 Ohm resistor, right?
I want to read the pots in differential mode on the ADS1115, so I'm wiring the 2.048V reference of LM4040 to ADS1115's analog pins 1 and 3. Since I'm powering up with 5V, I used Mike's calculations in the original post, which result in a 220 Ohm resistor (though his calculations are for pots with a 5V power and a 2.048V ref, not for the ADS1115). This setup though doesn't work. I believe that the 68 Ohm resistor I have chosen is correct, but there's something wrong with the 220 for the ADS1115. Perhaps the current flowing through in this case is not 204.8uA, but something else, and that's where the whole thing goes wrong?
As you can see, I don't have the necessary background in physics to really grasp all this... any help appreciated.

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adafruit_support_mike
 
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Re: How many pots can I wire to an LM4040, again...

Post by adafruit_support_mike »

alexdry wrote:If all the above is correct, I can get a variable number between 1 and 30 pots with one LM4040 with a 68 Ohm resistor, right?
Yep, you've got it.
alexdry wrote:I want to read the pots in differential mode on the ADS1115, so I'm wiring the 2.048V reference of LM4040 to ADS1115's analog pins 1 and 3. Since I'm powering up with 5V, I used Mike's calculations in the original post, which result in a 220 Ohm resistor
That's correct. 5V-2.048V=2.952V, and 2.952V/14mA=210.8 Ohms. Stepping up to the next higher standard value of 220 is correct, and will give you a total current of about 13.8mA.
alexdry wrote:This setup though doesn't work.
What does it do that it shouldn't, or doesn't do that it should?
alexdry wrote:Perhaps the current flowing through in this case is not 204.8uA, but something else, and that's where the whole thing goes wrong?
If you're still using 10k pots, 2.048V/10e3R=204.8uA. That's correct, and you should be able to connect about 60 10k pots in parallel with the LM4040 (204.8e-6*60=12.3mA).

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alexdry
 
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Re: How many pots can I wire to an LM4040, again...

Post by alexdry »

Well, the pots drop their range when I plug more of them. In Teensy's forum someone wrote that it sounds like a power issue. Also, the only LM4040 I have is Adafruit's breakout, so I cut the traces from the LM4040 chips to the resistors, to be able to add arbitrary resistor values. That is not ideal and could possible lead to issues, or couldn't it?

As for the total current flowing through the ADS1115 reference pins, I was told that the value I'm looking for is the "Differential input impedance" found in ADS1115's datasheet, page 7, table 7.5. That value is 4.9 MOhms which is quite huge, so the resistor used can be quite high in this case. So, I decided to use the 750 Ohm resistor found on Adafruit's LM4040 breakout. I guess I'm good to go with all the info I have for now. Thanks a million for all the information in this thread and the previous one.

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