alexandros
Well-known member
I'm trying to use multiple ADS1115 chips to read potentiometers at differential mode, to get the full 16-bit range. To achieve this I'm using the LM4040 voltage reference, passing a 4.096V to the pots (and ground to the other end), and 2.048V to the ADS1115 chips. I'm having a hard time calculating the resistors I need for the LM4040. I would like to have a variable number of pots with one LM4040, from 1 to 30 pots in total. I have already posted this question in Adafruit's forum, but it's now closed. The thread is here.
From the information I was given in this thread, I concluded that for 10K pots with a 4.096V reference, and a 5V supply, I need 68 Ohm resistors. I think I got this right by applying the following calculations:
If this is right, then what I can't get right is the resistor used for the 2.048V reference wired to the ADS1115 chips. Following the thread of the link above, from what I understand that should be 220 Ohms, but that doesn't work. As I add more pots to the circuit, the maximum value these pots output gets lower.
Here's the circuit I use for the two LM4040 references:
Can anyone help with the resistor values I have to use?
From the information I was given in this thread, I concluded that for 10K pots with a 4.096V reference, and a 5V supply, I need 68 Ohm resistors. I think I got this right by applying the following calculations:
Code:
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
Here's the circuit I use for the two LM4040 references:
Can anyone help with the resistor values I have to use?