I've got what I hope to be a simple question for all of you!
Let me preface with a little background. We are constructing an ADC circuit used in conjunction with a Raspberry PI to monitor cycles of some industrial machines. If a constant Voltage, Say Vi is sent to the machine, Vi will be returned if the machine is not in cycle, and a lesser voltage, Vo if it is.
To give a bit more detail on the machines, they are industrial plastic injection machines. They are all equipped with a small circuit which I've shown below. When the machine is not running, no current is flowing back from Vin. When the machine is running, the transistor is triggered, so some Vout < Vin is returned. Below is an image of the circuit that is used in the machine:
The warehouse is currently using a control system which, if we tap into the voltage differentials between the Vin & Vout to the machines, the circuit is quite simple. However, we need to remove the control system entirely so that isn't an option. We discovered that the impedance of the PI isn't enough to get accurate readings so an external voltage source is necessary for what we need, which leads us to my questions today.
where M0 - M6 represents the machines, and R0 = R1 = R2 ... = R7
A quick thing to note: the software I developed for this analyzes dV/dT to determine peaks and valleys so specific voltages are not required.
Here are my questions / concerns:
From Kirchoff's law, I believe the voltages sent to the machines will all be the same. Will the current be the same, or will the current draw when the machines are running throw off the other channels?
Does it make sense to attach Vref of the ADC to a channel from the external source?
Should the Analog Ground of the ADC be connected to ground of the external source?
Will we need to use any series references / OP Amps for this type of circuit?
Thank you in advance for any help. Please bare with me as I am EE noob (Software dude here).