Here's my issue. When the PID is in the aggressive state, whereby it's driving its output down to the setpoint, my entire current draw from the 12V supply by the whole board is around 4.25 A. When the transistor goes into high impedance, the current draw goes down to 0.4 A. As a result, the Trinamic board becomes inoperable (which is something I will hopefully address myself in the future) AND the LM35 analog input readings drop significantly.
The transistor (IRL530NS) is used to either allow or disallow the cool side of the Peliter sensor from receiving voltage (if that doesn't sound right, lingo-wise, could you reword it so I know how to voice it properly for the future, thanks!)
Are you keeping the high current wiring as twisted pairs? Its a very very good idea to do so,always keeping a supply wire twisted together with its ground return, etc. Open loops ofunpaired high current wiring are radiating magnetic interference all over the place.
The transistor (IRL530NS) is a low side switch to control current through the Peltier device.
Sounds like your AREF voltage is changing when you drop the 4 Amp load, causing the VREF voltage and ADC result to change, is your Arduino being supplied from the same PS as the Peltier? Might try using the internal VREF.