We are using the CN-0359 reference design to evaluate the potential to replace a currently configured, manual conductivity (i.e., non-autoscaling) system. The reference design performs very well, but we are confused by what we are seeing with the RTD circuit. When a PT100 or PT1000 is connected, the temperature displayed is correct. However, in one of our system test procedures, we typically substitute a precision resistor of known value (in our case, 1000 Ω ± 0.1%) to get a specific temperature reading. When we attach the precision resistor to the temperature sense inputs of the CN-0359 (2, 3, or 4 wire), the system identifies it as PT1000, which makes sense given its value, but the temperature displayed is slightly negative (e.g., -0.5C). Why would this be the case since we are only measuring a resistance?