I plan to use an AD7792 or AD7793 with a 2-wire PT100 sensor.
The connections will be like this:
I need to calculate the reference resistor value.
Example: PT100, 2-Wire, 0°C to +100°C
Resistance = ~ 100 ohms (0°C) to ~ 140 ohms (100°C)
Iexc = 1mA
Voltage across RTD:
At 0°C: 1mA x 100 ohms = 100 mV
At 100°C: 1mA x 140 ohms = 140 mV
Voltage in the comparator (PGA=16):
At 0°C: 16 x 100mV = 1.600V
At 100°C: 16 x 140mV = 2.240V
Rref = 2.240 V / 1 mA = 2.24 k ohms
Vref = 2.2K x 1mA = 2.2 V
Vtotal (IOUT to GND) = 140mV + 2.2V = 2.34V (OK, under 3.3V)
But at the voltage comparator we will have both positive and negative values:
At 0°C: 1.600V - 2.200V = -0.600V
At 100°C: 2.240V - 2.200V = 0.040V
Is this OK?
If yes, would it be better to use an 1.8 kohm resistor? (it would be next the middle of the range)
If no, should I use a reference resistor of 2.7 kohms?
Here is the calculation for a 2.7 kohms resistor:
Vref = 2.7K x 1mA = 2.7 V
Vtotal (IOUT to GND) = 140mV + 2.7V = 2.84V (OK, under 3.3V)
Voltage comparator difference:
At 0°C: 1.600V - 2.700V = -1.100V
At 100°C: 2.240V - 2.700V = -0.460V
Which would be the best reference resistor value for this case?