I think the gain of AD8551 in Figure 60 is R9/R8=124000/453=273.7.
But as show in datasheet, the output voltage track temperature at 10mV/℃, the
input voltage track temperature at 40.7uV/℃, Gain=10000/40.7=245.7.
The figure is showed below
Consider the thermocouple as a voltage source with zero ohms output impedance.
It is in series
with the 453 ohm resistor. The parallel combination of R2 and R3 is 52.57
ohms. This is in series
with the 453 ohm resistor, so the effective input resistance is 52.57 + 453 =
Now calculate 124k/505.57 = 245.27 gain.
The AD8551 datasheet was done over 10 years ago, so the apps engineer is not
so I cannot ask him about it. We don’t have any type K thermocouples in the
lab so I cannot
confirm that the circuit works, but I think if the customer builds the circuit,
he will find that
it is very close to what he wants.