AD590 and Op-Amp

[drkidd22]

Hello,

I was looking at a circuit that uses the AD590. Upon looking up the datasheet I see the circuit I saw is similar to this one. The only difference is that it uses +5V instead of 15 and on the AD590 V- it's -15V.
How is the gain determined here to get the 100mV/C output? Is it -(R2+97.6K) / (R1+35.7K)?
I was trying to look up other similar op-amp circuits, but can't get around this one.

 

[OBW0549]

How is the gain determined here to get the 100mV/C output? Is it -(R2+97.6K) / (R1+35.7K)?

No, the OP177 op amp simply acts as a summing current-to-voltage converter, not a voltage amplifier.

The 100 mV/°C scale factor on the output is equal to the 1 μA/K output from the AD590, multiplied by 100 kΩ (the 97.6 kΩ resistor, in series with the scale factor trimpot R2).

The 10.00 volt reference voltage from the AD581, together with the 35.7 kΩ resistor and offset trimpot R1, inject 273 μA into the op amp's summing junction to make the output zero at 0 °C.

 

[AnalogKid]

There are two things going on at the same time, DC offset and gain. If the left side of the 35.7K were grounded, your equation would be all there is too it. But to have an output voltage that goes negative for negative temperatures, you need to compensate for the DC output pedestal of the AD590. It sinks a current that is directly proportional to degrees kelvin, so at 0 degrees celsius the 590 is sinking 273.2 uA. The 581 provides this current through the 35.7K and R1.

The nice thing about this circuit is that you do not have to go through multiple temperature iterations to calibrate it. At one particular temperature, the gain and offset are independent. At 0C, both OP177 inputs should be at GND, and you adjust R1 to get there. If the input is perfectly balanced, the output will be 0 V no matter what the forward gain is, because 0 times anything is 0. After that, raise the sensor temperature to some other known value, and adjust R2 for the correct output.

ak