The amplifier’s offset voltage is multiplied by the circuit’s noise gain and appears as an output error. The DC noise gain of the circuit is 1+RF/RD. Noise gain expresses how the signal between the amplifier's input pins appears at the output. Imagine the feedback resistor to be 1 MΩ, and the diode resistance is the same. Then imagine an absurd amplifier offset voltage of -1 V. That means the negative pin is 1 V above the grounded positive pin. That 1 V will make a 1 µA current flow through the diode resistance. The amplifier will adjust its output to allow that exact current to flow from the right to left in the feedback resistor. A 1 µA current across the 1 MΩ feedback resistor makes a 1 V drop across the resistor. The positive side of the voltage is on the right side of the feedback resistor, since that is where the current is flowing to. Remember that the feedback resistor is “sitting atop” the 1 V offset voltage imagined at the amplifier negative pin. Therefore with the left side of the feedback resistor at 1 V, the right side is 1 V higher, hence the output must be 2 V. This is where the “1+” term comes from in the noise gain. The noise gain is 1+RF/RD. With RF equal to RD, that means the noise gain is two. That is why the absurd 1 V offset appears as 2 V at the output. Now imagine the diode resistance drops in half, to 500 kΩ. There is now 2 V across the feedback resistor, and the noise gain is three. The offset would create a 3 V output error. Diode resistance is usually hundreds of MΩ at room temperature, so if the feedback resistor is 100 MΩ, the ratio of feedback to diode resistance is 1/1, for a noise gain of two. But diode resistance drops a great deal at higher temperatures, some diodes halve the shunt resistance for every 10°C temperature rise. So at 70°C, the 100 MΩ diode resistance will fall to 4.3 MΩ, and the ratio between the feedback resistance and diode resistance is 23, so the noise gain is twenty four. That means the op-amp’s input offset voltage is multiplied by twenty four when it appears at the output of the circuit.