# How to calculate opamp input noise for DC input?

I'm trying to design a circuit with an operational amplifier (OPA625). The input signal is amplified and sent to an ADC. In order to estimate the input noise, I'm using the following equation from an Analog Devices tutorial:

$$v_{n,\text{rms}} (F_L, F_C) = v_{\text{nw}} \cdot \sqrt{F_C} \cdot \sqrt{\displaystyle\int_{F_L}^{F_C} \frac{1}{f} \mathrm{d} f}$$

$$v_{n,\text{rms}} (F_L, F_C) = v_{\text{nw}} \cdot \sqrt{F_C \ln{\dfrac{F_C}{F_L}}}$$

$v_{\text{n,rms}}$ — input noise RMS

$F_L, F_H$ — frequency bandwidth

$v_{\text{nw}}$ — voltage noise density in the white noise area

It's quite easy to calculate the noise for the specified bandwidth. But what happens when $F_L$ is close to 0? In the Analog Devices' tutorial I mentioned earlier, $0.1\text{ Hz}$ is used as a lower frequency limit, but how close it should be to 0? How do I estimate the noise for DC input?

TI's Analog Applications Journal includes an article about noise analysis for an op amp driving an ADC as in your application. The author uses $F_{\text{L}} = 0.1\text{Hz}$ with the following rationale:
Additionally, the OPA625 datasheet specifies the input noise voltage starting at $f = 0.1\text{Hz}$.
$F_{\text{L}} = 0.1\text{Hz}$ seems like a good choice since all three documents use it.