The JFET input noise may have a different spectral composition than the precision BJT parts. Often one is indeed interrested in the frequencies lower than 0.1 Hz. The 0.1 - 10 Hz range is more like choosen to be easy to measure (AC coupling still works and 2 decades is large enough so that the exact filter shape is not that critical (still a factor to cause condusion). For the lower frequency noise a relatively fat DMM (to work well in 1 PLC mode) should be a reasonable choice. If not with a low of DC offset like a voltage reference the amplification can be quite large in a DC coupled configuration.
I discovered the OPA140 when researching suitable improved replacements for the old AD542, which at one time was about the best precision JFET part. The LT1012 is the bipolar alternative that I would have used by default. My primary interests were high common mode rejection, low 0.1 to 10 Hz noise, low input bias current, and low input offset voltage drift.
The difference in noise is apparent. The JFET in this case has lower high frequency noise, but its higher corner frequency means comparable low frequency noise.
OPA140 140 dB cmrr 1 uV/C max 0.5 pA 0.1 to 10 Hz 230 nVpp 8 Hz 5.1 nV/SqrtHz
LT1012 132 dB cmrr 1 uV/C max 30 pA 0.1 to 10 Hz 500 nVpp 2.5 Hz 14 nV/SqrtHz
LT1001 126 dB cmrr 1 uV/C max 500 pA 0.1 to 10 Hz 300 nVpp 4 Hz 9.8 nV/SqrtHz
As you point out, it is straightforward to make a spot noise measurement over the 0.1 to 10 Hz range. Since the corner frequency usually lies within this range, it makes for a good proxy measurement of low frequency noise. However older chopper stabilized parts have *higher* noise within this range, so are more suitable for even lower frequency applications, where their 1/f noise is flat, and a 0.01 to 1 Hz test is required for proper evaluation which is not so easy.
I have used a high resolution DMM to make low frequency spot noise measurements before and got excellent results which agreed with the datasheet values. I am looking to improve on this and then run some tests on modern chopper parts.
A point to whatch for is thermal stability: temperature fluctuation combined with a TC may look a lot like 1/f noise.
Yea, at low frequencies it is difficult to distinguish 1/f noise and thermally caused drift.