Ken Smith a écrit :
I need a 10V reference with a noise level below 5nV/sqrt(Hz). My current
thinking is that the only way to get this is to add a huge amount of
filtering to a normal reference. Are there any references with a low
noise spec. A care most about the frequencies above 0.1Hz, Below that
point, the noise can increase by 20dB/decade without causing too much
trouble.
I had some similar requirements for really low noise PSUs two year ago
and I had to get down to 0.1Hz too but I managed to have this point
somewhat relaxed.
The best ref. you can come with (well, the best I could find) are the
XFETs one.
All the refs I found were about 100nV/rtHz but that's not hard to
filter... except for the low freq part. Also they are not all 10V, so
for you the interesting data is the ratio of low frequency noise to ref
voltage.
The good old AD586 at 4uVpp/10V is still a good value, which only
equaled by the more recent 5V ADR445 (2.25uVpp/5V)
Are you really wanting under 5nV/rtHz flat down to 0.1Hz? That's pretty
tough.
Over a 0.1Hz-10Hz BW this is about 20nVrms or 120nVpp which is just
within reach (guaranteed) of LT1028/1128 and AD797.
Unfortunately the low LPF corner frequency needed to filter the ref
voltage requires high impedances which won't do any good with these
bipolar opamps high current noise.
The OPA227 with 3nV/rtHz and 90nVpp(typ) has a 2.5 to 5 time better
current noise, but is still high at 0.4pA/rtHz and skyrockets at
20pA/rtHz at 0.1Hz which will require high value caps with their own
noise pbs (over 10000uF, no even speaking of LT1128 or AD797).
Jfet opamps are really bad at low frequency. The best is probably the
AD745, but will still miss the target by a 10 factor at 0.1Hz