First of, the standard referenced (MIL 202, M308) in the data sheet is not covering the tradiational 1/f noise frequency range (which is from DC to some ten Hz). The frequency range covered is geometrically centered arround 1KHz and is 1kHz wide. Also, the -55dB figure, as per spec, is defined as the ratio of the DC voltage applied accross the resistor, measured in V, to a RMS noise voltage, measured in uV. So we are talking about appr. 13nV RMS excess noise here.
Therein lies the problem with Datasheets and sketchy noise specs - and trying to filter out the noise introduced by LT5400 into an otherwise precision circuit.
The 1/F noise is the problem - If you've ever tried this: You build a 7-10V voltage booster with a clever application of LT5400. You realize it's a little too noisy for your ADC system. You low-pass filter your 10V output say to 5Hz, thinking you'll get rid of noise...Then you test everything and realize you got some noise energy at 1Hz and below. So you dink around with more filtering, say 0.5Hz. Now you realize you've got more noise down below .1Hz, and maybe even a bigger problem....and so on. Then you realize what a rat-hole that 1/f noise is doing to you.
It is fairly difficult to get rid of that noise and have your Vref stable for measurements over longer and longer time frames. Just speaking from experience here.
At some point you realize it was easier to just go with some good PWW's, or if your customer demands it a custom metal film ratio set for your boost amp divider. It is always better to -avoid noise when you can- rather than try to remove it from a signal later. At least that's what we try for.
The next thing you realize is: Do you really need 10V for your application?
But that is a story for another day.
Couple of other "Gotcha's" on LT5400 paks - just a general head's up if you haven't used these before:
1) The only thing going for them is pretty good TCR ratio matching. The resistance absolute value (20%) and TCR is absolute garbage compared to real precision resistors - these are designed to only be used as
ratio groups. Be careful of that if your amplifier feedback circuit also depends on a total -absolute- value (including individual TCR of around 8ppm) of the divider as well as the divider -ratio- itself.
Note that the TCR ratio of LT5400 isn't anything that can't be done with good wire-matched PWW that are thermally tied together. Or a good film resistor ratio set. The advantage of using real resistors here is you also get good control over the absolute resistance value AND much better control over individual TCR AND much better mechanical stress forgiveness. Your application may or may not need those characteristics.
2) The withstanding voltage of LT5400 is surprisingly low. Remember these are IC style, small diffused resistors. Be warned if you are using these as an input-signal divider; you might need surge protection -ahead- of the resistor pak. These are not very forgiving of an over-voltage stress.
3) These resistors - being small- have a surprising crosstalk especially on adjacent units. Be aware of that if you have any signal with higher dv/dt or sensitive AC signals - like for instance if you're using these to build a differential / instrument amp, etc. It's best to test on your own board before you believe anything on that datasheet.
4) Mounting a LT5400 is not a hand-soldering job. For minimal package stress these want to be properly reflowed and watch out for solder paste thickness - if you can get them to "float" slightly off the PCB that can help. The thermal pad is critical on the back side but remember this is a small capacitor to each resistor element also. Lead-Free solder can introduce different stress characteristics over regular solder - make sure to test your board out mechanically so you know what the effects are.
Lots of things to watch out for with those 5400's. They might be a good fit for some applications but for us they've always been somewhat disappointing and never helped the profit margin over other quality resistors. Your mileage may vary.
They are smaller though. <Grin>