Am 01.08.2016 um 22:16 schrieb David:
This duplicates the problems encountered when trying to quantify low
frequency noise from a voltage reference; it is difficult to make an
low frequency high pass filter with lower noise than the lowest noise
references and the capacitor is the problem.
I beg to differ. Voltage references are not that wonderful. Bandgaps
live from amplifying
small voltage differences and stable Zener references at 6 to 7 Volts
are plagued by
avalanche noise. And that includes the LT6655 band gap.
WRT short term stability all of these are eclipsed by 2.7 / 3.3 volt
zeners and by LEDs.
Even the LT6655 gains a lot of noise performance from an active filter
with AD797 /
ADA4898 op amps and even resistors and 6V/1000uF Nipon Chemi SMD
electrolytics.
I have made some absolute noise voltage measurements:
< https://www.flickr.com/photos/137684711@N07/albums/72157662535945536 >
LEDs and Zeners are measured with bias from a 1 or 2k wire resistor and a
14V NiMH battery. I find the HLMP6000 LED really impressive and the
LT3042 regulator.
The preamp is 20 ADA4898 op amps in parallel ( i.e. 220pV/sqrtHz), the 0
dB line is 1 nV/sqrt Hz.
Everything was fed from batteries in a box in box in a box and then
after +80 dB passed to
an 89441A vector signal analyzer.
The input capacitor of the preamp is 20 times 10uF WIMA foil, that is
not enough for the
low frequency corner as we do not see the real 1/f noise below 20 Hz.
What we see looks more like GR noise, spectrum-wise, and it is really
the insufficient shorting
of the 10K bias resistor through the input source and coupling cap.
I have bought some wet slug tantals as proposed by Jim Williams (see
below), 10000 uF bring
the right 1/f behaviour but at very substantial cost :-( At least for
small input voltages alu
electrolytics do not seem to make a difference. I did not test large
voltages.
I'm working on a new amplifier based on IF3602 or BF862 FETs that can
use 10u foil only.
When it's done I'll repeat these measurements.
In Linear Technology Application Note 124, Jim Williams discusses the
problems with electrolytic capacitors for this type of application. I
have read that you *can* get away with aluminum electrolytics if you
grade them for low leakage and low noise. The dielectric absorption
is also a problem unless you can wait hours for best performance.
JW has the added handicap that he wants to keep the the long term and
absolute stability
of his reference and so cannot afford any voltage drop on a series R. We
do not share that
problem on an EFC line because the C stands for control and if the
voltage there does
never change for some other reason we have probably made a bad decision
with regard
to loop gain.
And large resistors may feature more noise voltage, but that increases
only with the
root of the resistance. The filter corner drops in a linear way, so a
large resistor may
really help. The tiny noise voltage of a reasonable resistor must be
seen anyway in
the context of say, a 10811A that tunes +- 1 Hz for 10 Volts on the EFC.
regards, Gerhard
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