Question on input noise current

[john curl]

I use the paralleled equivalent of a 2SK146 and a 2SJ73. This is 4 big fets in noise parallel, BUT they tend to cancel their own bias current. I doubt that fets will ever cause much of a problem, but stock bipolars probably will. The bias current compensated AD797 should be just OK. I can't be sure, but 1uA or so is a pretty low current with a few windings as the source.

[john curl]

Fred, as you know, I often go by the initials 'JC' Once, at Harmon Kardon in the late '70's, some engineers and I were discussing a design problem. Someone said, "Even JC could not solve this problem" I chimed in that in fact, I could not. They reminded me that they were not referring to me. ;-)

[SY]

John, what's your feeling about using transformers to get the source impedance (and voltage) high enough that the noise characteristic of FETs is closer to optimal? There seems to be a dizzying number of high quality transformers out there with widely varying turns ratios.

[john curl]

In this case, transformers are a waste of time. It is possible to parallel fets and get the noise equivalent of transistor action without added distortion or extra series resistance that the transformer must have.

[peranders]

Christer, a sound card especially 24 bit, the better ones, hardly ever need a preamp in order to measure noise and when you have very low noise you can connect a pre amp but your problem is when you measure a noise source not related to ground. This is your problem and I think this is really hard to solve.

May I ask your interest in this measurement desires? Normally you are only interested in the total noise out and in an amp the place where it's important (really important) is the input stage. If you want to measure the noise in the power supply voltage you don't need any diff amp because the noise is referred to ground.

Now SY and Fred may continue their debate....

[Christer]

Per-Anders,

I appreciate that you are trying to help, but it is almost impossible
to have a discussion when you insist on making false assumptions
and jumping to conclusions and base your argumentation on
those. I do not have a battery-powered scope. I do not
have a battery-powered PC. I do not have a 24-bit
soundcard. I do not have have wall outlets with safety
ground (earth?). I do not intend to measure only noise
with this amplifier (and has clearly said so from the start).

Let's try to get som facts straight.

1) The ground of the DUT and the instruments are not necesarily
the same, but may be floating wrt. to each other even have
have a voltage difference of approx. 115V AC caused by PC
mains filter. (Safety note: This is a special situation where the
voltage is current limited. High voltages are generally very
dangerous. Don't try this at home unless you know what you
are doing.)

2) I can easily imagine cases where I want to measure a small
voltage difference between two points, neither being the DUT
ground, and possibly at the same time also use the other
scope/PC channel to measure some other voltage referenced
to DUT ground. (For those with poor imagination, measuring a
current by measuring the voltage over a resistor not connected
to ground at either end would be one such case).

I see the following two possibilities:

A) An instrumentation-type diff. amp where the inputs may
be AC-coupled to overcome limitations in common-mode voltage
range.

B) A single non-inverting op amp where the positive input is
connected through a capacitor to the op amp pos. input and
the negative input is connected via a capacitor to instrumentation
ground. There should probably also be a resistor between the
two inputs on the op-amp side of the capacitors to give a more
well-defined load.

Variant A should work without problems (or maybe not, if we
cannot connect the DUT ground to the amp).
I am not so sure
about variant B, though. Would it work as well? Are there
further possibilities that I am missing?

[peranders]

Christer, I see what you (did before also) but thing is this common mode thing. Eventually you may notice that what you are trying to do is not an easy task. You have for instance limited common mode rejection and you will also have capacitve coupling. This will not get you clean uV signals. I wonder if the only way is battery powered gear?

I imagine that your are interested in noise in different places but the most important is at the output. This is what counts.

I have a isolation transformer and a stablization transformer at work and I must say when I want to do floating measurements often at 230 VAC potential I have to have the tounge in the right place in the mouth.

I think you want an amp with gain of 1000 at least, with +- 50 V common mode range, 120-140 dB CMRR and 0.1-0.5 uV input noise. I suspect this is a hopeless equation.

[Christer]

I realize now I have been fooling myself that a capacitve
barrier would get around the common-mode problem. It
took half a bottle of Alsace Riesling and some Chévre to
realize I i won't work. (OK, there was some coffee and some
Spice simulations involved too ).

Well, that means it won't be possible to use such an amp
in quite as many ways as I thought. It doesn't matter that
much, I suppose. Since most of the cost will probably go into
a shileded box., connectors, swtiches etc. I thought I might
as well try to make it as versatile as possible, although I
seem to have overestimated the possibilities.

Thanks for you help and suggestions folks.

[peranders]

You bought my ideas finallly? Still, noise is mostly interesting at the output and maybe also the supply voltage. Both places requires only a good preamp (the thing I suggested) in front of a voltmeter or whatever. Bear in mind your ungrounded PC.

Capacitors can only block DC. They are useless for common mode problems, which you now have discovered.

[Fred Dieckmann]

"You bought my ideas finallly?"

Damn now he is trying to sell those too.....

From what I have seen so far I would expect them to be very inexpensive.