Apex HV opamp issues.

I don't really like that noise plot for the metal films... I did it a while ago. (Maybe I'll repeat it some day.)

But now you are making me think. 1E-7 V/V per decade, So with ten volts of bias and let's say a 1 Hz to 1 kHz bandwidth, that would be 3E-6 V(rms)(?) and a noise density (at 1 kHz) of ~1E-7V/rtHz... that seems huge! (even it I did loose a factor of pi in there somewhere.)

George H.

By my few measurements, the real number is at least 30 dB lower than that.

Cheers

Phil Hobbs

Yes. The "excess noise" I've observed is usually highly voltage dependent, proportional to voltage, etc. It may be non-existent at low voltages, like under 10V, but severe above 50 to 100V to 1kV. And it can vary dramatically from part to part. E.e., made some +/-2KV amplifiers, with sub 1ppm noise levels, and 20% of the 10kV feedback resistors were very noisy and had to be replaced.

But troublesome FET transistor noise may not have classic 1/f characteristics at all, e.g., "telegraph noise" and "popcorn noise" are not strictly 1/f. Let's talk after you've digested our measurements and the discussion in Chapter 8. The book is being shipped as we speak. My order is in.

Excellent! I look forward to the discussion once my piece of the 40 tons arrives. I wonder if the voltage dependence is a surface thing. (could be the outer surface or some layer underneath.)

George H.

All resistors have the same Johnson noise at zero volts. Some, not metal films, have shot noise with current flowing, which is simple electron arrival statistics.

I conjecture that some nonmetallic resistors, like cermets, may have shot noise when biased. They also have excess, 1/f ish, noise, maybe from their polycrystaline confused nature. Tiny temperature variations must play heck with a grainy cermet-like structure.

I've done a little testing on high-ohm resistors, but just enough to get me through a project. I needed 100 Mohms and wound up using two Dale custom-ordered 50M axials in series in mid-air, hopping over some surface-mount parts. Cermets were awful. This is fairly difficult stuff to measure, but would be a good student project or something.

I've never seen excess shot noise in resistors.. (excess noise that is white in frequency.) but I haven't looked hard either. According to Landauer a resistor should reduce the shot noise as 1/N, where N is the number of scattering events as a "single electron pulse" travels the length of the resistor.

I'm not sure how to think about "scattering events" and 10 ns pulses.

I wired up my LTC6090 HV opamps today. Fired it up on the test bench. no smoke, but all I saw noise-wise, was first the room lights, and then

George H.

Was that basketball? I never understood that game. It seems to be a bunch of guys with endocrine problems, running back and forth all night. Looks chaotic to me.

Well just a number out of the air... well typical PMT pulse width, that's more than one electron of course.

Yeah, I played it a lot when younger, which helps.. Team sports are fun. I wish I still had good knees, I hate running, but running after some ball is different. (I'm trying to win!)

Basketball, soccer, tennis, volleyball... I still play ping-pong, a pretty small ball, and not much running.

George H.

Cermets and high value carbon comp resistors rely on the spreading resistance of minute contacts between grains--IOW the effective length of the resistor is much shorter than its physical length.

Cheers

Phil Hobbs