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From: George Herold on 11 Aug 2010 19:26
On Aug 11, 7:10 pm, John Larkin
<jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> On Wed, 11 Aug 2010 06:10:15 -0700 (PDT), George Herold
>
>
>
>
>
> <ggher...(a)gmail.com> wrote:
> >On Aug 10, 8:57 pm, John Larkin
> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> >> On Tue, 10 Aug 2010 07:13:20 -0700 (PDT), George Herold
>
> >> <ggher...(a)gmail.com> wrote:
> >> >On Aug 9, 11:00 pm, John Larkin
> >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> >> >> On Mon, 9 Aug 2010 13:18:18 -0700 (PDT), George Herold
>
> >> >> <ggher...(a)gmail.com> wrote:
> >> >> >On Aug 8, 4:41 pm, John Larkin
> >> >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> >> >> >> On Sat, 7 Aug 2010 13:14:41 -0700 (PDT), George Herold
>
> >> >> >> <ggher...(a)gmail.com> wrote:
> >> >> >> >On Aug 7, 12:12 pm, John Larkin
> >> >> >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> >> >> >> >> On Thu, 5 Aug 2010 16:39:25 -0700 (PDT), Bret Cahill
>
> >> >> >> >> <BretCah...(a)peoplepc.com> wrote:
> >> >> >> >> >To get to a higher frequency, is it possible to just use a smaller cap
> >> >> >> >> >and/or resistor on op amp derivative taking circuits?
>
> >> >> >> >> What do you mean by "get to a higher frequency"? Do you mean "continue
> >> >> >> >> to be accurate at a higher signal frequency"?
>
> >> >> >> >> The size of the cap scales the constant K in
>
> >> >> >> >> OUT = K * (dIN/dt)
>
> >> >> >> >> but has nothing to do with how high a frequency the circuit will work
> >> >> >> >> at. The opamp determines that.
>
> >> >> >> >> The "pure" opamp differentiator, just a cap, a resistor, and an opamp,
> >> >> >> >> seldom works. It tends to be unstable and oscillate.
>
> >> >> >> >> Interestingly, its dual, the opamp integrator, has problems of its
> >> >> >> >> own.
>
> >> >> >> >> Do you have any specific performance goals in mind?
>
> >> >> >> >> John
>
> >> >> >> >What problems do you see with an integrator?  These always seem to
> >> >> >> >work just fine for me.
>
> >> >> >> They integrate their own voltage offset and bias current, of course.
> >> >> >> For something like a magnetic field probe coil, that gets to be the
> >> >> >> dominant error. Some cute periodic auto-zero becomes necessary.
> >> >> >> Chopper amps are great, but noisy.
>
> >> >> >> > I find the State Variable filter a bit 'scary'.  Whoever first
> >> >> >> >thought of putting to integrators in a row had a lot of 'guts'.  But I
> >> >> >> >love the outcome.
>
> >> >> >> We're just finishing up a product that jams 32 brutaly-pipelined
> >> >> >> 8-pole lowpass filters into one FPGA, sample rate 500 KHz per channel.
> >> >> >> The cutoff range is 50 KHz down to 1 Hz, and original concept, classic
> >> >> >> DSP butterfly stages, blew up mathematically. At 1 Hz we had allowable
> >> >> >> coefficients errors like one part in 10^40, and 2-pole stage gains
> >> >> >> like 10^17. This wasn't good. I suggested simulating a state-variable
> >> >> >> lowpass digitally, and that worked, using the 48 bit MACs in the
> >> >> >> Xilinx FPGA. The nice thing about state-variable filters is that you
> >> >> >> can make the 2-pole stage gains exactly 1, and the coefficients scale
> >> >> >> pretty much linearly on frequency.
>
> >> >> >" I like SV analog filters, but sometimes a Sallen-Key is better,
> >> >> > because the DC gain is 1 and doesn't depend on resistor accuracy."
>
> >> >> >I was measuring the DC gain of SV filters we are using a few months
> >> >> >ago.   I was amazed at how accurate they were.
> >> >> >I can't recall the exact numbers,  (My notebooks at work and I'm on
> >> >> >vacation.)  but gain error was much less than the 0.1% resistor
> >> >> >tolerance.
> >> >> >They all used the same 10k 0.1% Sumuso (sp) resistors, I guess the
> >> >> >resistors matched much better than 0.1%.  It's hard for me to measure
> >> >> >things to much better than 0.1%.  I need another digit on my
> >> >> >voltmeter.
>
> >> >> Susumu. They are fabulous, come from Digikey, and cost 1/10 of the
> >> >> Vishay stuff.
>
> >> >> >Say has anyone looked at the resistor values from 0.1% Sumuso (sp)
> >> >> >resistors?  I wonder if they have the same bimodal
> >> >> >distribution that was claimed for the old 10% tolerance carbon
> >> >> >resistors.  (where the 5% resistors were selected from the middle of
> >> >> >the
> >> >> >normal distribution.)  For those who don't know the better Sumuso
> >> >> >resistors also come in 0.05% tolerance.
>
> >> >> The actual available values are bizarre. Maybe they made what specific
> >> >> customers wanted, then put them on the market. Or used a random number
> >> >> generator.
>
> >> >> We tested some of the 0.05% parts, for TC. We got numbers like 5 and 8
> >> >> PPM/K.
>
> >> >> John
>
> >> >Ahh Susumu,  Thanks for the correction.  I wonder if the 0.1% are
> >> >rejects from 0.05% batches.  (The 0.1% cost something like 1/5 as
> >> >much.)  Maybe I'll try and measure some.... Say If I put them in a
> >> >bridge I can measure differences with a lot more resolution.  Is there
> >> >any easy way to swap chip resistors into some test jig?   I'll need to
> >> >keep the variations in the test jig resistance down below 0.1 ohm or
> >> >so.. (for 10k ohm samples).
>
> >> I think the only fair thing to do is solder them to real surface-mount
> >> pads on a board, especially to measure TC. You never know how stresses
> >> might effect things when you get to single digits of PPMs.
>
> >> John
>
> >Hmm If I want a quick measure of say 10 or 20 of them that sounds like
> >a lot of work.  Unsoldering surface mount is always a bit of a PITA.
> >I use solder wick and then push with the iron...
>
> >Maybe I could stand the R's on end, solder one end to a PCB and touch
> >solder a bit of wire to the other end.
>
> >The Sumusu data sheet does list TC's at the few ppm level.  Resistors
> >are pretty amazing.  What's the TC of a piece of copper?.. one part in
> >10^4 or something like that.
>
> Much worse, around 0.4% per K. Most metals are in that ballpark.
>
> John

Thanks, I knew it was big.

I should known, I see a few tenths of an ohm increase in our ~10ohm
Helmholtz coils with 3 amps going through them. They get warm maybe
60-70C.

George H.
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