Linearily of Derivative Circuits At High Frequency

[George Herold]

On Aug 11, 7:10 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 11 Aug 2010 06:10:15 -0700 (PDT), George Herold





ggher...@gmail.com> wrote:
On Aug 10, 8:57 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Tue, 10 Aug 2010 07:13:20 -0700 (PDT), George Herold

ggher...@gmail.com> wrote:
On Aug 9, 11:00 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 9 Aug 2010 13:18:18 -0700 (PDT), George Herold

ggher...@gmail.com> wrote:
On Aug 8, 4:41 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Sat, 7 Aug 2010 13:14:41 -0700 (PDT), George Herold

ggher...@gmail.com> wrote:
On Aug 7, 12:12 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Thu, 5 Aug 2010 16:39:25 -0700 (PDT), Bret Cahill

BretCah...@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.