Non-linear functions ...
in [Design]
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From: Spehro Pefhany on 26 Nov 2009 15:46 On Thu, 26 Nov 2009 21:09:28 +0100, Fred Bartoli <" "> wrote: >Fred Bartoli a �crit : >> Richard Rasker a �crit : >>> Fred Bartoli <" "> wrote: >>> >>>> Spehro Pefhany a �crit : >>>>> On Wed, 25 Nov 2009 16:10:31 +0100, Fred Bartoli <" "> wrote: >>>>> >>>>>> Hmmm, even simpler... >>> >>> [snip great, simple antilog circuit] >>> >>>> Thanks for calculating the values I was to lazy to compute. >>>> >>>> Now I can reveal the whole world the last simplification bit (ahem) : >>>> that is, if you make sure the duty cycle is far enough from 100%, which >>>> sure would be with a 100Hz clock, then you can just delete the 1V >>>> reference and make it a simple resistor (with a small bypass cap) so >>>> that the 6.02V reference with the 24K9/Rsource divider just gives you >>>> the wanted 1V at the capacitor top. >>>> Adjust the source bypass cap to optimize the circuit behavior WRT the >>>> mosfet charges. A value from the low tens of pF to maybe 1nF. >>>> >>>> Also, please applause the effort I made in disclosing the resistor's >>>> secret value (just had a cup of coffee :-). >>>> >>>> That would be: >>>> 24K9 10n >>>> ___ || >>>> 6.02V >--|___|--+--||---. >>>> | || | >>>> | === >>>> Vout >---------. .--------+ GND >>>> | | | >>>> .-----. | >>>> \+ -/ +-|| >>>> comparator \ / ->|| optim. >>>> V +-||---. .---||-----. >>>> >>>> | | | | ___ | >>>> | '--------+--|___|---+ >>>> >>>> .--o--. | 4K96 | >>>> GND -|D S Q|-------------' === >>>> | | GND >>>> 100Hz Clk >---|> -| ___ >>>> | R Q|---|___|--+-----> Flow >>>> '--o--' | 200mV full scale >>>> --- >>>> --- >>>> | >>>> === >>>> GND >>>> >>> >>> >>> Absolutely wonderful, I'm truly amazed how even elegant, simple >>> circuits can >>> often be optimized and refined even further! >>> Now talking about optimization: from this design, it would appear that >>> one >>> could swap the MOSFET and 4K96 resistor -- which would mean that the >>> internal discharge FET (using an ICM7555) can be used, doing away with an >>> external MOSFET altogether. >>> >> >> Of course you're absolutely right (I guess one cup of coffee wasn't >> enough this morning). >> Now you've really squizzed the last remaining bit of optimization out of >> this. >> >> > >Argh... And also don't forget to shift the 6.02V by one volt too and >make it 7.02V, which makes the resistor's secret value 4K14 It might be worth dividing down the input voltage to allow the 6.02V reference to be the same as the output reference (eg. FF power supply) which I assumed to be 5.00V). Then a series 5V 0.1% reference like the ADR395 could be used and a 1% NP0 cap. Stray capacitance would require a bit of fiddling to compensate for, but it should be pretty darn stable.
From: whit3rd on 28 Nov 2009 16:50
On Nov 24, 2:20 pm, Richard Rasker <spamt...(a)linetec.nl> wrote: > Jon Kirwan wrote: > >>Does anyone know of designs which provide a better fit for this type of > >>logarithmic function, and preferably a better temperature stability? > > Third was to consider recommending a micro, which is a rather common > > approach to conditioning sensors these days. > > I know. But I'm one of those old school die-hards who prefers hooking up a > dozen or so components to a meter i Well, just for YOU, then, how about using a straight two-slope converter except that the counter value selects a tap on an analog multiplexer, to a multiplicity of current sources. The 'discharge' part of the converter doesn't operate from a fixed reference current, but from a MODULATED reference current, programmed by the converter count value. Unlike the diode-feedback schemes, this makes a true polygon of the readout as a function of input. A '4051 analog switch will suffice for an octagon. A PAL does the digital, a '4051 and some op amps does the conversion, and the readout can be a decimal counter module. Eight or so fixed resistor values set the curve. |