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From: Grant on 22 Jul 2010 17:59
On Thu, 22 Jul 2010 06:57:55 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:

>On Thu, 22 Jul 2010 15:14:13 +1000, Grant <omg(a)grrr.id.au> wrote:
>
>>On Wed, 21 Jul 2010 20:33:03 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote:
>>
>>>On Wed, 21 Jul 2010 19:59:24 -0700, John Larkin
>>><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:
>>>
>>>>On Thu, 22 Jul 2010 12:44:18 +1000, Grant <omg(a)grrr.id.au> wrote:
>>...
>>>>>
>>>>>I settled for a 1N3595 recently as it leaks a lot less (1nA) than the
>>>>>'HC4053 analog mux chip (100nA, both datasheet 25'C numbers) it is
>>>>>protecting from signal over-voltage.
>>>>>
>>>>>For the curious, I have two signals, out of OP07s running from +12V
>>>>>and -5V, via 1k6 resistor, a 1N3595 diode to 4V stops the signal
>>>>>exceeding 5V as it goes into a 'hc4053 mux switch. There's a cap
>>>>>to analog ground as well. The 4V is a TL431 with scaling resistors
>>>>>(could use 3.9V zener) sinking several mA so it has low impedance.
>>>>
>>>>Good. Don't count on the HC40xx mux ESD diodes to clamp signals, even
>>>>if the source resistance is high. Once you get a little beyond the
>>>>rails, the internal series switch fets can be turned on, and all hell
>>>>breaks loose, many nanoamps of it.
>>
>>All sorts of chaos including power getting mixed with signals, I've
>>read the warnings in the various datasheets :) No way do I want to
>>temp that chip into making errors when I'm chasing 15 or 16 bit
>>resolution.
>>>>
>>>>Central Semi makes some nice dual super-low-leakage surface-mount
>>>>diodes, 10s of fA.
>>
>>Don't think I need to go that far, yet. Perhaps if I had to go to
>>better class analog mux chips and +/-15V rails.
>>
>>>>
>>>>John
>>>>
>>>
>>>Or do the job properly :-)
>>
>>Me? I don't want to try clamping the output via feedback network
>>around the OP07 as there's a set of four 0.1% resistors setting the
>>gain. I figure smallish resistor plus diode clamp was less evil
>>than passing signal through another opamp. Not too fussed about
>>noise since the ADC on other side of analog mux is a dual slope
>>thingy. Span is done via reference adjustment, null offset on
>>each OP07.
>>
>>Only running about ten conversions per second. PIC chip will run
>>the ADC front end chip, last piece of this little puzzle, I hope...
>>
>>So far: http://grrr.id.au/adc-hires/
>>
>>Grant.
>
>I used to do my own dual-slopes. Capacitor dielectric absorption was
>usually the major error source.

Yes, got a big 5% pp 470n cap in there for the integrator, try for
maximum integrator swing to get the resolution I'm after.

> Nowadays, you just buy a 24-bit
>delta-sigma and talk horrible SPI commands to it.

They tempt me too, but I like what seems like the last dual slope
converters out there, have used the 4 1/2 digit with counters, and
the same analog front end available on it's own (TC500) I'm trying
here, as then I can directly count to 32000 in the PIC, rather than
massage DC offset conversion with the 4 1/2 digit meter chip.
>
>John

Much of my recent work is recapping where I've been before in design,
(I'm returning after some years), in the '80s I made a 50,000 count
converter that worked well. We had one of those calibration boxes
you could set to any uV value up to 11V to check it with.

Don't have one of them here, but the application I'm working on is
more about resolution than absolute accuracy over time, so a Fluke
4 1/2 digit meter is good enough for the calibration standard.

Now I'm trying to see how far I can go with PIC chip and what's
easily available. I'll be using delta-sigma in hybrid power DAC
to get 15 or 16 bit resolution. This ADC is the feedback for that
project.

Funny thing is that I designed something like an unclocked delta-
sigma converter (had the integrator + comparator, no clocked flip-
flop), for accurately passing an analog signal through an optocoupler
back around 1981.

Grant.
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