On Fri, 26 Mar 2010 17:01:38 -0700 (PDT), " snipped-for-privacy@fonz.dk"
>
>
>
> wrote:
> >> >> On Fri, 26 Mar 2010 19:21:00 GMT, Jan Panteltje
>
> >> wrote:
> >> >On a sunny day (Fri, 26 Mar 2010 10:19:55 -0700) it happened John Larkin
> >> > wrote in
> >> >:
>
> >> >>ftp://jjlarkin.lmi.net/V220_reg.JPG
>
> >> >>John
>
> >> >No money for a real current source or current mirror?
> >> >Does that 1M depending on beta circuit change a lot over temperature?
> >> >Its Vbe will change, so will the Ib.
>
> >> The current into the reference will be more constant than if I just
> >> used a resistor with a nominal 0.3 volt drop across it. The BCX71K has
> >> a very tight beta spec. It would probably be better than using the
> >> current mirror discussed in another thread recently. And we have
> >> BCX71Ks in stock for under 4 cents each.
>
> >> The +15 is not very well regulated, and I'm tight for space.
>
> >> I guess I could use a depletion mode fet from +15. That's just one
> >> part. The current regulation would be fair, but I wouldn't have any
> >> flexibility on setting current. An LND150 is 33 cents, no problem
> >> there, but Idss is 1 to 3 mA and it may change a bunch with
> >> temperature.
>
> >> Or maybe there exists a 3 volt 3-wire reference that would work from
> >> 3.3 volts.
>
> >use the ~3.3V as reference, measure a 1.2V reference voltage and
> >correct for it?
>
> I could use a 1.2 volt shunt reference, with a reasonable resistor
> from 3.3, and use an opamp to scale the 1.2 up to 3. More parts! The
> National low-dropout reference is ideal, if it doesn't oscillate.
>
>
>
> >> This will be the ADC voltage reference for an NXP ARM processor. I'm
> >> doing a 12-channel 4-20 mA thing, electrically isolated per channel,
> >> with an ARM per channel. The ARM and other stuff might need 45 mA or
> >> so, less if we can sleep the code a lot.
>
> >why not pick an ARM with a build in reference? something like
> >stm32... ?
> >12 bits too, think the nxp is only 10bits
>
> The NXP LPC1768 we're going to use has a 12 bit mux'd ADC and a 10 bit
> DAC, and all the other stuff like ram, flash, SPI we need. It's a 100
> MHz CPU with single-cycle multiply, pretty impressive for around $7.
> We're going to be running a couple of PID loops as fast as the ADC can
> feed us data, 100K hits a second maybe.
>
> Some of the ST parts look nice, but we already have the compiler/jtag
> infrastructure in place for the NXP, and we're blinking LEDs, so we'll
> stick with that.
yep lots of ARM mcus now, very fast and with impressive amounts of memory for little money. but it can quickly take a few days to switch to another one if you use lots of peripherals
>
>
> >I'm sure analog has some too
>
> ARM seems to have won the embedded game. Freescale/Coldfire is rumored
> to be seriously ill.
>
> John
I meant analog devices, they have a few ARM7 devices, even some with
24 bits adcs and a 10ppm reference. Though I'm not sure how much they are used and thus how they will be available.Even freescale has some ARM7s, mac7xxx, they run from 5volt with an internal vreg and even have real 5volt IOs
-Lasse
-Lasse