reg + reference [Design]

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From: langwadt on 26 Mar 2010 22:45

On 27 Mar., 02:40, John Larkin
<jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> On Fri, 26 Mar 2010 17:01:38 -0700 (PDT), "langw...(a)fonz.dk"
>
>
>
> <langw...(a)fonz.dk> wrote:
> >On 26 Mar., 22:24, John Larkin
> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> >> On Fri, 26 Mar 2010 19:21:00 GMT, Jan Panteltje
>
> >> <pNaonStpealm...(a)yahoo.com> wrote:
> >> >On a sunny day (Fri, 26 Mar 2010 10:19:55 -0700) it happened John Larkin
> >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote in
> >> ><13rpq5lqd1lj1k9flb9orfr2vcl2tuu...(a)4ax.com>:
>
> >> >>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

From: Jan Panteltje on 27 Mar 2010 08:09

On a sunny day (Fri, 26 Mar 2010 18:27:35 -0700) it happened John Larkin
<jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in
<imnqq51aro61mta0ehao2copasiga90627(a)4ax.com>:

>>>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.
>>
>>It is senile,
>>the base will pick up all the noise it can get from the universe.
>
>I suppose all transistors do that. Low-noise preamps become
>impossible.

This circuit of yours is basically a rectifier (be junction).
In good opamps there is a current source in the emitter.
Indeed very few opamps are free from RF interference,
Dave had some chip on his video blog that actually could stand GSM signals close up, most cannot.

If you do not believe that, get a GSM phone, and put it next to your circuit.
Low pass the reference your create, and use your nnnn+ digit volt meter to see any changes,

As for creating the reference, you have plenty of volts on the input,
why not use something like a MCP1525?
Or simply a LM317, I use LM317 for everything, very stable output at low loads.
Some PICs have an internal programmable reference that you can output on a pin too IIRC.
Maybe it can replace some of the rest of your circuit :-)

>That's awfully good to know.
>
>John

I would have typed 'never to old to learn',
but of course in the case of Alzheimer that is no longer valid.

From: langwadt on 27 Mar 2010 10:08

From: Nico Coesel on 27 Mar 2010 10:21

John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:

>On Fri, 26 Mar 2010 14:34:19 -0700, Joerg <invalid(a)invalid.invalid>
>wrote:
>
>>John Larkin wrote:
>>> ftp://jjlarkin.lmi.net/V220_reg.JPG
>>>
>>
>>Just curious, why not replace all this with one of these:
>>
>>http://www.analog.com/static/imported-files/Data_Sheets/ADR01_02_03_06.pdf
>>
>
>>Much less space, cheaper. Unless you need the 3.3V somewhere.
>
>Yes, I need 3.3 to run an ARM processor, 40 mA or so maybe.

AFAIK the LPC1768 can run on lower voltages as well like down to 2.6V.
Not at full speed ofcourse. 40mA may be too optimistic though.

>The ADR could run off the +15, so maybe that's a better way to go. It
>would cost a little more, but the TC would be a lot better.
>
>Maybe I'll do that, the LM1117 for power and the ADR for the
>reference.
>
>Wait! LM4120 will work. +3 ref with 120 mV dropout!

Be sure to decouple the reference properly. The ADC sinks quite some
current from the reference pin when sampling.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico(a)nctdevpuntnl (punt=.)
--------------------------------------------------------------

From: Nico Coesel on 27 Mar 2010 10:25

Joerg <invalid(a)invalid.invalid> wrote:

>
>It's like press-fit where I sat in a meting where a company pitched it
>to a client. Tappa-di-tap went my calculator. "I calculated that 3 out
>of all the contacts will be open. Can you tell us which ones those will
>be?". The meeting and the sales pitch ended that instant ...

Sorry but that must be some calculating error on your side. All PABXes
I've seen had press-fit backplanes for a very good reason
(reliability). But just like soldering and crimping connections
mounting press-fit is something that needs to be done right.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico(a)nctdevpuntnl (punt=.)
--------------------------------------------------------------

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