Prev: Opto-isolated zero crossing detector
Next: MAKE UPTO $5000 MONTHLY! $2000 INYOUR FIRST 30 DAYS!
From: a7yvm109gf5d1 on 14 Jul 2010 21:49
I was thinking of using discrete single or dual transistors to make a
current mirror, for currents of about 1uA to 3mA.
Then I found that Zetex makes current mirror devices.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=ZDS1009CT-ND
Zetex ZDS1009TA
OK, fine, but what I don't get is the suggested application of high
side sensing as shown in the datasheet.
Why not just use a RR op amp? These current mirrors are 5$. That's
crazy, no?
Is it just the fact it can withstand a certain voltage differential
and can be used to level translate?

Anyways my initial application was to translate the current output
from an old 8 bit current output DAC in the range of 0 - 0.3mA to the
needed 0-3mA.

I suppose I can use this Zetex part and try to get a gain of 10, but I
have to admit I haven't looked at this stuff in ages. I can barely
handle Ohms law lately. I guess it's time to go into management?! :)
From: Michael Robinson on 14 Jul 2010 22:37

<a7yvm109gf5d1(a)netzero.com> wrote in message
news:bf518e90-de62-4b97-9370-3f8af823ac6c(a)w30g2000yqw.googlegroups.com...
>I was thinking of using discrete single or dual transistors to make a
> current mirror, for currents of about 1uA to 3mA.
> Then I found that Zetex makes current mirror devices.
> http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=ZDS1009CT-ND
> Zetex ZDS1009TA
> OK, fine, but what I don't get is the suggested application of high
> side sensing as shown in the datasheet.
> Why not just use a RR op amp? These current mirrors are 5$. That's
> crazy, no?
> Is it just the fact it can withstand a certain voltage differential
> and can be used to level translate?
>
> Anyways my initial application was to translate the current output
> from an old 8 bit current output DAC in the range of 0 - 0.3mA to the
> needed 0-3mA.
>
> I suppose I can use this Zetex part and try to get a gain of 10, but I
> have to admit I haven't looked at this stuff in ages. I can barely
> handle Ohms law lately. I guess it's time to go into management?! :)

Watch out. If one transistor gets as little as a degree or two warmer than
the one across from it, it will kill the accuracy. 3 mA could do that.
There's an interesting thread about current mirrors you can find by going on
google groups and searching for "accuracy of bcv62"


From: Bill Sloman on 15 Jul 2010 02:38
On Jul 15, 12:37 pm, "Michael Robinson" <kellrobin...(a)yahoo.com>
wrote:
> <a7yvm109gf...(a)netzero.com> wrote in message
>
> news:bf518e90-de62-4b97-9370-3f8af823ac6c(a)w30g2000yqw.googlegroups.com...
>
>
>
> >I was thinking of using discrete single or dual transistors to make a
> > current mirror, for currents of about 1uA to 3mA.
> > Then I found that Zetex makes current mirror devices.
> >http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=ZDS1....
> > Zetex ZDS1009TA
> > OK, fine, but what I don't get is the suggested application of high
> > side sensing as shown in the datasheet.
> > Why not just use a RR op amp? These current mirrors are 5$. That's
> > crazy, no?
> > Is it just the fact it can withstand a certain voltage differential
> > and can be used to level translate?
>
> > Anyways my initial application was to translate the current output
> > from an old 8 bit current output DAC in the range of 0 - 0.3mA to the
> > needed 0-3mA.
>
> > I suppose I can use this Zetex part and try to get a gain of 10, but I
> > have to admit I haven't looked at this stuff in ages. I can barely
> > handle Ohms law lately. I guess it's time to go into management?! :)
>
> Watch out.  If one transistor gets as little as a degree or two warmer than
> the one across from it, it will kill the accuracy.  3 mA could do that.
> There's an interesting thread about current mirrors you can find by going on
> google groups and searching for "accuracy of bcv62"

As soon as your current mirror becomes asymmetrical, you also have to
worry about intrinsic emitter reisitance - base resistance can also
screw you up, but not usually as much.

Cascode transistors can decrease the differential self-heating - if
you wanted top be really cute you could organise to have ten times the
emitter-collector voltage drop across the 0- 0.3mA device that you had
across the 0- 3mA device.

--

Bill Sloman, Nijmegen
 | 
Pages: 1
Prev: Opto-isolated zero crossing detector
Next: MAKE UPTO $5000 MONTHLY! $2000 INYOUR FIRST 30 DAYS!