Sub uA battery powered current source.

On Friday, January 12, 2018 at 4:01:49 PM UTC-5, whit3rd wrote:
> On Friday, January 12, 2018 at 12:01:32 PM UTC-8, George Herold wrote:
> > On Thursday, January 11, 2018 at 8:35:01 PM UTC-5, whit3rd wrote:
> > > On Thursday, January 11, 2018 at 3:08:06 PM UTC-8, George Herold wrote:
> > > 
> > > > ...  I really like the Vref, R, opamp
> > > > for it's simplicity.  
> > > 
> > > It's certainly likeable, but the unknown  load connected directly to
> > > an op amp input makes me itch.  I like my control knobs away from
> > > the playful kittens.   Thus, a current mirror.
> > 
> > Hmm I never worried about the load connection.  I guess you could put 
> > ~1k ohm in series with the (+) input.  Or if more worried a pair of 
> > LND150's (my new favorite input protection, 'hammer'.)  
> 
> My concern was more with output isolation.   There's a microsecond or five 
> while an op amp resettles, that could disturb pulse-shapes.

Oh, you mean the time response?  If the load changes or set point 
voltage is modulated.  I've never really looked at that. 
For this case there's a ~1meg resistor (changing ~1-10% with the light,
 IIRC) with cable capacitance down a probe... ~10-100pF?  The time 
response is audio.  

Oh I needed to give myself a head slap. Two circuits ago in the 
current project (at work), The 10 uA current source* is as above, 
with lnd150's on the the output line... in series with the output and 
not just the feedback to the opamp.  (at 10 uA you don't really care about 
a kilo ohm.) 

George H.   
*diode temp sensor