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[DPRG] Working with very small voltage margins & a op amp question

Subject: [DPRG] Working with very small voltage margins & a op amp question
From: Ray Xu rayxu123 at gmail.com
Date: Wed Aug 19 18:35:06 CDT 2009

Hi guys,

I'm now finalizing my schematic for my laser diode controller, and I have
some concerns for the design of it.  To start, part of my design looks like
this: a digital voltage reference generator that is connected to a op
amp/MOSFET version voltage-current converter.  The laser diode current would
be digitally programmable through the voltage reference generator by
adjusting the gain (of the op amps) and adjusting a digital potentiometer
(that drives a LM4121), where the voltage from the voltage reference
generator to the laser diode current ratio is 1V:100mA.

So for example if I was going to drive a small 25mA laser diode; that would
mean the reference voltage generator's output would equal 250mV, and a +/-
10mV change would mean the laser being over-currented by +/- 1mA.

My concern here would be that, in the real world, the electronic noise would
either offset the "drive" voltage, or it would generate transient spikes
enough to blow the laser diode (they are sensitive to being over-driven,
BTW).  On paper, my design would work flawlessly so far.

Also, I cannot use any heavy (large inductors/capacitors, etc) filtering
since the signal has to be modulated at 1KHz (part of the calibration

How do I go about this (besides the usual capacitors and inductors...)?  How
should I isolate/decouple/bypass all of my supply rails and the signals
around the "sensitive" area?  How should I lay out my PCB design?

So impractical... :(

Also a quick question about op amps: For example I'm using the
LMC6062...Does the input or output capacitance make it unstable?  As far as
the datasheet says, I think it means both makes it unstable.  I don't really
get it; since the capacitance at the input is not within the "feedback
loop", it should be able to handle any amount of capacitance.  Can anyone
please correct me here?

Thanks everyone for all the help

Ray Xu
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