Dave Dilatush gave great advice:

<snip>

> The only modification I'd make to it is to change R6 back to 20K
> ohms as in Olin's design, to preserve his original PWM-to-current
> scaling.

That was just a stupid mistake. I'll change it back.

> Getting REALLY nit-picky, I might recommend reducing the values
> of R5, R8, R9 and R14 down to 100K ohms or so; the LM6132 is a
> bipolar opamp (as opposed to a CMOS opamp) and has pretty
> considerable input bias currents and offset current (50 nA worst
> case).  Current accuracy will be a little better with the lower
> resistance values (but, I admit, not spectacularly better).

I really don't think I'd use the LM6132 for real. It is rather pricey and
overspecced for the requirements. As Olin mentioned, he used it only because
he had it on the shelf.

> If you do that, remember to make R14 lower than the other three
> resistors by 15K ohms (the PWM filter's equivalent output
> resistance) to keep everything balanced.

Makes sense.

> But like I said, that's a small nit.

> One thing to watch out for- the LM6132 is a real screamer of an
> opamp.  Its gain-bandwidth product is at least 10 MHz so careful
> layout is a must: keep all connections to its inputs as short as
> possible, or it can oscillate like a politician.

Again, I probably would use a slower amp.

Bob Ammerman
RAm Systems

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