Bob Ammerman wrote...

>Did I analyze this right?

Yup, pretty much.

>Does it have a chance of working?

Everything has a chance of working.  This seems to have a very
good chance of working: as others have pointed out, this circuit
is a variant of the Howland current source.

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.

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).

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.

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.

Dave

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