I need to operate a 1 milliamp panel meter using a 16C54.
The meter will be used to display measured periods, which
I have set up the timer to determine. The problem I am
having concerns driving the analog meter from 0 to 1 ma.
My first approach was to build a R-2R ladder using port_b
pins (yes, all 8 of them). The problem with this is that
the meter was loading down the ladder (the meter in series
with 5 Kohm), so I addded a unity-gain op-amp. But now I need
a rail-to-rail op-amp to drive the meter to its full
range (from 0 to 5 volts).

I started looking at other ways to accomplish the above meter
drive. I looked at commercial DAC chips, but all that I have found
need split supplies or 10 volts or more (I am driving the PIC
with 5 volts - no other supply is available). Is there a DAC
chip on the market which operates on single 5-volt supply?

I have also looked at using a PWM approach. The internal
timer is running at a frequency of 125 Khz, so I could set up
a PWM output on one of the pins. My questions are:

1) What filtering do I need on the output?  I only need around
10 Hz response on the meter, so I guess a simple RC would
work (right???)

2) Is this approach to generating analog outputs very accurate?

3) How do I work out the required components?

Thanks for any help - Bruce
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                 Bruce A. Bowling
                 Staff Scientist
   Thomas Jefferson National Accelerator Facility
    12000 Jefferson Ave - Newport News, VA 23602
                 (804) 249-7240
bowling@cebaf.gov  http://devserve.cebaf.gov/~bowling
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