At 04:51 PM 3/7/2010, you wrote:

>I need to think about using a PIC or other small micro. VFC chips are
>nice and simple, but they pretty much all require a trimpot for
>calibration, and NPO caps. If I use a micro I can get one with a bandgap
>reference and use a crystal for frequency accuracy and then I won't need
>to trim it. But each one will require programming.

Sounds like a reasonable way to go.. decent band gap references are not
expensive (maybe use a series one in place of a voltage regulator), and
that and a crystal (or maybe a ceramic resonator) frequency
reference would do the trick if you don't need high ADC resolution and
want to avoid any adjustments.  Maybe $2-3 in parts @ 500 qty.

Doesn't compare with the 16 bit+ resolution and 15 bit linearity you
can get out of a VCO though, so it might not be suitable as an input
for a PID controller, for example, at least using a uC with a 10-bit
ADC and no trickery. There are some cheap MSP430s with 16-bit delta-sigma
converters, but IIRC the guaranteed accuracy is not so great.

Again, if you could control the other side of the isolation barrier it
would be simple to store zero and span calibration numbers for each
channel and use jellybean parts.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist