Hi Gordon,

Too bad you can't run the sensors at 3.3 V!


> 1)  The sensor output (up to 5V) is greater than the allowable ADC input
> because the PIC is at 3.3V.  A voltage divider would have to use large
> resistances ~50 - 100 Kohm to limit the current from the sensor (max
> 0.1mA).
> This exceeds the PIC ADC input impedance.  Therefore what are the
> alternatives?  Could I use a capacitor in parallel with the bottom leg of
> the voltage divider to supply the ADC input impedance requirements. Do I
> need to use an op amp with a gain <1?  I have a few LM158 on hand.  Any
> other ideas?

Resistive divider and then the opamp as a buffer?

>
> 2)  What should I be using for the Vref for the ADC.  Ideally I would want
> to use the pressure sensor supply, but it will be at ~5 V.  As the sensor
> is
> ratiometric I should have the ADC ratiometric using the same voltage.

Got another opamp? Do the same thing to divide the +5 supply down to use
as + reference.

> 3)  What is the simplest way to create the 5 V for the two sensors (and
> opamps?).  The required supply will be up to 7-10 mA per sensor, so maybe
> around 20-25 mA in total.  I have a few pins available on the PIC if that
> is
> any use.  A LDO regulator will clean up the noise for the 3.3V PIC and
> peripherals.

MAX710 works great for me. And unlike many other Maxim parts, it has
always been available.

Cheerful regards,

Bob


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