Hi,

I have a couple of problems with a battery powered data logging system
design that I am putting together and I am hoping that someone can come up
with some ideas.

I am measuring the pressure with two freescale pressure sensors where the
output (~0 to 5 V) is ratiometric with Vs = 5 +/- 0.25 V.  The max. output
current at full scale is 0.1 mA.

I will be using a PIC for the ADC and I *think* that it is best that I run
this at 3.3V because I will also have a graphic LCD and an MMC memory that
take this voltage to make connecting easier.  To get the sensor resolution
that I need I will be oversampling and decimating to get an effective ADC
output of 12 bits.  I will be taking 16 samples in quick succession every
0.1 sec.

I will be using 4 NiMH or NiCad batteries for the power source.  This power
source (~4 to 6 V) will also be powering another device which will be
controlling and powering some small brushed DC motors (servos).  I haven't
measured the noise, but I expect that it will be quite noisy.

I'm looking to put together a very small and light package just a bit larger
than the MMC memory with a minimum parts.  And it has to be cheap :)

These are the problems that I am mulling over:

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?

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.

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.

Thanks for your suggestions.

Regards,

Gordon Williams

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