>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>Sent: 07 December 2006 19:22
>To: pic microcontroller discussion list
>Subject: [PIC] ADC voltage mismatch problem
>
>
>  I will be taking 16 samples in quick 
>succession every 0.1 sec.
>

Note that this method does depend on the input being suitably dithered! i.e. it won't work so well on a very clean input signal.

>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?

A capacitor would help with transient currents taken during conversion, but not with leakage currents (which are temperature dependant).

>
>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.

My suggestion would be one of the "flying capacitor" voltage doublers, made by Maxim, Linear, Nat-Semi etc.  These are small and efficient, perfect for low current applications like yours.  You can get regulated or unregulated varieties.  Unregulated followed by a micropower linear regulator would probably give the best noise performance.

Regards

Mike


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