Electronic – Reading 5v analog to 3V PIC

The first thing I'd look at is ripple in your voltage rail. Noisy voltages are killer with a voltage divider. A higher-quality measurement system might use a regulated constant current source to drive the RTD instead of a voltage divider. An LM317 can be used to do this - hook a resistor between the OUTPUT and ADJ terminal, and hook the RTD between ADJ and GND. The value of the resistor between output and adjust will set the current going through the RTD - use a precise resistor to be certain of the amount of current.

Otherwise, attempt to do the filtering in hardware if possible. You first have to figure out where the noise is coming from to make it effective. Determine what frequencies of noise you're seeing and then probe at the input to the gain stage, the output, and the input to the ADC. If there's noise everywhere, then it's in the source, otherwise it's being injected somewhere else. Make sure all of your IC's have bypass capacitors to start with. Then make sure that you don't have any long ground loops - make everything as direct (high current) of a connection to ground as possible. Don't daisy chain grounds - everything should get its own connection to ground that doesn't run through other chips.

If you're seeing noise at the source, chances are it's your voltage source for the divider. To combat this, you could put a capacitor in parallel with the RTD to make a simple filtering circuit. Just figure out what frequencies of noise you're seeing and match the capacitor to the resistance of the RTD and figure it out.

You would be better off with a barcode reader with serial or PS/2 interface. Sometimes it is as easy as swapping the cable for a different one to change the interface type, depends on your barcode scanner model.

If you really want to use the USB barcode reader, you need either

• a PIC that can be an USB host like PIC24FJ256GB106 or PIC32MX420FxxxH or
• a standalone USB host controller like MAX3421E
• a software implemented USB host stack - I know they exist for Atmel, not sure about PIC

Even with these hardware solutions it would still take some effort to implement enumeration, the HID device class and so on.

Outside the scope of your question and probably more expensive than a serial or PS/2 barcode scanner, but there is a precompiled firmware for FTDI's Vinculum chips that bridges an USB HID device to a microcontroller's SPI interface, see USB HID to SPI Master Sample Application ROM.