Analog Whacker

Analog Whacker -- An Analog Input Signal Conditioner for the UBW USB BitWhacker

 * Name: Analog Wacker ( Whacker )
 * Status: Concept, developing.
 * Technology: Operational Amplifiers
 * Author: russ_hensel ( where you can find an email address to reach me )


 * Purpose: to provide a board which will condition analog signals so that the USB Bit Whacker (UBW) can convert the analog signals to digital ones. We will also be providing specialized software to make working with the hardware more useful.  If you would like to participate in this project contact russ_hensel.

Issuses for Input to Analog Pins
Input to analog pins involves several issues. A few of the most basic and our approach to dealing with them will be discussed here.


 * The voltage you have may not be the voltage you want. The UBW typically wants a voltage from 0 to 5 volts.  We will use a gain controlled operational amplifier to either amplify or attenuate the input voltage.  Thus a analog therometer with an output of 0 to 1 volt will be amplified to 0 to 5 volts.


 * These inputs are not entirely passive. Often they present a high impedance, but during the sampling period they have a substantial capacitance.  This if they are driven from a high impedance you will not get an accurate sample of the input.  A common solution to this is to drive them from an operational amplifier.  This is what this circuit does.


 * A second problem is that the inputs measure only positive voltages. An op amp again offers a solution to this, either by inverting the input or by offsetting the voltage from zero.  We are using non inverting amplifier, but providing for offset.


 * Input pins should not be driven above the voltage of the PIC power supply or below zero. We will use diodes to limit these voltages.


 * A low pass filter is needed at the input. Idealy, the filter should cut off all frequencies above half the sample rate.  Maybe omitted for very slowly changing signals such as from a temperature sensor.

Guidelines for the Project

 * Circuit board should be single sided and be fairly easily etched using toner transfer.
 * Board should support different configurations depending on how it is populated and how jumpers are placed.
 * Common parts should be used.
 * Board/schematic in Eagle (free version). I have a first circuit that will autoroute.
 * Much of calibration adjustment will be done in software rather than high precision parts in the hardware.

Ideas
Single ended input, for ballanced input or current monitor, will need a seperate board.

single ended supply – invert in software do not accurately 0, do in software but use 1 percent to center on 0 ( maybe even power from a port so can center or not )

Add calibrate to the BitWhacker app

Just diodes for over under voltage protect

Current monitor on separate board

Offset for say diode temp – limit number

gain of 50 max, use jump to shift or what.

connect to BitWhacker how

5 volt reference from local 5v reg,