Some sensors output a current rather than a voltage. The most common sensor of this type is the photodiode which has a current output proportional to the amount of light shining on it. The purpose of the signal conditioning circuitry is to convert the current output of the sensor to a voltage.
In converting a current to a voltage, one usually uses an inverting amplifier configuration,
since a non-inverting amplifier draws very little current. Figure 31 shows a current
amplifier connected to a photodiode. Notice that the circuit is identical to the non-
inverting amplifier with the input resistor removed. As the light increases, the current
output of the photdiode increases, increasing 
proportionally:
Figure 31: Photocell Connected to a Current Amplifier
